zerocopy/

lib.rs

// SPDX-License-Identifier: (BSD-2-Clause OR Apache-2.0) OR MIT
//
// Copyright 2018 The Fuchsia Authors
//
// Licensed under the 2-Clause BSD License <LICENSE-BSD or
// https://opensource.org/license/bsd-2-clause>, Apache License, Version 2.0
// <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT
// license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.
// This file may not be copied, modified, or distributed except according to
// those terms.

// After updating the following doc comment, make sure to run the following
// command to update `README.md` based on its contents:
//
//   (cd .. && cargo -q run --manifest-path tools/Cargo.toml -p generate-readme) > README.md

//! ***<span style="font-size: 140%">Fast, safe, <span
//! style="color:red;">compile error</span>. Pick two.</span>***
//!
//! Zerocopy makes zero-cost memory manipulation effortless. We write `unsafe`
//! so you don't have to.
//!
//! *For an overview of what's changed from zerocopy 0.7, check out our [release
//! notes][release-notes], which include a step-by-step upgrading guide.*
//!
//! *Have questions? Need more out of zerocopy? Submit a [customer request
//! issue][customer-request-issue] or ask the maintainers on
//! [GitHub][github-q-a] or [Discord][discord]!*
//!
//! [customer-request-issue]: https://github.com/google/zerocopy/issues/new/choose
//! [release-notes]: https://github.com/google/zerocopy/discussions/1680
//! [github-q-a]: https://github.com/google/zerocopy/discussions/categories/q-a
//! [discord]: https://discord.gg/MAvWH2R6zk
//!
//! # Overview
//!
//! ##### Conversion Traits
//!
//! Zerocopy provides four derivable traits for zero-cost conversions:
//! - [`TryFromBytes`] indicates that a type may safely be converted from
//!   certain byte sequences (conditional on runtime checks)
//! - [`FromZeros`] indicates that a sequence of zero bytes represents a valid
//!   instance of a type
//! - [`FromBytes`] indicates that a type may safely be converted from an
//!   arbitrary byte sequence
//! - [`IntoBytes`] indicates that a type may safely be converted *to* a byte
//!   sequence
//!
//! These traits support sized types, slices, and [slice DSTs][slice-dsts].
//!
//! [slice-dsts]: KnownLayout#dynamically-sized-types
//!
//! ##### Marker Traits
//!
//! Zerocopy provides three derivable marker traits that do not provide any
//! functionality themselves, but are required to call certain methods provided
//! by the conversion traits:
//! - [`KnownLayout`] indicates that zerocopy can reason about certain layout
//!   qualities of a type
//! - [`Immutable`] indicates that a type is free from interior mutability,
//!   except by ownership or an exclusive (`&mut`) borrow
//! - [`Unaligned`] indicates that a type's alignment requirement is 1
//!
//! You should generally derive these marker traits whenever possible.
//!
//! ##### Conversion Macros
//!
//! Zerocopy provides six macros for safe casting between types:
//!
//! - ([`try_`][try_transmute])[`transmute`] (conditionally) converts a value of
//!   one type to a value of another type of the same size
//! - ([`try_`][try_transmute_mut])[`transmute_mut`] (conditionally) converts a
//!   mutable reference of one type to a mutable reference of another type of
//!   the same size
//! - ([`try_`][try_transmute_ref])[`transmute_ref`] (conditionally) converts a
//!   mutable or immutable reference of one type to an immutable reference of
//!   another type of the same size
//!
//! These macros perform *compile-time* size and alignment checks, meaning that
//! unconditional casts have zero cost at runtime. Conditional casts do not need
//! to validate size or alignment runtime, but do need to validate contents.
//!
//! These macros cannot be used in generic contexts. For generic conversions,
//! use the methods defined by the [conversion traits](#conversion-traits).
//!
//! ##### Byteorder-Aware Numerics
//!
//! Zerocopy provides byte-order aware integer types that support these
//! conversions; see the [`byteorder`] module. These types are especially useful
//! for network parsing.
//!
//! # Cargo Features
//!
//! - **`alloc`**
//!   By default, `zerocopy` is `no_std`. When the `alloc` feature is enabled,
//!   the `alloc` crate is added as a dependency, and some allocation-related
//!   functionality is added.
//!
//! - **`std`**
//!   By default, `zerocopy` is `no_std`. When the `std` feature is enabled, the
//!   `std` crate is added as a dependency (ie, `no_std` is disabled), and
//!   support for some `std` types is added. `std` implies `alloc`.
//!
//! - **`derive`**
//!   Provides derives for the core marker traits via the `zerocopy-derive`
//!   crate. These derives are re-exported from `zerocopy`, so it is not
//!   necessary to depend on `zerocopy-derive` directly.
//!
//!   However, you may experience better compile times if you instead directly
//!   depend on both `zerocopy` and `zerocopy-derive` in your `Cargo.toml`,
//!   since doing so will allow Rust to compile these crates in parallel. To do
//!   so, do *not* enable the `derive` feature, and list both dependencies in
//!   your `Cargo.toml` with the same leading non-zero version number; e.g:
//!
//!   ```toml
//!   [dependencies]
//!   zerocopy = "0.X"
//!   zerocopy-derive = "0.X"
//!   ```
//!
//!   To avoid the risk of [duplicate import errors][duplicate-import-errors] if
//!   one of your dependencies enables zerocopy's `derive` feature, import
//!   derives as `use zerocopy_derive::*` rather than by name (e.g., `use
//!   zerocopy_derive::FromBytes`).
//!
//! - **`simd`**
//!   When the `simd` feature is enabled, `FromZeros`, `FromBytes`, and
//!   `IntoBytes` impls are emitted for all stable SIMD types which exist on the
//!   target platform. Note that the layout of SIMD types is not yet stabilized,
//!   so these impls may be removed in the future if layout changes make them
//!   invalid. For more information, see the Unsafe Code Guidelines Reference
//!   page on the [layout of packed SIMD vectors][simd-layout].
//!
//! - **`simd-nightly`**
//!   Enables the `simd` feature and adds support for SIMD types which are only
//!   available on nightly. Since these types are unstable, support for any type
//!   may be removed at any point in the future.
//!
//! - **`float-nightly`**
//!   Adds support for the unstable `f16` and `f128` types. These types are
//!   not yet fully implemented and may not be supported on all platforms.
//!
//! [duplicate-import-errors]: https://github.com/google/zerocopy/issues/1587
//! [simd-layout]: https://rust-lang.github.io/unsafe-code-guidelines/layout/packed-simd-vectors.html
//!
//! # Build Tuning
//!
//! ## `--cfg zerocopy_inline_always`
//!
//! Upgrades `#[inline]` to `#[inline(always)]` on many of zerocopy's public
//! functions and methods. This provides a narrowly-scoped alternative that
//! *may* improve the optimization of hot paths using zerocopy without the broad
//! compile-time penalties of configuring `codegen-units=1`.
//!
//! # Security Ethos
//!
//! Zerocopy is expressly designed for use in security-critical contexts. We
//! strive to ensure that that zerocopy code is sound under Rust's current
//! memory model, and *any future memory model*. We ensure this by:
//! - **...not 'guessing' about Rust's semantics.**
//!   We annotate `unsafe` code with a precise rationale for its soundness that
//!   cites a relevant section of Rust's official documentation. When Rust's
//!   documented semantics are unclear, we work with the Rust Operational
//!   Semantics Team to clarify Rust's documentation.
//! - **...rigorously testing our implementation.**
//!   We run tests using [Miri], ensuring that zerocopy is sound across a wide
//!   array of supported target platforms of varying endianness and pointer
//!   width, and across both current and experimental memory models of Rust.
//! - **...formally proving the correctness of our implementation.**
//!   We apply formal verification tools like [Kani][kani] to prove zerocopy's
//!   correctness.
//!
//! For more information, see our full [soundness policy].
//!
//! [Miri]: https://github.com/rust-lang/miri
//! [Kani]: https://github.com/model-checking/kani
//! [soundness policy]: https://github.com/google/zerocopy/blob/main/zerocopy/POLICIES.md#soundness
//!
//! # Relationship to Project Safe Transmute
//!
//! [Project Safe Transmute] is an official initiative of the Rust Project to
//! develop language-level support for safer transmutation. The Project consults
//! with crates like zerocopy to identify aspects of safer transmutation that
//! would benefit from compiler support, and has developed an [experimental,
//! compiler-supported analysis][mcp-transmutability] which determines whether,
//! for a given type, any value of that type may be soundly transmuted into
//! another type. Once this functionality is sufficiently mature, zerocopy
//! intends to replace its internal transmutability analysis (implemented by our
//! custom derives) with the compiler-supported one. This change will likely be
//! an implementation detail that is invisible to zerocopy's users.
//!
//! Project Safe Transmute will not replace the need for most of zerocopy's
//! higher-level abstractions. The experimental compiler analysis is a tool for
//! checking the soundness of `unsafe` code, not a tool to avoid writing
//! `unsafe` code altogether. For the foreseeable future, crates like zerocopy
//! will still be required in order to provide higher-level abstractions on top
//! of the building block provided by Project Safe Transmute.
//!
//! [Project Safe Transmute]: https://rust-lang.github.io/rfcs/2835-project-safe-transmute.html
//! [mcp-transmutability]: https://github.com/rust-lang/compiler-team/issues/411
//!
//! # MSRV
//!
//! See our [MSRV policy].
//!
//! [MSRV policy]: https://github.com/google/zerocopy/blob/main/zerocopy/POLICIES.md#msrv
//!
//! # Changelog
//!
//! Zerocopy uses [GitHub Releases].
//!
//! [GitHub Releases]: https://github.com/google/zerocopy/releases
//!
//! # Thanks
//!
//! Zerocopy is maintained by engineers at Google with help from [many wonderful
//! contributors][contributors]. Thank you to everyone who has lent a hand in
//! making Rust a little more secure!
//!
//! [contributors]: https://github.com/google/zerocopy/graphs/contributors

// Sometimes we want to use lints which were added after our MSRV.
// `unknown_lints` is `warn` by default and we deny warnings in CI, so without
// this attribute, any unknown lint would cause a CI failure when testing with
// our MSRV.
#![allow(unknown_lints, non_local_definitions, unreachable_patterns)]
#![deny(renamed_and_removed_lints)]
#![deny(
    anonymous_parameters,
    deprecated_in_future,
    late_bound_lifetime_arguments,
    missing_copy_implementations,
    missing_debug_implementations,
    missing_docs,
    path_statements,
    patterns_in_fns_without_body,
    rust_2018_idioms,
    trivial_numeric_casts,
    unreachable_pub,
    unsafe_op_in_unsafe_fn,
    unused_extern_crates,
    // We intentionally choose not to deny `unused_qualifications`. When items
    // are added to the prelude (e.g., `core::mem::size_of`), this has the
    // consequence of making some uses trigger this lint on the latest toolchain
    // (e.g., `mem::size_of`), but fixing it (e.g. by replacing with `size_of`)
    // does not work on older toolchains.
    //
    // We tested a more complicated fix in #1413, but ultimately decided that,
    // since this lint is just a minor style lint, the complexity isn't worth it
    // - it's fine to occasionally have unused qualifications slip through,
    // especially since these do not affect our user-facing API in any way.
    variant_size_differences
)]
#![cfg_attr(
    __ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS,
    deny(implicit_provenance_casts)
)]
#![deny(
    clippy::all,
    clippy::alloc_instead_of_core,
    clippy::arithmetic_side_effects,
    clippy::as_underscore,
    clippy::assertions_on_result_states,
    clippy::as_conversions,
    clippy::correctness,
    clippy::dbg_macro,
    clippy::decimal_literal_representation,
    clippy::double_must_use,
    clippy::get_unwrap,
    clippy::indexing_slicing,
    clippy::missing_inline_in_public_items,
    clippy::missing_safety_doc,
    clippy::multiple_unsafe_ops_per_block,
    clippy::must_use_candidate,
    clippy::must_use_unit,
    clippy::obfuscated_if_else,
    clippy::perf,
    clippy::print_stdout,
    clippy::return_self_not_must_use,
    clippy::std_instead_of_core,
    clippy::style,
    clippy::suspicious,
    clippy::todo,
    clippy::undocumented_unsafe_blocks,
    clippy::unimplemented,
    clippy::unnested_or_patterns,
    clippy::unwrap_used,
    clippy::use_debug
)]
// `clippy::incompatible_msrv` (implied by `clippy::suspicious`): This sometimes
// has false positives, and we test on our MSRV in CI, so it doesn't help us
// anyway.
#![allow(clippy::needless_lifetimes, clippy::type_complexity, clippy::incompatible_msrv)]
#![deny(
    rustdoc::bare_urls,
    rustdoc::broken_intra_doc_links,
    rustdoc::invalid_codeblock_attributes,
    rustdoc::invalid_html_tags,
    rustdoc::invalid_rust_codeblocks,
    rustdoc::missing_crate_level_docs,
    rustdoc::private_intra_doc_links
)]
// In test code, it makes sense to weight more heavily towards concise, readable
// code over correct or debuggable code.
#![cfg_attr(any(test, kani), allow(
    // In tests, you get line numbers and have access to source code, so panic
    // messages are less important. You also often unwrap a lot, which would
    // make expect'ing instead very verbose.
    clippy::unwrap_used,
    // In tests, there's no harm to "panic risks" - the worst that can happen is
    // that your test will fail, and you'll fix it. By contrast, panic risks in
    // production code introduce the possibly of code panicking unexpectedly "in
    // the field".
    clippy::arithmetic_side_effects,
    clippy::indexing_slicing,
))]
#![cfg_attr(not(any(test, kani, feature = "std")), no_std)]
#![cfg_attr(
    all(feature = "simd-nightly", any(target_arch = "powerpc", target_arch = "powerpc64")),
    feature(stdarch_powerpc)
)]
#![cfg_attr(feature = "float-nightly", feature(f16, f128))]
#![cfg_attr(doc_cfg, feature(doc_cfg))]
#![cfg_attr(__ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS, feature(coverage_attribute))]
#![cfg_attr(all(test, __ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS), feature(test))]

// This is a hack to allow zerocopy-derive derives to work in this crate. They
// assume that zerocopy is linked as an extern crate, so they access items from
// it as `zerocopy::Xxx`. This makes that still work.
#[cfg(any(feature = "derive", test))]
extern crate self as zerocopy;

#[cfg(all(test, __ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS))]
extern crate test;

#[doc(hidden)]
#[macro_use]
pub mod util;

pub mod byte_slice;
pub mod byteorder;
mod deprecated;

#[cfg(__ZEROCOPY_INTERNAL_USE_ONLY_DEV_MODE)]
pub mod doctests;

// This module is `pub` so that zerocopy's error types and error handling
// documentation is grouped together in a cohesive module. In practice, we
// expect most users to use the re-export of `error`'s items to avoid identifier
// stuttering.
pub mod error;
mod impls;
#[doc(hidden)]
pub mod layout;
mod macros;
#[cfg_attr(not(zerocopy_unstable_ptr), doc(hidden))]
#[cfg_attr(doc_cfg, doc(cfg(zerocopy_unstable_ptr)))]
pub mod pointer;
mod r#ref;
mod split_at;
// FIXME(#252): If we make this pub, come up with a better name.
mod wrappers;

use core::{
    cell::{Cell, UnsafeCell},
    cmp::Ordering,
    fmt::{self, Debug, Display, Formatter},
    hash::Hasher,
    marker::PhantomData,
    mem::{self, ManuallyDrop, MaybeUninit as CoreMaybeUninit},
    num::{
        NonZeroI128, NonZeroI16, NonZeroI32, NonZeroI64, NonZeroI8, NonZeroIsize, NonZeroU128,
        NonZeroU16, NonZeroU32, NonZeroU64, NonZeroU8, NonZeroUsize, Wrapping,
    },
    ops::{Deref, DerefMut},
    ptr::{self, NonNull},
    slice,
};
#[cfg(feature = "std")]
use std::io;

#[doc(hidden)]
pub use crate::pointer::{
    invariant::{self, BecauseExclusive},
    PtrInner,
};
pub use crate::{
    byte_slice::*,
    byteorder::*,
    error::*,
    r#ref::*,
    split_at::{Split, SplitAt},
    wrappers::*,
};

#[cfg(any(feature = "alloc", test))]
extern crate alloc;
#[cfg(any(feature = "alloc", test))]
use alloc::{boxed::Box, vec::Vec};
#[cfg(any(feature = "alloc", test))]
use core::alloc::Layout;

// Used by `KnownLayout`.
#[doc(hidden)]
pub use crate::layout::*;
// Used by `TryFromBytes::is_safe`.
#[doc(hidden)]
pub use crate::pointer::{invariant::BecauseImmutable, Maybe, Ptr};
// For each trait polyfill, as soon as the corresponding feature is stable, the
// polyfill import will be unused because method/function resolution will prefer
// the inherent method/function over a trait method/function. Thus, we suppress
// the `unused_imports` warning.
//
// See the documentation on `util::polyfills` for more information.
#[allow(unused_imports)]
use crate::util::polyfills::{self, NonNullExt as _, NumExt as _};
#[cfg_attr(not(zerocopy_unstable_ptr), doc(hidden))]
#[cfg_attr(doc_cfg, doc(cfg(zerocopy_unstable_ptr)))]
pub use crate::util::MetadataOf;

#[cfg(all(test, not(__ZEROCOPY_INTERNAL_USE_ONLY_DEV_MODE)))]
const _: () = {
    #[deprecated = "Development of zerocopy using cargo is not supported. Please use `cargo.sh` or `win-cargo.bat` instead."]
    #[allow(unused)]
    const WARNING: () = ();
    #[warn(deprecated)]
    WARNING
};

#[cfg(all(any(feature = "derive", test), zerocopy_unstable_linux))]
pub use zerocopy_derive::most_traits;
/// Implements [`KnownLayout`].
///
/// This derive analyzes various aspects of a type's layout that are needed for
/// some of zerocopy's APIs. It can be applied to structs, enums, and unions;
/// e.g.:
///
/// ```
/// # use zerocopy_derive::KnownLayout;
/// #[derive(KnownLayout)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(KnownLayout)]
/// enum MyEnum {
/// #   V00,
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(KnownLayout)]
/// union MyUnion {
/// #   variant: u8,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// # Limitations
///
/// This derive cannot currently be applied to unsized structs without an
/// explicit `repr` attribute.
///
/// Some invocations of this derive run afoul of a [known bug] in Rust's type
/// privacy checker. For example, this code:
///
/// ```compile_fail,E0446
/// use zerocopy::*;
/// # use zerocopy_derive::*;
///
/// #[derive(KnownLayout)]
/// #[repr(C)]
/// pub struct PublicType {
///     leading: Foo,
///     trailing: Bar,
/// }
///
/// #[derive(KnownLayout)]
/// struct Foo;
///
/// #[derive(KnownLayout)]
/// struct Bar;
/// ```
///
/// ...results in a compilation error:
///
/// ```text
/// error[E0446]: private type `Bar` in public interface
///  --> examples/bug.rs:3:10
///    |
/// 3  | #[derive(KnownLayout)]
///    |          ^^^^^^^^^^^ can't leak private type
/// ...
/// 14 | struct Bar;
///    | ---------- `Bar` declared as private
///    |
///    = note: this error originates in the derive macro `KnownLayout` (in Nightly builds, run with -Z macro-backtrace for more info)
/// ```
///
/// This issue arises when `#[derive(KnownLayout)]` is applied to `repr(C)`
/// structs whose trailing field type is less public than the enclosing struct.
///
/// To work around this, mark the trailing field type `pub` and annotate it with
/// `#[doc(hidden)]`; e.g.:
///
/// ```no_run
/// use zerocopy::*;
/// # use zerocopy_derive::*;
///
/// #[derive(KnownLayout)]
/// #[repr(C)]
/// pub struct PublicType {
///     leading: Foo,
///     trailing: Bar,
/// }
///
/// #[derive(KnownLayout)]
/// struct Foo;
///
/// #[doc(hidden)]
/// #[derive(KnownLayout)]
/// pub struct Bar; // <- `Bar` is now also `pub`
/// ```
///
/// [known bug]: https://github.com/rust-lang/rust/issues/45713
#[cfg(any(feature = "derive", test))]
#[cfg_attr(doc_cfg, doc(cfg(feature = "derive")))]
pub use zerocopy_derive::KnownLayout;
// These exist so that code which was written against the old names will get
// less confusing error messages when they upgrade to a more recent version of
// zerocopy. On our MSRV toolchain, the error messages read, for example:
//
//   error[E0603]: trait `FromZeroes` is private
//       --> examples/deprecated.rs:1:15
//        |
//   1    | use zerocopy::FromZeroes;
//        |               ^^^^^^^^^^ private trait
//        |
//   note: the trait `FromZeroes` is defined here
//       --> /Users/josh/workspace/zerocopy/src/lib.rs:1845:5
//        |
//   1845 | use FromZeros as FromZeroes;
//        |     ^^^^^^^^^^^^^^^^^^^^^^^
//
// The "note" provides enough context to make it easy to figure out how to fix
// the error.
#[allow(unused)]
use {FromZeros as FromZeroes, IntoBytes as AsBytes, Ref as LayoutVerified};

/// Indicates that zerocopy can reason about certain aspects of a type's layout.
///
/// This trait is required by many of zerocopy's APIs. It supports sized types,
/// slices, and [slice DSTs](#dynamically-sized-types).
///
/// # Implementation
///
/// **Do not implement this trait yourself!** Instead, use
/// [`#[derive(KnownLayout)]`][derive]; e.g.:
///
/// ```
/// # use zerocopy_derive::KnownLayout;
/// #[derive(KnownLayout)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(KnownLayout)]
/// enum MyEnum {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(KnownLayout)]
/// union MyUnion {
/// #   variant: u8,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// This derive performs a sophisticated analysis to deduce the layout
/// characteristics of types. You **must** implement this trait via the derive.
///
/// # Dynamically-sized types
///
/// `KnownLayout` supports slice-based dynamically sized types ("slice DSTs").
///
/// A slice DST is a type whose trailing field is either a slice or another
/// slice DST, rather than a type with fixed size. For example:
///
/// ```
/// #[repr(C)]
/// struct PacketHeader {
/// # /*
///     ...
/// # */
/// }
///
/// #[repr(C)]
/// struct Packet {
///     header: PacketHeader,
///     body: [u8],
/// }
/// ```
///
/// It can be useful to think of slice DSTs as a generalization of slices - in
/// other words, a normal slice is just the special case of a slice DST with
/// zero leading fields. In particular:
/// - Like slices, slice DSTs can have different lengths at runtime
/// - Like slices, slice DSTs cannot be passed by-value, but only by reference
///   or via other indirection such as `Box`
/// - Like slices, a reference (or `Box`, or other pointer type) to a slice DST
///   encodes the number of elements in the trailing slice field
///
/// ## Slice DST layout
///
/// Just like other composite Rust types, the layout of a slice DST is not
/// well-defined unless it is specified using an explicit `#[repr(...)]`
/// attribute such as `#[repr(C)]`. [Other representations are
/// supported][reprs], but in this section, we'll use `#[repr(C)]` as our
/// example.
///
/// A `#[repr(C)]` slice DST is laid out [just like sized `#[repr(C)]`
/// types][repr-c-structs], but the presence of a variable-length field
/// introduces the possibility of *dynamic padding*. In particular, it may be
/// necessary to add trailing padding *after* the trailing slice field in order
/// to satisfy the outer type's alignment, and the amount of padding required
/// may be a function of the length of the trailing slice field. This is just a
/// natural consequence of the normal `#[repr(C)]` rules applied to slice DSTs,
/// but it can result in surprising behavior. For example, consider the
/// following type:
///
/// ```
/// #[repr(C)]
/// struct Foo {
///     a: u32,
///     b: u8,
///     z: [u16],
/// }
/// ```
///
/// Assuming that `u32` has alignment 4 (this is not true on all platforms),
/// then `Foo` has alignment 4 as well. Here is the smallest possible value for
/// `Foo`:
///
/// ```text
/// byte offset | 01234567
///       field | aaaab---
///                    ><
/// ```
///
/// In this value, `z` has length 0. Abiding by `#[repr(C)]`, the lowest offset
/// that we can place `z` at is 5, but since `z` has alignment 2, we need to
/// round up to offset 6. This means that there is one byte of padding between
/// `b` and `z`, then 0 bytes of `z` itself (denoted `><` in this diagram), and
/// then two bytes of padding after `z` in order to satisfy the overall
/// alignment of `Foo`. The size of this instance is 8 bytes.
///
/// What about if `z` has length 1?
///
/// ```text
/// byte offset | 01234567
///       field | aaaab-zz
/// ```
///
/// In this instance, `z` has length 1, and thus takes up 2 bytes. That means
/// that we no longer need padding after `z` in order to satisfy `Foo`'s
/// alignment. We've now seen two different values of `Foo` with two different
/// lengths of `z`, but they both have the same size - 8 bytes.
///
/// What about if `z` has length 2?
///
/// ```text
/// byte offset | 012345678901
///       field | aaaab-zzzz--
/// ```
///
/// Now `z` has length 2, and thus takes up 4 bytes. This brings our un-padded
/// size to 10, and so we now need another 2 bytes of padding after `z` to
/// satisfy `Foo`'s alignment.
///
/// Again, all of this is just a logical consequence of the `#[repr(C)]` rules
/// applied to slice DSTs, but it can be surprising that the amount of trailing
/// padding becomes a function of the trailing slice field's length, and thus
/// can only be computed at runtime.
///
/// [reprs]: https://doc.rust-lang.org/reference/type-layout.html#representations
/// [repr-c-structs]: https://doc.rust-lang.org/reference/type-layout.html#reprc-structs
///
/// ## What is a valid size?
///
/// There are two places in zerocopy's API that we refer to "a valid size" of a
/// type. In normal casts or conversions, where the source is a byte slice, we
/// need to know whether the source byte slice is a valid size of the
/// destination type. In prefix or suffix casts, we need to know whether *there
/// exists* a valid size of the destination type which fits in the source byte
/// slice and, if so, what the largest such size is.
///
/// As outlined above, a slice DST's size is defined by the number of elements
/// in its trailing slice field. However, there is not necessarily a 1-to-1
/// mapping between trailing slice field length and overall size. As we saw in
/// the previous section with the type `Foo`, instances with both 0 and 1
/// elements in the trailing `z` field result in a `Foo` whose size is 8 bytes.
///
/// When we say "x is a valid size of `T`", we mean one of two things:
/// - If `T: Sized`, then we mean that `x == size_of::<T>()`
/// - If `T` is a slice DST, then we mean that there exists a `len` such that the instance of
///   `T` with `len` trailing slice elements has size `x`
///
/// When we say "largest possible size of `T` that fits in a byte slice", we
/// mean one of two things:
/// - If `T: Sized`, then we mean `size_of::<T>()` if the byte slice is at least
///   `size_of::<T>()` bytes long
/// - If `T` is a slice DST, then we mean to consider all values, `len`, such
///   that the instance of `T` with `len` trailing slice elements fits in the
///   byte slice, and to choose the largest such `len`, if any
///
///
/// # Safety
///
/// This trait does not convey any safety guarantees to code outside this crate.
///
/// You must not rely on the `#[doc(hidden)]` internals of `KnownLayout`. Future
/// releases of zerocopy may make backwards-breaking changes to these items,
/// including changes that only affect soundness, which may cause code which
/// uses those items to silently become unsound.
///
#[cfg_attr(feature = "derive", doc = "[derive]: zerocopy_derive::KnownLayout")]
#[cfg_attr(
    not(feature = "derive"),
    doc = concat!("[derive]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.KnownLayout.html"),
)]
#[cfg_attr(
    not(no_zerocopy_diagnostic_on_unimplemented_1_78_0),
    diagnostic::on_unimplemented(note = "Consider adding `#[derive(KnownLayout)]` to `{Self}`")
)]
pub unsafe trait KnownLayout {
    // The `Self: Sized` bound makes it so that `KnownLayout` can still be
    // object safe. It's not currently object safe thanks to `const LAYOUT`, and
    // it likely won't be in the future, but there's no reason not to be
    // forwards-compatible with object safety.
    #[doc(hidden)]
    fn only_derive_is_allowed_to_implement_this_trait()
    where
        Self: Sized;

    /// The type of metadata stored in a pointer to `Self`.
    ///
    /// This is `()` for sized types and [`usize`] for slice DSTs.
    type PointerMetadata: PointerMetadata;

    /// A maybe-uninitialized analog of `Self`
    ///
    /// # Safety
    ///
    /// `Self::LAYOUT` and `Self::MaybeUninit::LAYOUT` are identical.
    /// `Self::MaybeUninit` admits uninitialized bytes in all positions.
    #[doc(hidden)]
    type MaybeUninit: ?Sized + KnownLayout<PointerMetadata = Self::PointerMetadata>;

    /// The layout of `Self`.
    ///
    /// # Safety
    ///
    /// Callers may assume that `LAYOUT` accurately reflects the layout of
    /// `Self`. In particular:
    /// - `LAYOUT.align` is equal to `Self`'s alignment
    /// - If `Self: Sized`, then `LAYOUT.size_info == SizeInfo::Sized { size }`
    ///   where `size == size_of::<Self>()`
    /// - If `Self` is a slice DST, then `LAYOUT.size_info ==
    ///   SizeInfo::SliceDst(slice_layout)` where:
    ///   - `slice_layout.size_rounding_align_and_phase` decodes to an alignment
    ///     that is a power of two and a phase which is less than that alignment
    ///   - The size, `size`, of an instance of `Self` with `elems` trailing
    ///     slice elements is equal to `slice_layout.size_base +
    ///     round_up(size_phase + slice_layout.elem_size * elems, size_align)`,
    ///     where `(size_align, size_phase)` is decoded from
    ///     `slice_layout.size_rounding_align_and_phase`
    ///   - The trailing slice begins at byte offset `slice_layout.offset`
    ///   - `slice_layout.offset + slice_layout.elem_size * elems` does not
    ///     overflow and is no greater than `size`
    ///   - For such an instance, any bytes in the range `[slice_layout.offset +
    ///     slice_layout.elem_size * elems, size)` are padding and must not be
    ///     assumed to be initialized
    #[doc(hidden)]
    const LAYOUT: DstLayout;

    /// SAFETY: The returned pointer has the same address and provenance as
    /// `bytes`. If `Self` is a DST, the returned pointer's referent has `elems`
    /// elements in its trailing slice.
    #[doc(hidden)]
    fn raw_from_ptr_len(bytes: NonNull<u8>, meta: Self::PointerMetadata) -> NonNull<Self>;

    /// Extracts the metadata from a pointer to `Self`.
    ///
    /// # Safety
    ///
    /// `pointer_to_metadata` always returns the correct metadata stored in
    /// `ptr`.
    #[doc(hidden)]
    fn pointer_to_metadata(ptr: *mut Self) -> Self::PointerMetadata;

    /// Computes the length of the byte range addressed by `ptr`.
    ///
    /// Returns `None` if the resulting length would not fit in an `usize`.
    ///
    /// # Safety
    ///
    /// Callers may assume that `size_of_val_raw` always returns the correct
    /// size.
    ///
    /// Callers may assume that, if `ptr` addresses a byte range whose length
    /// fits in an `usize`, this will return `Some`.
    #[doc(hidden)]
    #[must_use]
    #[inline(always)]
    fn size_of_val_raw(ptr: NonNull<Self>) -> Option<usize> {
        let meta = Self::pointer_to_metadata(ptr.as_ptr());
        // SAFETY: `size_for_metadata` promises to only return `None` if the
        // resulting size would not fit in a `usize`.
        Self::size_for_metadata(meta)
    }

    #[doc(hidden)]
    #[must_use]
    #[inline(always)]
    fn raw_dangling() -> NonNull<Self> {
        let meta = Self::PointerMetadata::from_elem_count(0);
        Self::raw_from_ptr_len(NonNull::dangling(), meta)
    }

    /// Computes the size of an object of type `Self` with the given pointer
    /// metadata.
    ///
    /// # Safety
    ///
    /// `size_for_metadata` promises to return `None` if and only if the
    /// resulting size would not fit in a [`usize`]. Note that the returned size
    /// could exceed the actual maximum valid size of an allocated object,
    /// [`isize::MAX`].
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::KnownLayout;
    ///
    /// assert_eq!(u8::size_for_metadata(()), Some(1));
    /// assert_eq!(u16::size_for_metadata(()), Some(2));
    /// assert_eq!(<[u8]>::size_for_metadata(42), Some(42));
    /// assert_eq!(<[u16]>::size_for_metadata(42), Some(84));
    ///
    /// // This size exceeds the maximum valid object size (`isize::MAX`):
    /// assert_eq!(<[u8]>::size_for_metadata(usize::MAX), Some(usize::MAX));
    ///
    /// // This size, if computed, would exceed `usize::MAX`:
    /// assert_eq!(<[u16]>::size_for_metadata(usize::MAX), None);
    /// ```
    #[inline(always)]
    fn size_for_metadata(meta: Self::PointerMetadata) -> Option<usize> {
        meta.size_for_metadata(Self::LAYOUT)
    }

    /// Computes whether `meta` can describe a valid allocation of `Self`.
    ///
    /// # Safety
    ///
    /// `is_valid_metadata` promises to return `true` if and only if the size of
    /// an allocation of `Self` with `meta` would not overflow an
    /// [`isize::MAX`].
    #[doc(hidden)]
    #[inline(always)]
    fn is_valid_metadata(meta: Self::PointerMetadata) -> bool {
        meta.to_elem_count() <= maximum_trailing_slice_len::<Self>().to_elem_count()
    }
}

/// Efficiently produces the [`TrailingSliceLayout`] of `T`.
#[inline(always)]
pub(crate) fn trailing_slice_layout<T>() -> TrailingSliceLayout
where
    T: ?Sized + KnownLayout<PointerMetadata = usize>,
{
    trait LayoutFacts {
        const SIZE_INFO: TrailingSliceLayout;
    }

    impl<T: ?Sized> LayoutFacts for T
    where
        T: KnownLayout<PointerMetadata = usize>,
    {
        const SIZE_INFO: TrailingSliceLayout = match T::LAYOUT.size_info {
            crate::SizeInfo::Sized { .. } => const_panic!("unreachable"),
            crate::SizeInfo::SliceDst(info) => info,
        };
    }

    T::SIZE_INFO
}

/// Efficiently produces the maximum trailing slice length `T`.
#[inline(always)]
pub(crate) fn maximum_trailing_slice_len<T>() -> usize
where
    T: ?Sized + KnownLayout,
{
    trait LayoutFacts {
        const MAX_LEN: usize;
    }

    impl<T: ?Sized> LayoutFacts for T
    where
        T: KnownLayout,
    {
        const MAX_LEN: usize = match T::LAYOUT.size_info {
            SizeInfo::SliceDst(TrailingSliceLayout { elem_size: 0, .. }) => usize::MAX,
            _ => match T::LAYOUT.validate_cast_and_convert_metadata(
                T::LAYOUT.align.get(),
                DstLayout::MAX_SIZE,
                CastType::Prefix,
            ) {
                Ok((elems, _)) => elems,
                Err(_) => const_panic!("unreachable"),
            },
        };
    }

    T::MAX_LEN
}

/// The metadata associated with a [`KnownLayout`] type.
#[doc(hidden)]
pub trait PointerMetadata: Copy + Eq + Debug + Ord {
    /// Constructs a `Self` from an element count.
    ///
    /// If `Self = ()`, this returns `()`. If `Self = usize`, this returns
    /// `elems`. No other types are currently supported.
    fn from_elem_count(elems: usize) -> Self;

    /// Converts `self` to an element count.
    ///
    /// If `Self = ()`, this returns `0`. If `Self = usize`, this returns
    /// `self`. No other types are currently supported.
    fn to_elem_count(self) -> usize;

    /// Computes the size of the object with the given layout and pointer
    /// metadata.
    ///
    /// # Panics
    ///
    /// If `Self = ()`, `layout` must describe a sized type. If `Self = usize`,
    /// `layout` must describe a slice DST. Otherwise, `size_for_metadata` may
    /// panic.
    ///
    /// # Safety
    ///
    /// `size_for_metadata` promises to only return `None` if the resulting size
    /// would not fit in a `usize`.
    fn size_for_metadata(self, layout: DstLayout) -> Option<usize>;
}

impl PointerMetadata for () {
    #[inline]
    #[allow(clippy::unused_unit)]
    fn from_elem_count(_elems: usize) -> () {}

    #[inline]
    fn to_elem_count(self) -> usize {
        0
    }

    #[inline]
    fn size_for_metadata(self, layout: DstLayout) -> Option<usize> {
        match layout.size_info {
            SizeInfo::Sized { size } => Some(size),
            // NOTE: This branch is unreachable, but we return `None` rather
            // than `unreachable!()` to avoid generating panic paths.
            SizeInfo::SliceDst(_) => None,
        }
    }
}

impl PointerMetadata for usize {
    #[inline]
    fn from_elem_count(elems: usize) -> usize {
        elems
    }

    #[inline]
    fn to_elem_count(self) -> usize {
        self
    }

    #[inline]
    fn size_for_metadata(self, layout: DstLayout) -> Option<usize> {
        match layout.size_info {
            SizeInfo::SliceDst(trailing) => trailing.size_for_elems(self),
            // NOTE: This branch is unreachable, but we return `None` rather
            // than `unreachable!()` to avoid generating panic paths.
            SizeInfo::Sized { .. } => None,
        }
    }
}

// SAFETY: Delegates safety to `DstLayout::for_slice`.
unsafe impl<T> KnownLayout for [T] {
    #[allow(clippy::missing_inline_in_public_items, dead_code)]
    #[cfg_attr(
        all(coverage_nightly, __ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS),
        coverage(off)
    )]
    fn only_derive_is_allowed_to_implement_this_trait()
    where
        Self: Sized,
    {
    }

    type PointerMetadata = usize;

    // SAFETY: `CoreMaybeUninit<T>::LAYOUT` and `T::LAYOUT` are identical
    // because `CoreMaybeUninit<T>` has the same size and alignment as `T` [1].
    // Consequently, `[CoreMaybeUninit<T>]::LAYOUT` and `[T]::LAYOUT` are
    // identical, because they both lack a fixed-sized prefix and because they
    // inherit the alignments of their inner element type (which are identical)
    // [2][3].
    //
    // `[CoreMaybeUninit<T>]` admits uninitialized bytes at all positions
    // because `CoreMaybeUninit<T>` admits uninitialized bytes at all positions
    // and because the inner elements of `[CoreMaybeUninit<T>]` are laid out
    // back-to-back [2][3].
    //
    // [1] Per https://doc.rust-lang.org/1.81.0/std/mem/union.MaybeUninit.html#layout-1:
    //
    //   `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as
    //   `T`
    //
    // [2] Per https://doc.rust-lang.org/1.82.0/reference/type-layout.html#slice-layout:
    //
    //   Slices have the same layout as the section of the array they slice.
    //
    // [3] Per https://doc.rust-lang.org/1.82.0/reference/type-layout.html#array-layout:
    //
    //   An array of `[T; N]` has a size of `size_of::<T>() * N` and the same
    //   alignment of `T`. Arrays are laid out so that the zero-based `nth`
    //   element of the array is offset from the start of the array by `n *
    //   size_of::<T>()` bytes.
    type MaybeUninit = [CoreMaybeUninit<T>];

    const LAYOUT: DstLayout = DstLayout::for_slice::<T>();

    // SAFETY: `.cast` preserves address and provenance. The returned pointer
    // refers to an object with `elems` elements by construction.
    #[inline(always)]
    fn raw_from_ptr_len(data: NonNull<u8>, elems: usize) -> NonNull<Self> {
        // FIXME(#67): Remove this allow. See NonNullExt for more details.
        #[allow(unstable_name_collisions)]
        NonNull::slice_from_raw_parts(data.cast::<T>(), elems)
    }

    #[inline(always)]
    fn pointer_to_metadata(ptr: *mut [T]) -> usize {
        #[cfg(not(no_zerocopy_slice_ptr_len_1_79_0))]
        {
            ptr.len()
        }

        #[cfg(no_zerocopy_slice_ptr_len_1_79_0)]
        {
            // `*mut [T]::len` was not stable before Rust 1.79. In every Rust
            // version from our 1.56 MSRV through 1.78, `Hash for *mut T`
            // decomposes a raw pointer and passes its address and metadata to
            // the hasher in that order [1]. `Hash for usize` passes its value
            // to `Hasher::write_usize` [2]. Capture those two values to obtain
            // a candidate for the slice length without dereferencing `ptr` or
            // constructing a reference.
            //
            // This historical `Hash` implementation is only used to produce a
            // candidate. Before returning it, we reconstruct a raw slice and
            // authenticate the candidate with `ptr::eq`, which compares slice
            // lengths as well as addresses [3]. Thus, an unexpected `Hash`
            // implementation cannot cause us to return incorrect metadata; it
            // can only fail to produce an authenticated candidate.
            //
            // [1] Per https://doc.rust-lang.org/1.56.0/src/core/hash/mod.rs.html#776-782:
            //
            //   let (address, metadata) = self.to_raw_parts();
            //   state.write_usize(address as usize);
            //   metadata.hash(state);
            //
            // [2] Per https://doc.rust-lang.org/1.56.0/src/core/hash/mod.rs.html#628-656:
            //
            //   fn hash<H: Hasher>(&self, state: &mut H) {
            //       state.$meth(*self)
            //   }
            //   ...
            //   (usize, write_usize),
            //
            // [3] Per https://doc.rust-lang.org/1.56.0/std/ptr/fn.eq.html:
            //
            //   Slices are also compared by their length (fat pointers).
            struct MetadataHasher {
                values: [usize; 2],
                writes: usize,
                valid: bool,
            }

            impl Hasher for MetadataHasher {
                #[inline(always)]
                fn finish(&self) -> u64 {
                    0
                }

                #[inline(always)]
                fn write(&mut self, _bytes: &[u8]) {
                    self.valid = false;
                }

                #[inline(always)]
                fn write_usize(&mut self, value: usize) {
                    match self.values.get_mut(self.writes) {
                        Some(slot) => *slot = value,
                        None => self.valid = false,
                    }
                    self.writes = self.writes.saturating_add(1);
                }
            }

            let mut hasher = MetadataHasher { values: [0; 2], writes: 0, valid: true };
            core::hash::Hash::hash(&ptr, &mut hasher);
            assert!(
                hasher.valid && hasher.writes == 2,
                "unexpected raw-pointer Hash implementation"
            );

            let elems = hasher.values[1];
            #[allow(clippy::as_conversions)]
            let reconstructed = ptr::slice_from_raw_parts_mut(ptr as *mut T, elems);
            assert!(ptr::eq(ptr, reconstructed), "captured value is not raw-slice metadata");
            elems
        }
    }
}

#[rustfmt::skip]
impl_known_layout!(
    (),
    u8, i8, u16, i16, u32, i32, u64, i64, u128, i128, usize, isize, f32, f64,
    bool, char,
    NonZeroU8, NonZeroI8, NonZeroU16, NonZeroI16, NonZeroU32, NonZeroI32,
    NonZeroU64, NonZeroI64, NonZeroU128, NonZeroI128, NonZeroUsize, NonZeroIsize
);
#[rustfmt::skip]
#[cfg(feature = "float-nightly")]
impl_known_layout!(
    #[cfg_attr(doc_cfg, doc(cfg(feature = "float-nightly")))]
    f16,
    #[cfg_attr(doc_cfg, doc(cfg(feature = "float-nightly")))]
    f128
);
#[rustfmt::skip]
impl_known_layout!(
    T         => Option<T>,
    T: ?Sized => PhantomData<T>,
    T         => Wrapping<T>,
    T         => CoreMaybeUninit<T>,
    T: ?Sized => *const T,
    T: ?Sized => *mut T,
    T: ?Sized => &'_ T,
    T: ?Sized => &'_ mut T,
);
impl_known_layout!(const N: usize, T => [T; N]);

// SAFETY: `str` has the same representation as `[u8]`. `ManuallyDrop<T>` [1],
// `UnsafeCell<T>` [2], and `Cell<T>` [3] have the same representation as `T`.
//
// [1] Per https://doc.rust-lang.org/1.85.0/std/mem/struct.ManuallyDrop.html:
//
//   `ManuallyDrop<T>` is guaranteed to have the same layout and bit validity as
//   `T`
//
// [2] Per https://doc.rust-lang.org/1.85.0/core/cell/struct.UnsafeCell.html#memory-layout:
//
//   `UnsafeCell<T>` has the same in-memory representation as its inner type
//   `T`.
//
// [3] Per https://doc.rust-lang.org/1.85.0/core/cell/struct.Cell.html#memory-layout:
//
//   `Cell<T>` has the same in-memory representation as `T`.
#[allow(clippy::multiple_unsafe_ops_per_block)]
const _: () = unsafe {
    unsafe_impl_known_layout!(
        #[repr([u8])]
        str
    );
    unsafe_impl_known_layout!(T: ?Sized + KnownLayout => #[repr(T)] ManuallyDrop<T>);
    unsafe_impl_known_layout!(T: ?Sized + KnownLayout => #[repr(T)] UnsafeCell<T>);
    unsafe_impl_known_layout!(T: ?Sized + KnownLayout => #[repr(T)] Cell<T>);
};

// SAFETY:
// - By consequence of the invariant on `T::MaybeUninit` that `T::LAYOUT` and
//   `T::MaybeUninit::LAYOUT` are equal, `T` and `T::MaybeUninit` have the same:
//   - Fixed prefix size
//   - Alignment
//   - (For DSTs) trailing slice element size
// - By consequence of the above, referents `T::MaybeUninit` and `T` have the
//   require the same kind of pointer metadata, and thus it is valid to perform
//   an `as` cast from `*mut T` and `*mut T::MaybeUninit`, and this operation
//   preserves referent size (ie, `size_of_val_raw`).
const _: () = unsafe {
    unsafe_impl_known_layout!(T: ?Sized + KnownLayout => #[repr(T::MaybeUninit)] MaybeUninit<T>)
};

// FIXME(#196, #2856): Eventually, we'll want to support enums variants and
// union fields being treated uniformly since they behave similarly to each
// other in terms of projecting validity – specifically, for a type `T` with
// validity `V`, if `T` is a struct type, then its fields straightforwardly also
// have validity `V`. By contrast, if `T` is an enum or union type, then
// validity is not straightforwardly recursive in this way.
#[doc(hidden)]
pub const STRUCT_VARIANT_ID: i128 = -1;
#[doc(hidden)]
pub const UNION_VARIANT_ID: i128 = -2;
#[doc(hidden)]
pub const REPR_C_UNION_VARIANT_ID: i128 = -3;

/// Marker types used to disambiguate implementations of projection traits that
/// would otherwise conflict.
#[doc(hidden)]
#[allow(missing_copy_implementations, missing_debug_implementations)]
pub mod project_clients {
    pub enum TryFromBytesDerive {}

    pub enum ProjectDerive {}
}

#[cfg(any(feature = "derive", test))]
#[cfg_attr(doc_cfg, doc(cfg(feature = "derive")))]
#[doc(hidden)]
pub use zerocopy_derive::Project;

/// # Safety
///
/// `<Self as HasTag<Client>>::ProjectToTag` must satisfy its safety invariant.
///
/// The `Client` parameter exists solely to disambiguate between implementations
/// of `HasTag` that would otherwise conflict.
#[doc(hidden)]
pub unsafe trait HasTag<Client = project_clients::TryFromBytesDerive> {
    fn only_derive_is_allowed_to_implement_this_trait()
    where
        Self: Sized;

    /// The type's enum tag, or `()` for non-enum types.
    type Tag: Immutable;

    /// A pointer projection from `Self` to its tag.
    ///
    /// # Safety
    ///
    /// It must be the case that, for all `slf: Ptr<'_, Self, I>` where
    /// `I::Aliasing` is `Shared`, it is sound to project it using this
    /// projection to a `Ptr<'_, Self::Tag, (Shared, I::Alignment,
    /// I::Validity)>`.
    type ProjectToTag: pointer::cast::Project<Self, Self::Tag>;
}

/// Projects a given field from `Self`.
///
/// All implementations of `HasField` for a particular `Client` and field `f`
/// in `Self` should use the same `Field` type; this ensures that `Field` is
/// inferable given an explicit `Client`, `VARIANT_ID`, and `FIELD_ID`.
///
/// The `Client` parameter exists solely to disambiguate between implementations
/// of `HasField` that would otherwise conflict.
///
/// # Safety
///
/// A field `f` is `HasField` for `Self` if and only if:
///
/// - If `Self` has the layout of a struct type, `VARIANT_ID` is
///   `STRUCT_VARIANT_ID`. If `Self` has the layout of a `repr(C)` union type,
///   `VARIANT_ID` is `REPR_C_UNION_VARIANT_ID`; for other union layouts, it is
///   `UNION_VARIANT_ID`. Otherwise, if `Self` has the layout of an enum type and
///   `f` appears in a variant named `v`, `VARIANT_ID` is
///   `zerocopy::ident_id!(v)`. Note that `Self` does not need to actually *be*
///   such a type – it just needs to have the same layout as such a type. For
///   example, a `#[repr(transparent)]` wrapper around an enum has the same
///   layout as that enum.
/// - If `f` has name `n`, `FIELD_ID` is `zerocopy::ident_id!(n)`; otherwise,
///   if `f` is at index `i`, `FIELD_ID` is `zerocopy::ident_id!(i)`.
/// - `Field` is a type with the same visibility as `f`.
/// - `Type` has the same type as `f`.
///
/// The caller must **not** assume that a pointer's referent being aligned
/// implies that calling `project` on that pointer will result in a pointer to
/// an aligned referent. For example, `HasField` may be implemented for
/// `#[repr(packed)]` structs.
///
/// The implementation of `project` must satisfy its safety post-condition.
#[doc(hidden)]
pub unsafe trait HasField<Client, Field, const VARIANT_ID: i128, const FIELD_ID: i128>:
    HasTag<Client>
{
    fn only_derive_is_allowed_to_implement_this_trait()
    where
        Self: Sized;

    /// The type of the field.
    type Type: ?Sized;

    /// Projects from `slf` to the field.
    ///
    /// Users should generally not call `project` directly, and instead should
    /// use high-level APIs like [`PtrInner::project`] or [`Ptr::project`].
    ///
    /// # Safety
    ///
    /// The returned pointer refers to a non-strict subset of the bytes of
    /// `slf`'s referent, and has the same provenance as `slf`.
    #[must_use]
    fn project(slf: PtrInner<'_, Self>) -> *mut Self::Type;
}

/// Projects a given field from `Self`.
///
/// Implementations of this trait encode the conditions under which a field can
/// be projected from a `Ptr<'_, Self, I>`, and how the invariants of that
/// [`Ptr`] (`I`) determine the invariants of pointers projected from it. In
/// other words, it is a type-level function over invariants; `I` goes in,
/// `Self::Invariants` comes out.
///
/// The `Client` parameter exists solely to disambiguate between implementations
/// of `ProjectField` (and their corresponding `HasField` implementations) that
/// would otherwise conflict.
///
/// # Safety
///
/// `T: ProjectField<Client, Field, I, VARIANT_ID, FIELD_ID>` if, for a
/// `ptr: Ptr<'_, T, I>` such that `T::is_projectable` returns `Ok(())`
/// when passed the tag pointer projected from `ptr`,
/// `<T as HasField<Client, Field, VARIANT_ID, FIELD_ID>>::project(ptr.as_inner())`
/// conforms to `T::Invariants`.
#[doc(hidden)]
pub unsafe trait ProjectField<Client, Field, I, const VARIANT_ID: i128, const FIELD_ID: i128>:
    HasField<Client, Field, VARIANT_ID, FIELD_ID>
where
    I: invariant::Invariants,
{
    fn only_derive_is_allowed_to_implement_this_trait()
    where
        Self: Sized;

    /// The invariants of the projected field pointer, with respect to the
    /// invariants, `I`, of the containing pointer. The aliasing dimension of
    /// the invariants is guaranteed to remain unchanged.
    type Invariants: invariant::Invariants<Aliasing = I::Aliasing>;

    /// The failure mode of projection. `()` if the projection is fallible,
    /// otherwise [`core::convert::Infallible`].
    type Error;

    /// Is the given field projectable from `ptr`?
    ///
    /// If a field with [`Self::Invariants`] is projectable from the containing
    /// value whose projected tag is `ptr`, this function produces `Ok(())`;
    /// otherwise it produces `Err`.
    ///
    /// This method must be overriden if the field's projectability depends on
    /// the value of the bytes in `ptr`.
    #[inline(always)]
    fn is_projectable<'a>(
        _ptr: Ptr<
            'a,
            <Self as HasTag<Client>>::Tag,
            (invariant::Shared, I::Alignment, I::Validity),
        >,
    ) -> Result<(), Self::Error> {
        trait IsInfallible {
            const IS_INFALLIBLE: bool;
        }

        struct Projection<T, Client, Field, I, const VARIANT_ID: i128, const FIELD_ID: i128>(
            PhantomData<(Client, Field, I, T)>,
        )
        where
            T: ?Sized + HasField<Client, Field, VARIANT_ID, FIELD_ID>,
            I: invariant::Invariants;

        impl<T, Client, Field, I, const VARIANT_ID: i128, const FIELD_ID: i128> IsInfallible
            for Projection<T, Client, Field, I, VARIANT_ID, FIELD_ID>
        where
            T: ?Sized + HasField<Client, Field, VARIANT_ID, FIELD_ID>,
            I: invariant::Invariants,
        {
            const IS_INFALLIBLE: bool = {
                let is_infallible = match VARIANT_ID {
                    // For nondestructive projections of struct and union
                    // fields, the projected field's satisfaction of
                    // `Invariants` does not depend on the value of the
                    // referent. This default implementation of `is_projectable`
                    // is non-destructive, as it does not overwrite any part of
                    // the referent.
                    crate::STRUCT_VARIANT_ID
                    | crate::UNION_VARIANT_ID
                    | crate::REPR_C_UNION_VARIANT_ID => true,
                    _enum_variant => {
                        use crate::invariant::{Validity, ValidityKind};
                        match I::Validity::KIND {
                            // The `Uninit` and `Initialized` validity
                            // invariants do not depend on the enum's tag. In
                            // particular, we don't actually care about what
                            // variant is present – we can treat *any* range of
                            // uninitialized or initialized memory as containing
                            // an uninitialized or initialized instance of *any*
                            // type – the type itself is irrelevant.
                            ValidityKind::Uninit | ValidityKind::Initialized => true,
                            // The projectability of an enum field from an
                            // `AsInitialized` or `Safe` state is a dynamic
                            // property of its tag.
                            ValidityKind::AsInitialized | ValidityKind::Safe => false,
                        }
                    }
                };
                const_assert!(is_infallible);
                is_infallible
            };
        }

        const_assert!(
            <Projection<Self, Client, Field, I, VARIANT_ID, FIELD_ID> as IsInfallible>::IS_INFALLIBLE
        );

        Ok(())
    }
}

/// Analyzes whether a type is [`FromZeros`].
///
/// This derive analyzes, at compile time, whether the annotated type satisfies
/// the [safety conditions] of `FromZeros` and implements `FromZeros` and its
/// supertraits if it is sound to do so. This derive can be applied to structs,
/// enums, and unions; e.g.:
///
/// ```
/// # use zerocopy_derive::{FromZeros, Immutable};
/// #[derive(FromZeros)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(FromZeros)]
/// #[repr(u8)]
/// enum MyEnum {
/// #   Variant0,
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(FromZeros, Immutable)]
/// union MyUnion {
/// #   variant: u8,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// [safety conditions]: trait@FromZeros#safety
///
/// # Analysis
///
/// *This section describes, roughly, the analysis performed by this derive to
/// determine whether it is sound to implement `FromZeros` for a given type.
/// Unless you are modifying the implementation of this derive, or attempting to
/// manually implement `FromZeros` for a type yourself, you don't need to read
/// this section.*
///
/// If a type has the following properties, then this derive can implement
/// `FromZeros` for that type:
///
/// - If the type is a struct, all of its fields must be `FromZeros`.
/// - If the type is an enum:
///   - It must have a defined representation (`repr`s `C`, `u8`, `u16`, `u32`,
///     `u64`, `usize`, `i8`, `i16`, `i32`, `i64`, or `isize`).
///   - It must have a variant with a discriminant/tag of `0`, and its fields
///     must be `FromZeros`. See [the reference] for a description of
///     discriminant values are specified.
///   - The fields of that variant must be `FromZeros`.
///
/// This analysis is subject to change. Unsafe code may *only* rely on the
/// documented [safety conditions] of `FromZeros`, and must *not* rely on the
/// implementation details of this derive.
///
/// [the reference]: https://doc.rust-lang.org/reference/items/enumerations.html#custom-discriminant-values-for-fieldless-enumerations
///
/// ## Why isn't an explicit representation required for structs?
///
/// Neither this derive, nor the [safety conditions] of `FromZeros`, requires
/// that structs are marked with `#[repr(C)]`.
///
/// Per the [Rust reference](reference),
///
/// > The representation of a type can change the padding between fields, but
/// > does not change the layout of the fields themselves.
///
/// [reference]: https://doc.rust-lang.org/reference/type-layout.html#representations
///
/// Since the layout of structs only consists of padding bytes and field bytes,
/// a struct is soundly `FromZeros` if:
/// 1. its padding is soundly `FromZeros`, and
/// 2. its fields are soundly `FromZeros`.
///
/// The answer to the first question is always yes: padding bytes do not have
/// any validity constraints. A [discussion] of this question in the Unsafe Code
/// Guidelines Working Group concluded that it would be virtually unimaginable
/// for future versions of rustc to add validity constraints to padding bytes.
///
/// [discussion]: https://github.com/rust-lang/unsafe-code-guidelines/issues/174
///
/// Whether a struct is soundly `FromZeros` therefore solely depends on whether
/// its fields are `FromZeros`.
// FIXME(#146): Document why we don't require an enum to have an explicit `repr`
// attribute.
#[cfg(any(feature = "derive", test))]
#[cfg_attr(doc_cfg, doc(cfg(feature = "derive")))]
pub use zerocopy_derive::FromZeros;
/// Analyzes whether a type is [`Immutable`].
///
/// This derive analyzes, at compile time, whether the annotated type satisfies
/// the [safety conditions] of `Immutable` and implements `Immutable` if it is
/// sound to do so. This derive can be applied to structs, enums, and unions;
/// e.g.:
///
/// ```
/// # use zerocopy_derive::Immutable;
/// #[derive(Immutable)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(Immutable)]
/// enum MyEnum {
/// #   Variant0,
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(Immutable)]
/// union MyUnion {
/// #   variant: u8,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// # Analysis
///
/// *This section describes, roughly, the analysis performed by this derive to
/// determine whether it is sound to implement `Immutable` for a given type.
/// Unless you are modifying the implementation of this derive, you don't need
/// to read this section.*
///
/// If a type has the following properties, then this derive can implement
/// `Immutable` for that type:
///
/// - All fields must be `Immutable`.
///
/// This analysis is subject to change. Unsafe code may *only* rely on the
/// documented [safety conditions] of `Immutable`, and must *not* rely on the
/// implementation details of this derive.
///
/// [safety conditions]: trait@Immutable#safety
#[cfg(any(feature = "derive", test))]
#[cfg_attr(doc_cfg, doc(cfg(feature = "derive")))]
pub use zerocopy_derive::Immutable;

/// Types which are free from interior mutability.
///
/// `T: Immutable` indicates that `T` does not permit interior mutation, except
/// by ownership or an exclusive (`&mut`) borrow.
///
/// # Implementation
///
/// **Do not implement this trait yourself!** Instead, use
/// [`#[derive(Immutable)]`][derive] (requires the `derive` Cargo feature);
/// e.g.:
///
/// ```
/// # use zerocopy_derive::Immutable;
/// #[derive(Immutable)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(Immutable)]
/// enum MyEnum {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(Immutable)]
/// union MyUnion {
/// #   variant: u8,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// This derive performs a sophisticated, compile-time safety analysis to
/// determine whether a type is `Immutable`.
///
/// # Safety
///
/// Unsafe code outside of this crate must not make any assumptions about `T`
/// based on `T: Immutable`. We reserve the right to relax the requirements for
/// `Immutable` in the future, and if unsafe code outside of this crate makes
/// assumptions based on `T: Immutable`, future relaxations may cause that code
/// to become unsound.
///
// # Safety (Internal)
//
// If `T: Immutable`, unsafe code *inside of this crate* may assume that, given
// `t: &T`, `t` does not permit interior mutation of its referent. Because
// [`UnsafeCell`] is the only type which permits interior mutation, it is
// sufficient (though not necessary) to guarantee that `T` contains no
// `UnsafeCell`s.
//
// [`UnsafeCell`]: core::cell::UnsafeCell
#[cfg_attr(
    feature = "derive",
    doc = "[derive]: zerocopy_derive::Immutable",
    doc = "[derive-analysis]: zerocopy_derive::Immutable#analysis"
)]
#[cfg_attr(
    not(feature = "derive"),
    doc = concat!("[derive]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.Immutable.html"),
    doc = concat!("[derive-analysis]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.Immutable.html#analysis"),
)]
#[cfg_attr(
    not(no_zerocopy_diagnostic_on_unimplemented_1_78_0),
    diagnostic::on_unimplemented(note = "Consider adding `#[derive(Immutable)]` to `{Self}`")
)]
pub unsafe trait Immutable {
    // The `Self: Sized` bound makes it so that `Immutable` is still object
    // safe.
    #[doc(hidden)]
    fn only_derive_is_allowed_to_implement_this_trait()
    where
        Self: Sized;
}

/// Implements [`TryFromBytes`].
///
/// This derive synthesizes the runtime checks required to check whether a
/// sequence of initialized bytes corresponds to a valid instance of a type.
/// This derive can be applied to structs, enums, and unions; e.g.:
///
/// ```
/// # use zerocopy_derive::{TryFromBytes, Immutable};
/// #[derive(TryFromBytes)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(TryFromBytes)]
/// #[repr(u8)]
/// enum MyEnum {
/// #   V00,
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(TryFromBytes, Immutable)]
/// union MyUnion {
/// #   variant: u8,
/// # /*
///     ...
/// # */
/// }
/// ```
///
#[cfg_attr(
    zerocopy_unstable_ptr,
    doc = r#"
# Field invariants

This experimental feature requires `--cfg zerocopy_unstable_ptr`.

Named fields of structs, enum variants, and unions can specify additional
runtime checks using `#[zerocopy(invariant(expression))]`:

```
# use zerocopy_derive::TryFromBytes;
#[derive(TryFromBytes)]
struct Foo {
    a: u8,
    #[zerocopy(invariant((*a.read() % 2) == (*b.read() as u8)))]
    b: bool,
    #[zerocopy(invariant(*c.read() > 0))]
    c: i16,
}
```

Each expression must return a `bool`. It has access to validated, read-only
[`Ptr`]s to the current field and all preceding fields of the struct or
variant, using their field names. A union's invariants have access only to
the current field. The expression can use the existing [`Ptr`] APIs to
inspect those fields. In this example, `read()` copies each field without
requiring alignment, and dereferencing the resulting [`ReadOnly`] accesses
the copied value.

Rust's usual restrictions on local bindings apply; for example, a field name
cannot shadow an in-scope constant.

Fields are checked in declaration order. Each field's bit validity is
checked before its invariants run. Multiple invariants on a field run in
attribute order. For structs and enums, validation stops at the first invalid
field or invariant that returns `false`. For unions, a failed bit-validity
check or invariant causes validation to try the next field; validation
succeeds as soon as one field and all its invariants pass. Each expression
runs in its own closure; `return` returns from that expression, and panics
propagate to the caller. Only the selected enum variant's fields and
invariants are checked. Expressions may have arbitrary side effects, even
when the conversion fails or its result is discarded.

These predicates are additional acceptance checks beyond Rust's bit validity;
bit-valid bytes may still be rejected. See [What is a "valid instance"?] for
the distinction.

[What is a "valid instance"?]: trait@TryFromBytes#what-is-a-valid-instance

Invariants are not supported on tuple fields. Types with invariants cannot
derive [`FromZeros`] or [`FromBytes`], whose conversions do not perform runtime
validation. These checks apply to conversions through [`TryFromBytes`]; they
do not restrict ordinary construction or mutation of Rust values.

"#
)]
///
/// # Portability
///
/// To ensure consistent endianness for enums with multi-byte representations,
/// explicitly specify and convert each discriminant using `.to_le()` or
/// `.to_be()`; e.g.:
///
/// ```
/// # use zerocopy_derive::TryFromBytes;
/// // `DataStoreVersion` is encoded in little-endian.
/// #[derive(TryFromBytes)]
/// #[repr(u32)]
/// pub enum DataStoreVersion {
///     /// Version 1 of the data store.
///     V1 = 9u32.to_le(),
///
///     /// Version 2 of the data store.
///     V2 = 10u32.to_le(),
/// }
/// ```
///
/// [safety conditions]: trait@TryFromBytes#safety
#[cfg(any(feature = "derive", test))]
#[cfg_attr(doc_cfg, doc(cfg(feature = "derive")))]
pub use zerocopy_derive::TryFromBytes;

/// Types for which some bit patterns are valid.
///
/// A memory region of the appropriate length which contains initialized bytes
/// can be viewed as a `TryFromBytes` type so long as the runtime value of those
/// bytes corresponds to a [*valid instance*] of that type. For example,
/// [`bool`] is `TryFromBytes`, so zerocopy can transmute a [`u8`] into a
/// [`bool`] so long as it first checks that the value of the [`u8`] is `0` or
/// `1`.
///
/// # Implementation
///
/// **Do not implement this trait yourself!** Instead, use
/// [`#[derive(TryFromBytes)]`][derive]; e.g.:
///
/// ```
/// # use zerocopy_derive::{TryFromBytes, Immutable};
/// #[derive(TryFromBytes)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(TryFromBytes)]
/// #[repr(u8)]
/// enum MyEnum {
/// #   V00,
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(TryFromBytes, Immutable)]
/// union MyUnion {
/// #   variant: u8,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// This derive ensures that the runtime check of whether bytes correspond to a
/// valid instance is sound. You **must** implement this trait via the derive.
///
/// # What is a "valid instance"?
///
/// In Rust, each type has *bit validity*, which refers to the set of bit
/// patterns which may appear in an instance of that type. It is impossible for
/// safe Rust code to produce values which violate bit validity (ie, values
/// outside of the "valid" set of bit patterns). If `unsafe` code produces an
/// invalid value, this is considered [undefined behavior].
///
/// Rust's bit validity rules are currently being decided, which means that some
/// types have three classes of bit patterns: those which are definitely valid,
/// and whose validity is documented in the language; those which may or may not
/// be considered valid at some point in the future; and those which are
/// definitely invalid.
///
/// Zerocopy takes a conservative approach, and only considers a bit pattern to
/// be valid if its validity is a documented guarantee provided by the
/// language.
///
/// For most use cases, Rust's current guarantees align with programmers'
/// intuitions about what ought to be valid. As a result, zerocopy's
/// conservatism should not affect most users.
///
/// If you are negatively affected by lack of support for a particular type,
/// we encourage you to let us know by [filing an issue][github-repo].
///
/// In this trait's conversion methods, a "valid instance" must also pass any
/// configured field invariants, including those on nested fields. Rust bit
/// validity alone does not guarantee that a conversion succeeds: an invariant
/// may reject otherwise bit-valid bytes. Such predicates check acceptance at
/// conversion time; they do not constrain subsequent mutation or ordinary Rust
/// construction. See the [derive's field invariants][derive] documentation for
/// the experimental attribute's syntax, evaluation order, and side effects.
///
/// # `TryFromBytes` is not symmetrical with [`IntoBytes`]
///
/// There are some types which implement both `TryFromBytes` and [`IntoBytes`],
/// but for which `TryFromBytes` is not guaranteed to accept all byte sequences
/// produced by `IntoBytes`. In other words, for some `T: TryFromBytes +
/// IntoBytes`, there exist values of `t: T` such that
/// `TryFromBytes::try_ref_from_bytes(t.as_bytes()) == None`. Code should not
/// generally assume that values produced by `IntoBytes` will necessarily be
/// accepted as valid by `TryFromBytes`.
///
/// # Safety
///
/// On its own, `T: TryFromBytes` does not make any guarantees about the layout
/// or representation of `T`. It merely provides the ability to perform a
/// validity check at runtime via methods like [`try_ref_from_bytes`].
///
/// You must not rely on the `#[doc(hidden)]` internals of `TryFromBytes`.
/// Future releases of zerocopy may make backwards-breaking changes to these
/// items, including changes that only affect soundness, which may cause code
/// which uses those items to silently become unsound.
///
/// [undefined behavior]: https://raphlinus.github.io/programming/rust/2018/08/17/undefined-behavior.html
/// [github-repo]: https://github.com/google/zerocopy
/// [`try_ref_from_bytes`]: TryFromBytes::try_ref_from_bytes
/// [*valid instance*]: #what-is-a-valid-instance
#[cfg_attr(feature = "derive", doc = "[derive]: zerocopy_derive::TryFromBytes")]
#[cfg_attr(
    not(feature = "derive"),
    doc = concat!("[derive]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.TryFromBytes.html"),
)]
#[cfg_attr(
    not(no_zerocopy_diagnostic_on_unimplemented_1_78_0),
    diagnostic::on_unimplemented(note = "Consider adding `#[derive(TryFromBytes)]` to `{Self}`")
)]
pub unsafe trait TryFromBytes {
    // The `Self: Sized` bound makes it so that `TryFromBytes` is still object
    // safe.
    #[doc(hidden)]
    fn only_derive_is_allowed_to_implement_this_trait()
    where
        Self: Sized;

    /// Does a given memory range contain a valid instance of `Self`?
    ///
    /// # Safety
    ///
    /// Unsafe code may assume that, if `is_safe(candidate)` returns true,
    /// `*candidate` contains a valid `Self`.
    ///
    /// # Panics
    ///
    /// `is_safe` may panic. Callers are responsible for ensuring that any
    /// `unsafe` code remains sound even in the face of `is_safe` panicking. (We
    /// support user-defined validation routines; so long as these routines are
    /// not required to be `unsafe`, there is no way to ensure that these do not
    /// generate panics.)
    ///
    /// Besides user-defined validation routines panicking, `is_safe` will either
    /// panic or fail to compile if called on a pointer with [`Shared`] aliasing
    /// when `Self: !Immutable`.
    ///
    /// [`UnsafeCell`]: core::cell::UnsafeCell
    /// [`Shared`]: invariant::Shared
    #[doc(hidden)]
    fn is_safe<A>(candidate: Maybe<'_, Self, A>) -> bool
    where
        A: invariant::Alignment;

    /// Attempts to interpret the given `source` as a `&Self`.
    ///
    /// If the bytes of `source` are a valid instance of `Self`, this method
    /// returns a reference to those bytes interpreted as a `Self`. If the
    /// length of `source` is not a [valid size of `Self`][valid-size], or if
    /// `source` is not appropriately aligned, or if `source` is not a valid
    /// instance of `Self`, this returns `Err`. If [`Self:
    /// Unaligned`][self-unaligned], you can [infallibly discard the alignment
    /// error][ConvertError::from].
    ///
    /// `Self` may be a sized type, a slice, or a [slice DST][slice-dst].
    ///
    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
    /// [self-unaligned]: Unaligned
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Compile-Time Assertions
    ///
    /// This method cannot yet be used on unsized types whose dynamically-sized
    /// component is zero-sized. Attempting to use this method on such types
    /// results in a compile-time assertion error; e.g.:
    ///
    /// ```compile_fail,E0080
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(TryFromBytes, Immutable, KnownLayout)]
    /// #[repr(C)]
    /// struct ZSTy {
    ///     leading_sized: u16,
    ///     trailing_dst: [()],
    /// }
    ///
    /// let _ = ZSTy::try_ref_from_bytes(0u16.as_bytes()); // ⚠ Compile Error!
    /// ```
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the byte sequence `0xC0C0`.
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    ///     marshmallows: [[u8; 2]],
    /// }
    ///
    /// let bytes = &[0xC0, 0xC0, 240, 77, 0, 1, 2, 3, 4, 5][..];
    ///
    /// let packet = Packet::try_ref_from_bytes(bytes).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(packet.marshmallows, [[0, 1], [2, 3], [4, 5]]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &[0x10, 0xC0, 240, 77, 0, 1, 2, 3, 4, 5][..];
    /// assert!(Packet::try_ref_from_bytes(bytes).is_err());
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "try_ref_from_bytes",
        format = "coco",
        arity = 3,
        [
            open
            @index 1
            @title "Sized"
            @variant "static_size"
        ],
        [
            @index 2
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 3
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_ref_from_bytes(source: &[u8]) -> Result<&Self, TryCastError<&[u8], Self>>
    where
        Self: KnownLayout + Immutable,
    {
        static_assert_dst_is_not_zst!(Self);
        match Ptr::from_ref(source).try_cast_into_no_leftover::<Self, BecauseImmutable>(None) {
            Ok(source) => {
                // This call may panic. If that happens, it doesn't cause any soundness
                // issues, as we have not generated any invalid state which we need to
                // fix before returning.
                match source.try_into_safe() {
                    Ok(valid) => Ok(valid.as_ref()),
                    Err(e) => {
                        Err(e.map_src(|src| src.as_bytes::<BecauseImmutable>().as_ref()).into())
                    }
                }
            }
            Err(e) => Err(e.map_src(Ptr::as_ref).into()),
        }
    }

    /// Attempts to interpret the prefix of the given `source` as a `&Self`.
    ///
    /// This method computes the [largest possible size of `Self`][valid-size]
    /// that can fit in the leading bytes of `source`. If that prefix is a valid
    /// instance of `Self`, this method returns a reference to those bytes
    /// interpreted as `Self`, and a reference to the remaining bytes. If there
    /// are insufficient bytes, or if `source` is not appropriately aligned, or
    /// if those bytes are not a valid instance of `Self`, this returns `Err`.
    /// If [`Self: Unaligned`][self-unaligned], you can [infallibly discard the
    /// alignment error][ConvertError::from].
    ///
    /// `Self` may be a sized type, a slice, or a [slice DST][slice-dst].
    ///
    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
    /// [self-unaligned]: Unaligned
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Compile-Time Assertions
    ///
    /// This method cannot yet be used on unsized types whose dynamically-sized
    /// component is zero-sized. Attempting to use this method on such types
    /// results in a compile-time assertion error; e.g.:
    ///
    /// ```compile_fail,E0080
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(TryFromBytes, Immutable, KnownLayout)]
    /// #[repr(C)]
    /// struct ZSTy {
    ///     leading_sized: u16,
    ///     trailing_dst: [()],
    /// }
    ///
    /// let _ = ZSTy::try_ref_from_prefix(0u16.as_bytes()); // ⚠ Compile Error!
    /// ```
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    ///     marshmallows: [[u8; 2]],
    /// }
    ///
    /// // These are more bytes than are needed to encode a `Packet`.
    /// let bytes = &[0xC0, 0xC0, 240, 77, 0, 1, 2, 3, 4, 5, 6][..];
    ///
    /// let (packet, suffix) = Packet::try_ref_from_prefix(bytes).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(packet.marshmallows, [[0, 1], [2, 3], [4, 5]]);
    /// assert_eq!(suffix, &[6u8][..]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &[0x10, 0xC0, 240, 77, 0, 1, 2, 3, 4, 5, 6][..];
    /// assert!(Packet::try_ref_from_prefix(bytes).is_err());
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "try_ref_from_prefix",
        format = "coco",
        arity = 3,
        [
            open
            @index 1
            @title "Sized"
            @variant "static_size"
        ],
        [
            @index 2
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 3
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_ref_from_prefix(source: &[u8]) -> Result<(&Self, &[u8]), TryCastError<&[u8], Self>>
    where
        Self: KnownLayout + Immutable,
    {
        static_assert_dst_is_not_zst!(Self);
        try_ref_from_prefix_suffix(source, CastType::Prefix, None)
    }

    /// Attempts to interpret the suffix of the given `source` as a `&Self`.
    ///
    /// This method computes the [largest possible size of `Self`][valid-size]
    /// that can fit in the trailing bytes of `source`. If that suffix is a
    /// valid instance of `Self`, this method returns a reference to those bytes
    /// interpreted as `Self`, and a reference to the preceding bytes. If there
    /// are insufficient bytes, or if the suffix of `source` would not be
    /// appropriately aligned, or if the suffix is not a valid instance of
    /// `Self`, this returns `Err`. If [`Self: Unaligned`][self-unaligned], you
    /// can [infallibly discard the alignment error][ConvertError::from].
    ///
    /// `Self` may be a sized type, a slice, or a [slice DST][slice-dst].
    ///
    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
    /// [self-unaligned]: Unaligned
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Compile-Time Assertions
    ///
    /// This method cannot yet be used on unsized types whose dynamically-sized
    /// component is zero-sized. Attempting to use this method on such types
    /// results in a compile-time assertion error; e.g.:
    ///
    /// ```compile_fail,E0080
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(TryFromBytes, Immutable, KnownLayout)]
    /// #[repr(C)]
    /// struct ZSTy {
    ///     leading_sized: u16,
    ///     trailing_dst: [()],
    /// }
    ///
    /// let _ = ZSTy::try_ref_from_suffix(0u16.as_bytes()); // ⚠ Compile Error!
    /// ```
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    ///     marshmallows: [[u8; 2]],
    /// }
    ///
    /// // These are more bytes than are needed to encode a `Packet`.
    /// let bytes = &[0, 0xC0, 0xC0, 240, 77, 2, 3, 4, 5, 6, 7][..];
    ///
    /// let (prefix, packet) = Packet::try_ref_from_suffix(bytes).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(packet.marshmallows, [[2, 3], [4, 5], [6, 7]]);
    /// assert_eq!(prefix, &[0u8][..]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &[0, 1, 2, 3, 4, 5, 6, 77, 240, 0xC0, 0x10][..];
    /// assert!(Packet::try_ref_from_suffix(bytes).is_err());
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "try_ref_from_suffix",
        format = "coco",
        arity = 3,
        [
            open
            @index 1
            @title "Sized"
            @variant "static_size"
        ],
        [
            @index 2
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 3
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_ref_from_suffix(source: &[u8]) -> Result<(&[u8], &Self), TryCastError<&[u8], Self>>
    where
        Self: KnownLayout + Immutable,
    {
        static_assert_dst_is_not_zst!(Self);
        try_ref_from_prefix_suffix(source, CastType::Suffix, None).map(swap)
    }

    /// Attempts to interpret the given `source` as a `&mut Self` without
    /// copying.
    ///
    /// If the bytes of `source` are a valid instance of `Self`, this method
    /// returns a reference to those bytes interpreted as a `Self`. If the
    /// length of `source` is not a [valid size of `Self`][valid-size], or if
    /// `source` is not appropriately aligned, or if `source` is not a valid
    /// instance of `Self`, this returns `Err`. If [`Self:
    /// Unaligned`][self-unaligned], you can [infallibly discard the alignment
    /// error][ConvertError::from].
    ///
    /// `Self` may be a sized type, a slice, or a [slice DST][slice-dst].
    ///
    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
    /// [self-unaligned]: Unaligned
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Compile-Time Assertions
    ///
    /// This method cannot yet be used on unsized types whose dynamically-sized
    /// component is zero-sized. Attempting to use this method on such types
    /// results in a compile-time assertion error; e.g.:
    ///
    /// ```compile_fail,E0080
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct ZSTy {
    ///     leading_sized: [u8; 2],
    ///     trailing_dst: [()],
    /// }
    ///
    /// let mut source = [85, 85];
    /// let _ = ZSTy::try_mut_from_bytes(&mut source[..]); // ⚠ Compile Error!
    /// ```
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    ///     marshmallows: [[u8; 2]],
    /// }
    ///
    /// let bytes = &mut [0xC0, 0xC0, 240, 77, 0, 1, 2, 3, 4, 5][..];
    ///
    /// let packet = Packet::try_mut_from_bytes(bytes).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(packet.marshmallows, [[0, 1], [2, 3], [4, 5]]);
    ///
    /// packet.temperature = 111;
    ///
    /// assert_eq!(bytes, [0xC0, 0xC0, 240, 111, 0, 1, 2, 3, 4, 5]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &mut [0x10, 0xC0, 240, 77, 0, 1, 2, 3, 4, 5, 6][..];
    /// assert!(Packet::try_mut_from_bytes(bytes).is_err());
    /// ```
    ///
    #[doc = codegen_header!("h5", "try_mut_from_bytes")]
    ///
    /// See [`TryFromBytes::try_ref_from_bytes`](#method.try_ref_from_bytes.codegen).
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_mut_from_bytes(bytes: &mut [u8]) -> Result<&mut Self, TryCastError<&mut [u8], Self>>
    where
        Self: KnownLayout + IntoBytes,
    {
        static_assert_dst_is_not_zst!(Self);
        match Ptr::from_mut(bytes).try_cast_into_no_leftover::<Self, BecauseExclusive>(None) {
            Ok(source) => {
                // This call may panic. If that happens, it doesn't cause any soundness
                // issues, as we have not generated any invalid state which we need to
                // fix before returning.
                match source.try_into_safe() {
                    Ok(source) => Ok(source.as_mut()),
                    Err(e) => Err(e.map_src(|src| src.as_bytes().as_mut()).into()),
                }
            }
            Err(e) => Err(e.map_src(Ptr::as_mut).into()),
        }
    }

    /// Attempts to interpret the prefix of the given `source` as a `&mut
    /// Self`.
    ///
    /// This method computes the [largest possible size of `Self`][valid-size]
    /// that can fit in the leading bytes of `source`. If that prefix is a valid
    /// instance of `Self`, this method returns a reference to those bytes
    /// interpreted as `Self`, and a reference to the remaining bytes. If there
    /// are insufficient bytes, or if `source` is not appropriately aligned, or
    /// if the bytes are not a valid instance of `Self`, this returns `Err`. If
    /// [`Self: Unaligned`][self-unaligned], you can [infallibly discard the
    /// alignment error][ConvertError::from].
    ///
    /// `Self` may be a sized type, a slice, or a [slice DST][slice-dst].
    ///
    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
    /// [self-unaligned]: Unaligned
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Compile-Time Assertions
    ///
    /// This method cannot yet be used on unsized types whose dynamically-sized
    /// component is zero-sized. Attempting to use this method on such types
    /// results in a compile-time assertion error; e.g.:
    ///
    /// ```compile_fail,E0080
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct ZSTy {
    ///     leading_sized: [u8; 2],
    ///     trailing_dst: [()],
    /// }
    ///
    /// let mut source = [85, 85];
    /// let _ = ZSTy::try_mut_from_prefix(&mut source[..]); // ⚠ Compile Error!
    /// ```
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    ///     marshmallows: [[u8; 2]],
    /// }
    ///
    /// // These are more bytes than are needed to encode a `Packet`.
    /// let bytes = &mut [0xC0, 0xC0, 240, 77, 0, 1, 2, 3, 4, 5, 6][..];
    ///
    /// let (packet, suffix) = Packet::try_mut_from_prefix(bytes).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(packet.marshmallows, [[0, 1], [2, 3], [4, 5]]);
    /// assert_eq!(suffix, &[6u8][..]);
    ///
    /// packet.temperature = 111;
    /// suffix[0] = 222;
    ///
    /// assert_eq!(bytes, [0xC0, 0xC0, 240, 111, 0, 1, 2, 3, 4, 5, 222]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &mut [0x10, 0xC0, 240, 77, 0, 1, 2, 3, 4, 5, 6][..];
    /// assert!(Packet::try_mut_from_prefix(bytes).is_err());
    /// ```
    ///
    #[doc = codegen_header!("h5", "try_mut_from_prefix")]
    ///
    /// See [`TryFromBytes::try_ref_from_prefix`](#method.try_ref_from_prefix.codegen).
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_mut_from_prefix(
        source: &mut [u8],
    ) -> Result<(&mut Self, &mut [u8]), TryCastError<&mut [u8], Self>>
    where
        Self: KnownLayout + IntoBytes,
    {
        static_assert_dst_is_not_zst!(Self);
        try_mut_from_prefix_suffix(source, CastType::Prefix, None)
    }

    /// Attempts to interpret the suffix of the given `source` as a `&mut
    /// Self`.
    ///
    /// This method computes the [largest possible size of `Self`][valid-size]
    /// that can fit in the trailing bytes of `source`. If that suffix is a
    /// valid instance of `Self`, this method returns a reference to those bytes
    /// interpreted as `Self`, and a reference to the preceding bytes. If there
    /// are insufficient bytes, or if the suffix of `source` would not be
    /// appropriately aligned, or if the suffix is not a valid instance of
    /// `Self`, this returns `Err`. If [`Self: Unaligned`][self-unaligned], you
    /// can [infallibly discard the alignment error][ConvertError::from].
    ///
    /// `Self` may be a sized type, a slice, or a [slice DST][slice-dst].
    ///
    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
    /// [self-unaligned]: Unaligned
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Compile-Time Assertions
    ///
    /// This method cannot yet be used on unsized types whose dynamically-sized
    /// component is zero-sized. Attempting to use this method on such types
    /// results in a compile-time assertion error; e.g.:
    ///
    /// ```compile_fail,E0080
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct ZSTy {
    ///     leading_sized: u16,
    ///     trailing_dst: [()],
    /// }
    ///
    /// let mut source = [85, 85];
    /// let _ = ZSTy::try_mut_from_suffix(&mut source[..]); // ⚠ Compile Error!
    /// ```
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    ///     marshmallows: [[u8; 2]],
    /// }
    ///
    /// // These are more bytes than are needed to encode a `Packet`.
    /// let bytes = &mut [0, 0xC0, 0xC0, 240, 77, 2, 3, 4, 5, 6, 7][..];
    ///
    /// let (prefix, packet) = Packet::try_mut_from_suffix(bytes).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(packet.marshmallows, [[2, 3], [4, 5], [6, 7]]);
    /// assert_eq!(prefix, &[0u8][..]);
    ///
    /// prefix[0] = 111;
    /// packet.temperature = 222;
    ///
    /// assert_eq!(bytes, [111, 0xC0, 0xC0, 240, 222, 2, 3, 4, 5, 6, 7]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &mut [0, 1, 2, 3, 4, 5, 6, 77, 240, 0xC0, 0x10][..];
    /// assert!(Packet::try_mut_from_suffix(bytes).is_err());
    /// ```
    ///
    #[doc = codegen_header!("h5", "try_mut_from_suffix")]
    ///
    /// See [`TryFromBytes::try_ref_from_suffix`](#method.try_ref_from_suffix.codegen).
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_mut_from_suffix(
        source: &mut [u8],
    ) -> Result<(&mut [u8], &mut Self), TryCastError<&mut [u8], Self>>
    where
        Self: KnownLayout + IntoBytes,
    {
        static_assert_dst_is_not_zst!(Self);
        try_mut_from_prefix_suffix(source, CastType::Suffix, None).map(swap)
    }

    /// Attempts to interpret the given `source` as a `&Self` with a DST length
    /// equal to `count`.
    ///
    /// This method attempts to return a reference to `source` interpreted as a
    /// `Self` with `count` trailing elements. If the length of `source` is not
    /// equal to the size of `Self` with `count` elements, if `source` is not
    /// appropriately aligned, or if `source` does not contain a valid instance
    /// of `Self`, this returns `Err`. If [`Self: Unaligned`][self-unaligned],
    /// you can [infallibly discard the alignment error][ConvertError::from].
    ///
    /// [self-unaligned]: Unaligned
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Examples
    ///
    /// ```
    /// # #![allow(non_camel_case_types)] // For C0::xC0
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    ///     marshmallows: [[u8; 2]],
    /// }
    ///
    /// let bytes = &[0xC0, 0xC0, 240, 77, 2, 3, 4, 5, 6, 7][..];
    ///
    /// let packet = Packet::try_ref_from_bytes_with_elems(bytes, 3).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(packet.marshmallows, [[2, 3], [4, 5], [6, 7]]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &[0, 1, 2, 3, 4, 5, 6, 77, 240, 0xC0, 0xC0][..];
    /// assert!(Packet::try_ref_from_bytes_with_elems(bytes, 3).is_err());
    /// ```
    ///
    /// Since an explicit `count` is provided, this method supports types with
    /// zero-sized trailing slice elements. Methods such as [`try_ref_from_bytes`]
    /// which do not take an explicit count do not support such types.
    ///
    /// ```
    /// use core::num::NonZeroU16;
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(TryFromBytes, Immutable, KnownLayout)]
    /// #[repr(C)]
    /// struct ZSTy {
    ///     leading_sized: NonZeroU16,
    ///     trailing_dst: [()],
    /// }
    ///
    /// let src = 0xCAFEu16.as_bytes();
    /// let zsty = ZSTy::try_ref_from_bytes_with_elems(src, 42).unwrap();
    /// assert_eq!(zsty.trailing_dst.len(), 42);
    /// ```
    ///
    /// [`try_ref_from_bytes`]: TryFromBytes::try_ref_from_bytes
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "try_ref_from_bytes_with_elems",
        format = "coco",
        arity = 2,
        [
            open
            @index 1
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 2
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_ref_from_bytes_with_elems(
        source: &[u8],
        count: usize,
    ) -> Result<&Self, TryCastError<&[u8], Self>>
    where
        Self: KnownLayout<PointerMetadata = usize> + Immutable,
    {
        match Ptr::from_ref(source).try_cast_into_no_leftover::<Self, BecauseImmutable>(Some(count))
        {
            Ok(source) => {
                // This call may panic. If that happens, it doesn't cause any soundness
                // issues, as we have not generated any invalid state which we need to
                // fix before returning.
                match source.try_into_safe() {
                    Ok(source) => Ok(source.as_ref()),
                    Err(e) => {
                        Err(e.map_src(|src| src.as_bytes::<BecauseImmutable>().as_ref()).into())
                    }
                }
            }
            Err(e) => Err(e.map_src(Ptr::as_ref).into()),
        }
    }

    /// Attempts to interpret the prefix of the given `source` as a `&Self` with
    /// a DST length equal to `count`.
    ///
    /// This method attempts to return a reference to the prefix of `source`
    /// interpreted as a `Self` with `count` trailing elements, and a reference
    /// to the remaining bytes. If the length of `source` is less than the size
    /// of `Self` with `count` elements, if `source` is not appropriately
    /// aligned, or if the prefix of `source` does not contain a valid instance
    /// of `Self`, this returns `Err`. If [`Self: Unaligned`][self-unaligned],
    /// you can [infallibly discard the alignment error][ConvertError::from].
    ///
    /// [self-unaligned]: Unaligned
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Examples
    ///
    /// ```
    /// # #![allow(non_camel_case_types)] // For C0::xC0
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    ///     marshmallows: [[u8; 2]],
    /// }
    ///
    /// let bytes = &[0xC0, 0xC0, 240, 77, 2, 3, 4, 5, 6, 7, 8][..];
    ///
    /// let (packet, suffix) = Packet::try_ref_from_prefix_with_elems(bytes, 3).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(packet.marshmallows, [[2, 3], [4, 5], [6, 7]]);
    /// assert_eq!(suffix, &[8u8][..]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &mut [0, 1, 2, 3, 4, 5, 6, 7, 8, 77, 240, 0xC0, 0xC0][..];
    /// assert!(Packet::try_ref_from_prefix_with_elems(bytes, 3).is_err());
    /// ```
    ///
    /// Since an explicit `count` is provided, this method supports types with
    /// zero-sized trailing slice elements. Methods such as [`try_ref_from_prefix`]
    /// which do not take an explicit count do not support such types.
    ///
    /// ```
    /// use core::num::NonZeroU16;
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(TryFromBytes, Immutable, KnownLayout)]
    /// #[repr(C)]
    /// struct ZSTy {
    ///     leading_sized: NonZeroU16,
    ///     trailing_dst: [()],
    /// }
    ///
    /// let src = 0xCAFEu16.as_bytes();
    /// let (zsty, _) = ZSTy::try_ref_from_prefix_with_elems(src, 42).unwrap();
    /// assert_eq!(zsty.trailing_dst.len(), 42);
    /// ```
    ///
    /// [`try_ref_from_prefix`]: TryFromBytes::try_ref_from_prefix
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "try_ref_from_prefix_with_elems",
        format = "coco",
        arity = 2,
        [
            open
            @index 1
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 2
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_ref_from_prefix_with_elems(
        source: &[u8],
        count: usize,
    ) -> Result<(&Self, &[u8]), TryCastError<&[u8], Self>>
    where
        Self: KnownLayout<PointerMetadata = usize> + Immutable,
    {
        try_ref_from_prefix_suffix(source, CastType::Prefix, Some(count))
    }

    /// Attempts to interpret the suffix of the given `source` as a `&Self` with
    /// a DST length equal to `count`.
    ///
    /// This method attempts to return a reference to the suffix of `source`
    /// interpreted as a `Self` with `count` trailing elements, and a reference
    /// to the preceding bytes. If the length of `source` is less than the size
    /// of `Self` with `count` elements, if the suffix of `source` is not
    /// appropriately aligned, or if the suffix of `source` does not contain a
    /// valid instance of `Self`, this returns `Err`. If [`Self:
    /// Unaligned`][self-unaligned], you can [infallibly discard the alignment
    /// error][ConvertError::from].
    ///
    /// [self-unaligned]: Unaligned
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Examples
    ///
    /// ```
    /// # #![allow(non_camel_case_types)] // For C0::xC0
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    ///     marshmallows: [[u8; 2]],
    /// }
    ///
    /// let bytes = &[123, 0xC0, 0xC0, 240, 77, 2, 3, 4, 5, 6, 7][..];
    ///
    /// let (prefix, packet) = Packet::try_ref_from_suffix_with_elems(bytes, 3).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(packet.marshmallows, [[2, 3], [4, 5], [6, 7]]);
    /// assert_eq!(prefix, &[123u8][..]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 77, 240, 0xC0, 0xC0][..];
    /// assert!(Packet::try_ref_from_suffix_with_elems(bytes, 3).is_err());
    /// ```
    ///
    /// Since an explicit `count` is provided, this method supports types with
    /// zero-sized trailing slice elements. Methods such as [`try_ref_from_prefix`]
    /// which do not take an explicit count do not support such types.
    ///
    /// ```
    /// use core::num::NonZeroU16;
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(TryFromBytes, Immutable, KnownLayout)]
    /// #[repr(C)]
    /// struct ZSTy {
    ///     leading_sized: NonZeroU16,
    ///     trailing_dst: [()],
    /// }
    ///
    /// let src = 0xCAFEu16.as_bytes();
    /// let (_, zsty) = ZSTy::try_ref_from_suffix_with_elems(src, 42).unwrap();
    /// assert_eq!(zsty.trailing_dst.len(), 42);
    /// ```
    ///
    /// [`try_ref_from_prefix`]: TryFromBytes::try_ref_from_prefix
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "try_ref_from_suffix_with_elems",
        format = "coco",
        arity = 2,
        [
            open
            @index 1
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 2
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_ref_from_suffix_with_elems(
        source: &[u8],
        count: usize,
    ) -> Result<(&[u8], &Self), TryCastError<&[u8], Self>>
    where
        Self: KnownLayout<PointerMetadata = usize> + Immutable,
    {
        try_ref_from_prefix_suffix(source, CastType::Suffix, Some(count)).map(swap)
    }

    /// Attempts to interpret the given `source` as a `&mut Self` with a DST
    /// length equal to `count`.
    ///
    /// This method attempts to return a reference to `source` interpreted as a
    /// `Self` with `count` trailing elements. If the length of `source` is not
    /// equal to the size of `Self` with `count` elements, if `source` is not
    /// appropriately aligned, or if `source` does not contain a valid instance
    /// of `Self`, this returns `Err`. If [`Self: Unaligned`][self-unaligned],
    /// you can [infallibly discard the alignment error][ConvertError::from].
    ///
    /// [self-unaligned]: Unaligned
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Examples
    ///
    /// ```
    /// # #![allow(non_camel_case_types)] // For C0::xC0
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    ///     marshmallows: [[u8; 2]],
    /// }
    ///
    /// let bytes = &mut [0xC0, 0xC0, 240, 77, 2, 3, 4, 5, 6, 7][..];
    ///
    /// let packet = Packet::try_mut_from_bytes_with_elems(bytes, 3).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(packet.marshmallows, [[2, 3], [4, 5], [6, 7]]);
    ///
    /// packet.temperature = 111;
    ///
    /// assert_eq!(bytes, [0xC0, 0xC0, 240, 111, 2, 3, 4, 5, 6, 7]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &mut [0, 1, 2, 3, 4, 5, 6, 77, 240, 0xC0, 0xC0][..];
    /// assert!(Packet::try_mut_from_bytes_with_elems(bytes, 3).is_err());
    /// ```
    ///
    /// Since an explicit `count` is provided, this method supports types with
    /// zero-sized trailing slice elements. Methods such as [`try_mut_from_bytes`]
    /// which do not take an explicit count do not support such types.
    ///
    /// ```
    /// use core::num::NonZeroU16;
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct ZSTy {
    ///     leading_sized: NonZeroU16,
    ///     trailing_dst: [()],
    /// }
    ///
    /// let mut src = 0xCAFEu16;
    /// let src = src.as_mut_bytes();
    /// let zsty = ZSTy::try_mut_from_bytes_with_elems(src, 42).unwrap();
    /// assert_eq!(zsty.trailing_dst.len(), 42);
    /// ```
    ///
    /// [`try_mut_from_bytes`]: TryFromBytes::try_mut_from_bytes
    ///
    #[doc = codegen_header!("h5", "try_mut_from_bytes_with_elems")]
    ///
    /// See [`TryFromBytes::try_ref_from_bytes_with_elems`](#method.try_ref_from_bytes_with_elems.codegen).
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_mut_from_bytes_with_elems(
        source: &mut [u8],
        count: usize,
    ) -> Result<&mut Self, TryCastError<&mut [u8], Self>>
    where
        Self: KnownLayout<PointerMetadata = usize> + IntoBytes,
    {
        match Ptr::from_mut(source).try_cast_into_no_leftover::<Self, BecauseExclusive>(Some(count))
        {
            Ok(source) => {
                // This call may panic. If that happens, it doesn't cause any soundness
                // issues, as we have not generated any invalid state which we need to
                // fix before returning.
                match source.try_into_safe() {
                    Ok(source) => Ok(source.as_mut()),
                    Err(e) => Err(e.map_src(|src| src.as_bytes().as_mut()).into()),
                }
            }
            Err(e) => Err(e.map_src(Ptr::as_mut).into()),
        }
    }

    /// Attempts to interpret the prefix of the given `source` as a `&mut Self`
    /// with a DST length equal to `count`.
    ///
    /// This method attempts to return a reference to the prefix of `source`
    /// interpreted as a `Self` with `count` trailing elements, and a reference
    /// to the remaining bytes. If the length of `source` is less than the size
    /// of `Self` with `count` elements, if `source` is not appropriately
    /// aligned, or if the prefix of `source` does not contain a valid instance
    /// of `Self`, this returns `Err`. If [`Self: Unaligned`][self-unaligned],
    /// you can [infallibly discard the alignment error][ConvertError::from].
    ///
    /// [self-unaligned]: Unaligned
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Examples
    ///
    /// ```
    /// # #![allow(non_camel_case_types)] // For C0::xC0
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    ///     marshmallows: [[u8; 2]],
    /// }
    ///
    /// let bytes = &mut [0xC0, 0xC0, 240, 77, 2, 3, 4, 5, 6, 7, 8][..];
    ///
    /// let (packet, suffix) = Packet::try_mut_from_prefix_with_elems(bytes, 3).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(packet.marshmallows, [[2, 3], [4, 5], [6, 7]]);
    /// assert_eq!(suffix, &[8u8][..]);
    ///
    /// packet.temperature = 111;
    /// suffix[0] = 222;
    ///
    /// assert_eq!(bytes, [0xC0, 0xC0, 240, 111, 2, 3, 4, 5, 6, 7, 222]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &mut [0, 1, 2, 3, 4, 5, 6, 7, 8, 77, 240, 0xC0, 0xC0][..];
    /// assert!(Packet::try_mut_from_prefix_with_elems(bytes, 3).is_err());
    /// ```
    ///
    /// Since an explicit `count` is provided, this method supports types with
    /// zero-sized trailing slice elements. Methods such as [`try_mut_from_prefix`]
    /// which do not take an explicit count do not support such types.
    ///
    /// ```
    /// use core::num::NonZeroU16;
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct ZSTy {
    ///     leading_sized: NonZeroU16,
    ///     trailing_dst: [()],
    /// }
    ///
    /// let mut src = 0xCAFEu16;
    /// let src = src.as_mut_bytes();
    /// let (zsty, _) = ZSTy::try_mut_from_prefix_with_elems(src, 42).unwrap();
    /// assert_eq!(zsty.trailing_dst.len(), 42);
    /// ```
    ///
    /// [`try_mut_from_prefix`]: TryFromBytes::try_mut_from_prefix
    ///
    #[doc = codegen_header!("h5", "try_mut_from_prefix_with_elems")]
    ///
    /// See [`TryFromBytes::try_ref_from_prefix_with_elems`](#method.try_ref_from_prefix_with_elems.codegen).
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_mut_from_prefix_with_elems(
        source: &mut [u8],
        count: usize,
    ) -> Result<(&mut Self, &mut [u8]), TryCastError<&mut [u8], Self>>
    where
        Self: KnownLayout<PointerMetadata = usize> + IntoBytes,
    {
        try_mut_from_prefix_suffix(source, CastType::Prefix, Some(count))
    }

    /// Attempts to interpret the suffix of the given `source` as a `&mut Self`
    /// with a DST length equal to `count`.
    ///
    /// This method attempts to return a reference to the suffix of `source`
    /// interpreted as a `Self` with `count` trailing elements, and a reference
    /// to the preceding bytes. If the length of `source` is less than the size
    /// of `Self` with `count` elements, if the suffix of `source` is not
    /// appropriately aligned, or if the suffix of `source` does not contain a
    /// valid instance of `Self`, this returns `Err`. If [`Self:
    /// Unaligned`][self-unaligned], you can [infallibly discard the alignment
    /// error][ConvertError::from].
    ///
    /// [self-unaligned]: Unaligned
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Examples
    ///
    /// ```
    /// # #![allow(non_camel_case_types)] // For C0::xC0
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    ///     marshmallows: [[u8; 2]],
    /// }
    ///
    /// let bytes = &mut [123, 0xC0, 0xC0, 240, 77, 2, 3, 4, 5, 6, 7][..];
    ///
    /// let (prefix, packet) = Packet::try_mut_from_suffix_with_elems(bytes, 3).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(packet.marshmallows, [[2, 3], [4, 5], [6, 7]]);
    /// assert_eq!(prefix, &[123u8][..]);
    ///
    /// prefix[0] = 111;
    /// packet.temperature = 222;
    ///
    /// assert_eq!(bytes, [111, 0xC0, 0xC0, 240, 222, 2, 3, 4, 5, 6, 7]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &mut [0, 1, 2, 3, 4, 5, 6, 7, 8, 77, 240, 0xC0, 0xC0][..];
    /// assert!(Packet::try_mut_from_suffix_with_elems(bytes, 3).is_err());
    /// ```
    ///
    /// Since an explicit `count` is provided, this method supports types with
    /// zero-sized trailing slice elements. Methods such as [`try_mut_from_prefix`]
    /// which do not take an explicit count do not support such types.
    ///
    /// ```
    /// use core::num::NonZeroU16;
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(TryFromBytes, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct ZSTy {
    ///     leading_sized: NonZeroU16,
    ///     trailing_dst: [()],
    /// }
    ///
    /// let mut src = 0xCAFEu16;
    /// let src = src.as_mut_bytes();
    /// let (_, zsty) = ZSTy::try_mut_from_suffix_with_elems(src, 42).unwrap();
    /// assert_eq!(zsty.trailing_dst.len(), 42);
    /// ```
    ///
    /// [`try_mut_from_prefix`]: TryFromBytes::try_mut_from_prefix
    ///
    #[doc = codegen_header!("h5", "try_mut_from_suffix_with_elems")]
    ///
    /// See [`TryFromBytes::try_ref_from_suffix_with_elems`](#method.try_ref_from_suffix_with_elems.codegen).
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_mut_from_suffix_with_elems(
        source: &mut [u8],
        count: usize,
    ) -> Result<(&mut [u8], &mut Self), TryCastError<&mut [u8], Self>>
    where
        Self: KnownLayout<PointerMetadata = usize> + IntoBytes,
    {
        try_mut_from_prefix_suffix(source, CastType::Suffix, Some(count)).map(swap)
    }

    /// Attempts to read the given `source` as a `Self`.
    ///
    /// If `source.len() != size_of::<Self>()` or the bytes are not a valid
    /// instance of `Self`, this returns `Err`.
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes)]
    /// #[repr(C)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    /// }
    ///
    /// let bytes = &[0xC0, 0xC0, 240, 77][..];
    ///
    /// let packet = Packet::try_read_from_bytes(bytes).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &mut [0x10, 0xC0, 240, 77][..];
    /// assert!(Packet::try_read_from_bytes(bytes).is_err());
    /// ```
    ///
    /// # Performance Considerations
    ///
    /// In this version of zerocopy, this method reads the `source` into a
    /// well-aligned stack allocation and *then* validates that the allocation
    /// is a valid `Self`. This ensures that validation can be performed using
    /// aligned reads (which carry a performance advantage over unaligned reads
    /// on many platforms) at the cost of an unconditional copy.
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "try_read_from_bytes",
        format = "coco_static_size",
    )]
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_read_from_bytes(source: &[u8]) -> Result<Self, TryReadError<&[u8], Self>>
    where
        Self: Sized,
    {
        try_read_from(source)
    }

    /// Attempts to read a `Self` from the prefix of the given `source`.
    ///
    /// This attempts to read a `Self` from the first `size_of::<Self>()` bytes
    /// of `source`, returning that `Self` and any remaining bytes. If
    /// `source.len() < size_of::<Self>()` or the bytes are not a valid instance
    /// of `Self`, it returns `Err`.
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes)]
    /// #[repr(C)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    /// }
    ///
    /// // These are more bytes than are needed to encode a `Packet`.
    /// let bytes = &[0xC0, 0xC0, 240, 77, 0, 1, 2, 3, 4, 5, 6][..];
    ///
    /// let (packet, suffix) = Packet::try_read_from_prefix(bytes).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(suffix, &[0u8, 1, 2, 3, 4, 5, 6][..]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &[0x10, 0xC0, 240, 77, 0, 1, 2, 3, 4, 5, 6][..];
    /// assert!(Packet::try_read_from_prefix(bytes).is_err());
    /// ```
    ///
    /// # Performance Considerations
    ///
    /// In this version of zerocopy, this method reads the `source` into a
    /// well-aligned stack allocation and *then* validates that the allocation
    /// is a valid `Self`. This ensures that validation can be performed using
    /// aligned reads (which carry a performance advantage over unaligned reads
    /// on many platforms) at the cost of an unconditional copy.
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "try_read_from_prefix",
        format = "coco_static_size",
    )]
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_read_from_prefix(source: &[u8]) -> Result<(Self, &[u8]), TryReadError<&[u8], Self>>
    where
        Self: Sized,
    {
        let (prefix, suffix) = match SplitAt::split_at(source, mem::size_of::<Self>()) {
            Some(split) => split.via_immutable(),
            None => return Err(SizeError::new(source).into()),
        };
        match try_read_from(prefix) {
            Ok(slf) => Ok((slf, suffix)),
            Err(e) => Err(e.map_src(
                #[inline(always)]
                |_| source,
            )),
        }
    }

    /// Attempts to read a `Self` from the suffix of the given `source`.
    ///
    /// This attempts to read a `Self` from the last `size_of::<Self>()` bytes
    /// of `source`, returning that `Self` and any preceding bytes. If
    /// `source.len() < size_of::<Self>()` or the bytes are not a valid instance
    /// of `Self`, it returns `Err`.
    ///
    /// # Examples
    ///
    /// ```
    /// # #![allow(non_camel_case_types)] // For C0::xC0
    /// use zerocopy::TryFromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// // The only valid value of this type is the byte `0xC0`
    /// #[derive(TryFromBytes)]
    /// #[repr(u8)]
    /// enum C0 { xC0 = 0xC0 }
    ///
    /// // The only valid value of this type is the bytes `0xC0C0`.
    /// #[derive(TryFromBytes)]
    /// #[repr(C)]
    /// struct C0C0(C0, C0);
    ///
    /// #[derive(TryFromBytes)]
    /// #[repr(C)]
    /// struct Packet {
    ///     magic_number: C0C0,
    ///     mug_size: u8,
    ///     temperature: u8,
    /// }
    ///
    /// // These are more bytes than are needed to encode a `Packet`.
    /// let bytes = &[0, 1, 2, 3, 4, 5, 0xC0, 0xC0, 240, 77][..];
    ///
    /// let (prefix, packet) = Packet::try_read_from_suffix(bytes).unwrap();
    ///
    /// assert_eq!(packet.mug_size, 240);
    /// assert_eq!(packet.temperature, 77);
    /// assert_eq!(prefix, &[0u8, 1, 2, 3, 4, 5][..]);
    ///
    /// // These bytes are not valid instance of `Packet`.
    /// let bytes = &[0, 1, 2, 3, 4, 5, 0x10, 0xC0, 240, 77][..];
    /// assert!(Packet::try_read_from_suffix(bytes).is_err());
    /// ```
    ///
    /// # Performance Considerations
    ///
    /// In this version of zerocopy, this method reads the `source` into a
    /// well-aligned stack allocation and *then* validates that the allocation
    /// is a valid `Self`. This ensures that validation can be performed using
    /// aligned reads (which carry a performance advantage over unaligned reads
    /// on many platforms) at the cost of an unconditional copy.
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "try_read_from_suffix",
        format = "coco_static_size",
    )]
    #[must_use = "the conversion result must be checked"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn try_read_from_suffix(source: &[u8]) -> Result<(&[u8], Self), TryReadError<&[u8], Self>>
    where
        Self: Sized,
    {
        let split_at = match source.len().checked_sub(mem::size_of::<Self>()) {
            Some(split_at) => split_at,
            None => return Err(SizeError::new(source).into()),
        };
        // SAFETY: `checked_sub` returned the difference without overflow [1],
        // so `split_at = source.len() - size_of::<Self>() <= source.len()`.
        // This satisfies `SplitAt::split_at_unchecked`'s precondition.
        //
        // [1] Per https://doc.rust-lang.org/1.56.0/std/primitive.usize.html#method.checked_sub:
        //
        //   Checked integer subtraction. Computes `self - rhs`, returning
        //   `None` if overflow occurred.
        let (prefix, suffix) =
            unsafe { SplitAt::split_at_unchecked(source, split_at) }.via_immutable();
        match try_read_from(suffix) {
            Ok(slf) => Ok((prefix, slf)),
            Err(e) => Err(e.map_src(
                #[inline(always)]
                |_| source,
            )),
        }
    }
}

/// Validates and interprets the given affix of `source`'s bytes as a `&T`.
///
/// Returns the destination reference and excess bytes on success. All errors,
/// including validity errors, contain the original `&S`.
#[inline(always)]
fn try_ref_from_prefix_suffix<S, T>(
    source: &S,
    cast_type: CastType,
    meta: Option<T::PointerMetadata>,
) -> Result<(&T, &[u8]), TryCastError<&S, T>>
where
    S: IntoBytes + Immutable + ?Sized,
    T: TryFromBytes + KnownLayout + Immutable + ?Sized,
{
    match Ptr::from_ref(source.as_bytes()).try_cast_into::<T, BecauseImmutable>(cast_type, meta) {
        Ok((candidate, prefix_suffix)) => {
            // This call may panic. If that happens, it doesn't cause any soundness
            // issues, as we have not generated any invalid state which we need to
            // fix before returning.
            match candidate.try_into_safe() {
                Ok(valid) => Ok((valid.as_ref(), prefix_suffix.as_ref())),
                Err(e) => Err(e
                    .map_src(
                        #[inline(always)]
                        |_| source,
                    )
                    .into()),
            }
        }
        Err(e) => Err(e
            .map_src(
                #[inline(always)]
                |_| source,
            )
            .into()),
    }
}

#[inline(always)]
fn try_mut_from_prefix_suffix<T: IntoBytes + TryFromBytes + KnownLayout + ?Sized>(
    candidate: &mut [u8],
    cast_type: CastType,
    meta: Option<T::PointerMetadata>,
) -> Result<(&mut T, &mut [u8]), TryCastError<&mut [u8], T>> {
    match Ptr::from_mut(candidate).try_cast_into::<T, BecauseExclusive>(cast_type, meta) {
        Ok((candidate, prefix_suffix)) => {
            // This call may panic. If that happens, it doesn't cause any soundness
            // issues, as we have not generated any invalid state which we need to
            // fix before returning.
            match candidate.try_into_safe() {
                Ok(valid) => Ok((valid.as_mut(), prefix_suffix.as_mut())),
                Err(e) => Err(e.map_src(|src| src.as_bytes().as_mut()).into()),
            }
        }
        Err(e) => Err(e.map_src(Ptr::as_mut).into()),
    }
}

#[inline(always)]
fn swap<T, U>((t, u): (T, U)) -> (U, T) {
    (u, t)
}

#[inline(always)]
fn try_read_from<S, T>(source: &S) -> Result<T, TryReadError<&S, T>>
where
    S: IntoBytes + Immutable + ?Sized,
    T: TryFromBytes,
{
    let bytes = source.as_bytes();

    if bytes.len() != mem::size_of::<T>() {
        return Err(SizeError::new(source).into());
    }

    // FIXME(#2981): Avoid the validation copy when validation can safely use
    // a pointer derived from `source`'s shared borrow.

    // Initialize the candidate in its final location. A typed move of a
    // `MaybeUninit<T>` may discard initialized bytes at `T`'s padding offsets
    // [1], but validation requires every byte to be initialized. Do not move
    // `candidate` between this copy and validation.
    //
    // [1] Per https://doc.rust-lang.org/1.93.1/std/mem/union.MaybeUninit.html#validity:
    //
    //   Moving or copying a value of type `MaybeUninit<T>` (i.e., performing a
    //   "typed copy") will exactly preserve the contents, including the
    //   provenance, of all non-padding bytes of type `T` in the value's
    //   representation.
    let mut candidate = CoreMaybeUninit::<T>::uninit();

    // SAFETY: The earlier `if bytes.len() != mem::size_of::<T>()` returns on a
    // size mismatch, so reaching this copy implies that `bytes.len()` equals
    // `mem::size_of::<T>()`. Copying that many `u8`s cannot overrun `bytes`.
    // `candidate.as_mut_ptr()` is writable for the same number of bytes because
    // `MaybeUninit<T>` has `T`'s size. Both pointers are non-null and aligned
    // for `u8`, including when `T` is zero-sized. The fresh local allocation
    // cannot overlap `source`, which is an argument. Thus the read, write,
    // alignment, and non-overlap requirements of [2] hold. Copying `u8`s
    // initializes every destination byte, including those at `T`'s padding
    // offsets.
    //
    // These writes cannot violate `candidate`'s bit validity because every bit
    // pattern is valid for `MaybeUninit<T>` [3], even if it is invalid for `T`.
    //
    // [2] Per https://doc.rust-lang.org/1.56.0/std/ptr/fn.copy_nonoverlapping.html:
    //
    //   Copies `count * size_of::<T>()` bytes from `src` to `dst`. The source
    //   and destination must *not* overlap.
    //
    // [3] Per https://doc.rust-lang.org/1.56.0/std/mem/union.MaybeUninit.html#layout:
    //
    //   ... any bit value is valid for a `MaybeUninit<T>` ...
    unsafe {
        ptr::copy_nonoverlapping(
            bytes.as_ptr(),
            candidate.as_mut_ptr().cast::<u8>(),
            mem::size_of::<T>(),
        );
    }

    // We use `from_mut` despite not mutating via `c_ptr` so that we don't need
    // to add a `T: Immutable` bound.
    let c_ptr = Ptr::from_mut(&mut candidate);

    // SAFETY: `c_ptr` has no uninitialized sub-ranges because it derived from
    // `candidate`, whose bytes were all initialized by the copy above and which
    // has not been moved since. Since `candidate` is a `MaybeUninit`, it has no
    // validity requirements, and so no values written to an `Initialized`
    // `c_ptr` can violate its validity. Since `c_ptr` has `Exclusive` aliasing,
    // no mutations may happen except via `c_ptr` so long as it is live, so we
    // don't need to worry about the fact that `c_ptr` may have more restricted
    // validity than `candidate`.
    let c_ptr = unsafe { c_ptr.assume_validity::<invariant::Initialized>() };

    let c_ptr = c_ptr.cast::<_, crate::pointer::cast::CastSized, _>();

    // SAFETY: `c_ptr` originated from a reference to `candidate`, so its
    // address is aligned for `MaybeUninit<T>`, which has `T`'s alignment [1].
    // `CastSized` preserves that address. `ReadOnly<T>` is `repr(transparent)`
    // with a single `T` field, so it also has `T`'s alignment [2], including
    // when `T` is zero-sized.
    //
    // [1] Per https://doc.rust-lang.org/1.56.0/std/mem/union.MaybeUninit.html#layout:
    //
    //   `MaybeUninit<T>` is guaranteed to have the same size, alignment, and
    //   ABI as `T`:
    //
    // [2] Per https://doc.rust-lang.org/1.93.1/reference/type-layout.html#the-transparent-representation:
    //
    //   ... same layout and ABI as the only non-size 0 non-alignment 1 field,
    //   if present, or unit otherwise.
    let mut c_ptr = unsafe { c_ptr.assume_alignment::<invariant::Aligned>() };

    // This call may panic. If that happens, it doesn't cause any soundness
    // issues, as we have not generated any invalid state which we need to fix
    // before returning.
    if !T::is_safe(c_ptr.reborrow_shared()) {
        return Err(ValidityError::new(source).into());
    }

    // SAFETY: `T::is_safe` returned true for `candidate`'s initialized bytes,
    // so it contains a valid `T`, as required by [1]. Validation used a shared
    // `ReadOnly<T>` pointer, and the bytes have not been modified since.
    //
    // [1] Per https://doc.rust-lang.org/1.56.0/std/mem/union.MaybeUninit.html#method.assume_init:
    //
    //   It is up to the caller to guarantee that the `MaybeUninit<T>` really is
    //   in an initialized state.
    Ok(unsafe { candidate.assume_init() })
}

/// Types for which a sequence of `0` bytes is a valid instance.
///
/// Any memory region of the appropriate length which is guaranteed to contain
/// only zero bytes can be viewed as any `FromZeros` type with no runtime
/// overhead. This is useful whenever memory is known to be in a zeroed state,
/// such memory returned from some allocation routines.
///
/// # Warning: Padding bytes
///
/// Note that, when a value is moved or copied, only the non-padding bytes of
/// that value are guaranteed to be preserved. It is unsound to assume that
/// values written to padding bytes are preserved after a move or copy. For more
/// details, see the [`FromBytes` docs][frombytes-warning-padding-bytes].
///
/// [frombytes-warning-padding-bytes]: FromBytes#warning-padding-bytes
///
/// # Implementation
///
/// **Do not implement this trait yourself!** Instead, use
/// [`#[derive(FromZeros)]`][derive]; e.g.:
///
/// ```
/// # use zerocopy_derive::{FromZeros, Immutable};
/// #[derive(FromZeros)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(FromZeros)]
/// #[repr(u8)]
/// enum MyEnum {
/// #   Variant0,
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(FromZeros, Immutable)]
/// union MyUnion {
/// #   variant: u8,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// This derive performs a sophisticated, compile-time safety analysis to
/// determine whether a type is `FromZeros`.
///
/// # Safety
///
/// *This section describes what is required in order for `T: FromZeros`, and
/// what unsafe code may assume of such types. If you don't plan on implementing
/// `FromZeros` manually, and you don't plan on writing unsafe code that
/// operates on `FromZeros` types, then you don't need to read this section.*
///
/// If `T: FromZeros`, then unsafe code may assume that it is sound to produce a
/// `T` whose bytes are all initialized to zero. If a type is marked as
/// `FromZeros` which violates this contract, it may cause undefined behavior.
///
/// `#[derive(FromZeros)]` only permits [types which satisfy these
/// requirements][derive-analysis].
///
#[cfg_attr(
    feature = "derive",
    doc = "[derive]: zerocopy_derive::FromZeros",
    doc = "[derive-analysis]: zerocopy_derive::FromZeros#analysis"
)]
#[cfg_attr(
    not(feature = "derive"),
    doc = concat!("[derive]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.FromZeros.html"),
    doc = concat!("[derive-analysis]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.FromZeros.html#analysis"),
)]
#[cfg_attr(
    not(no_zerocopy_diagnostic_on_unimplemented_1_78_0),
    diagnostic::on_unimplemented(note = "Consider adding `#[derive(FromZeros)]` to `{Self}`")
)]
pub unsafe trait FromZeros: TryFromBytes {
    // The `Self: Sized` bound makes it so that `FromZeros` is still object
    // safe.
    #[doc(hidden)]
    fn only_derive_is_allowed_to_implement_this_trait()
    where
        Self: Sized;

    /// Overwrites `self` with zeros.
    ///
    /// Sets every byte in `self` to 0. While this is similar to doing `*self =
    /// Self::new_zeroed()`, it differs in that `zero` does not semantically
    /// drop the current value and replace it with a new one — it simply
    /// modifies the bytes of the existing value.
    ///
    /// # Examples
    ///
    /// ```
    /// # use zerocopy::FromZeros;
    /// # use zerocopy_derive::*;
    /// #
    /// #[derive(FromZeros)]
    /// #[repr(C)]
    /// struct PacketHeader {
    ///     src_port: [u8; 2],
    ///     dst_port: [u8; 2],
    ///     length: [u8; 2],
    ///     checksum: [u8; 2],
    /// }
    ///
    /// let mut header = PacketHeader {
    ///     src_port: 100u16.to_be_bytes(),
    ///     dst_port: 200u16.to_be_bytes(),
    ///     length: 300u16.to_be_bytes(),
    ///     checksum: 400u16.to_be_bytes(),
    /// };
    ///
    /// header.zero();
    ///
    /// assert_eq!(header.src_port, [0, 0]);
    /// assert_eq!(header.dst_port, [0, 0]);
    /// assert_eq!(header.length, [0, 0]);
    /// assert_eq!(header.checksum, [0, 0]);
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "zero",
        format = "coco",
        arity = 3,
        [
            open
            @index 1
            @title "Sized"
            @variant "static_size"
        ],
        [
            @index 2
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 3
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[inline(always)]
    fn zero(&mut self) {
        let slf: *mut Self = self;
        let len = mem::size_of_val(self);
        // SAFETY:
        // - `self` is guaranteed by the type system to be valid for writes of
        //   size `size_of_val(self)`.
        // - `u8`'s alignment is 1, and thus `self` is guaranteed to be aligned
        //   as required by `u8`.
        // - Since `Self: FromZeros`, the all-zeros instance is a valid instance
        //   of `Self.`
        //
        // FIXME(#429): Add references to docs and quotes.
        unsafe { ptr::write_bytes(slf.cast::<u8>(), 0, len) };
    }

    /// Creates an instance of `Self` from zeroed bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// # use zerocopy::FromZeros;
    /// # use zerocopy_derive::*;
    /// #
    /// #[derive(FromZeros)]
    /// #[repr(C)]
    /// struct PacketHeader {
    ///     src_port: [u8; 2],
    ///     dst_port: [u8; 2],
    ///     length: [u8; 2],
    ///     checksum: [u8; 2],
    /// }
    ///
    /// let header: PacketHeader = FromZeros::new_zeroed();
    ///
    /// assert_eq!(header.src_port, [0, 0]);
    /// assert_eq!(header.dst_port, [0, 0]);
    /// assert_eq!(header.length, [0, 0]);
    /// assert_eq!(header.checksum, [0, 0]);
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "new_zeroed",
        format = "coco_static_size",
    )]
    #[must_use = "has no side effects"]
    #[inline(always)]
    fn new_zeroed() -> Self
    where
        Self: Sized,
    {
        // SAFETY: `FromZeros` says that the all-zeros bit pattern is legal.
        unsafe { mem::zeroed() }
    }

    /// Creates a `Box<Self>` from zeroed bytes.
    ///
    /// This function is useful for allocating large values on the heap and
    /// zero-initializing them, without ever creating a temporary instance of
    /// `Self` on the stack. For example, `<[u8; 1048576]>::new_box_zeroed()`
    /// will allocate `[u8; 1048576]` directly on the heap; it does not require
    /// storing `[u8; 1048576]` in a temporary variable on the stack.
    ///
    /// On systems that use a heap implementation that supports allocating from
    /// pre-zeroed memory, using `new_box_zeroed` (or related functions) may
    /// have performance benefits.
    ///
    /// # Errors
    ///
    /// Returns an error on allocation failure. Allocation failure is guaranteed
    /// never to cause a panic or an abort.
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "new_box_zeroed",
        format = "coco_static_size",
    )]
    #[must_use = "has no side effects (other than allocation)"]
    #[cfg(any(feature = "alloc", test))]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
    #[inline]
    fn new_box_zeroed() -> Result<Box<Self>, AllocError>
    where
        Self: Sized,
    {
        // If `T` is a ZST, then return a proper boxed instance of it. There is
        // no allocation, but `Box` does require a correct dangling pointer.
        let layout = Layout::new::<Self>();
        if layout.size() == 0 {
            // Construct the `Box` from a dangling pointer to avoid calling
            // `Self::new_zeroed`. This ensures that stack space is never
            // allocated for `Self` even on lower opt-levels where this branch
            // might not get optimized out.

            // SAFETY: Per [1], when `T` is a ZST, `Box<T>`'s only validity
            // requirements are that the pointer is non-null and sufficiently
            // aligned. Per [2], `NonNull::dangling` produces a pointer which
            // is sufficiently aligned. Since the produced pointer is a
            // `NonNull`, it is non-null.
            //
            // [1] Per https://doc.rust-lang.org/1.81.0/std/boxed/index.html#memory-layout:
            //
            //   For zero-sized values, the `Box` pointer has to be non-null and sufficiently aligned.
            //
            // [2] Per https://doc.rust-lang.org/std/ptr/struct.NonNull.html#method.dangling:
            //
            //   Creates a new `NonNull` that is dangling, but well-aligned.
            return Ok(unsafe { Box::from_raw(NonNull::dangling().as_ptr()) });
        }

        // FIXME(#429): Add a "SAFETY" comment and remove this `allow`.
        #[allow(clippy::undocumented_unsafe_blocks)]
        let ptr = unsafe { alloc::alloc::alloc_zeroed(layout).cast::<Self>() };
        if ptr.is_null() {
            return Err(AllocError);
        }
        // FIXME(#429): Add a "SAFETY" comment and remove this `allow`.
        #[allow(clippy::undocumented_unsafe_blocks)]
        Ok(unsafe { Box::from_raw(ptr) })
    }

    /// Creates a `Box<[Self]>` (a boxed slice) from zeroed bytes.
    ///
    /// This function is useful for allocating large values of `[Self]` on the
    /// heap and zero-initializing them, without ever creating a temporary
    /// instance of `[Self; _]` on the stack. For example,
    /// `u8::new_box_slice_zeroed(1048576)` will allocate the slice directly on
    /// the heap; it does not require storing the slice on the stack.
    ///
    /// On systems that use a heap implementation that supports allocating from
    /// pre-zeroed memory, using `new_box_slice_zeroed` may have performance
    /// benefits.
    ///
    /// If `Self` is a zero-sized type, then this function will return a
    /// `Box<[Self]>` that has the correct `len`. Such a box cannot contain any
    /// actual information, but its `len()` property will report the correct
    /// value.
    ///
    /// # Errors
    ///
    /// Returns an error on allocation failure. Allocation failure is
    /// guaranteed never to cause a panic or an abort.
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "new_box_zeroed_with_elems",
        format = "coco",
        arity = 2,
        [
            open
            @index 1
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 2
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[must_use = "has no side effects (other than allocation)"]
    #[cfg(feature = "alloc")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
    #[inline]
    fn new_box_zeroed_with_elems(count: usize) -> Result<Box<Self>, AllocError>
    where
        Self: KnownLayout<PointerMetadata = usize>,
    {
        // SAFETY: `alloc::alloc::alloc_zeroed` is a valid argument of
        // `new_box`. The referent of the pointer returned by `alloc_zeroed`
        // (and, consequently, the `Box` derived from it) is a valid instance of
        // `Self`, because `Self` is `FromZeros`.
        unsafe { crate::util::new_box(count, alloc::alloc::alloc_zeroed) }
    }

    #[deprecated(since = "0.8.0", note = "renamed to `FromZeros::new_box_zeroed_with_elems`")]
    #[doc(hidden)]
    #[cfg(feature = "alloc")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
    #[must_use = "has no side effects (other than allocation)"]
    #[inline(always)]
    fn new_box_slice_zeroed(len: usize) -> Result<Box<[Self]>, AllocError>
    where
        Self: Sized,
    {
        <[Self]>::new_box_zeroed_with_elems(len)
    }

    /// Creates a `Vec<Self>` from zeroed bytes.
    ///
    /// This function is useful for allocating large values of `Vec`s and
    /// zero-initializing them, without ever creating a temporary instance of
    /// `[Self; _]` (or many temporary instances of `Self`) on the stack. For
    /// example, `u8::new_vec_zeroed(1048576)` will allocate directly on the
    /// heap; it does not require storing intermediate values on the stack.
    ///
    /// On systems that use a heap implementation that supports allocating from
    /// pre-zeroed memory, using `new_vec_zeroed` may have performance benefits.
    ///
    /// If `Self` is a zero-sized type, then this function will return a
    /// `Vec<Self>` that has the correct `len`. Such a `Vec` cannot contain any
    /// actual information, but its `len()` property will report the correct
    /// value.
    ///
    /// # Errors
    ///
    /// Returns an error on allocation failure. Allocation failure is
    /// guaranteed never to cause a panic or an abort.
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "new_vec_zeroed",
        format = "coco_static_size",
    )]
    #[must_use = "has no side effects (other than allocation)"]
    #[cfg(feature = "alloc")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
    #[inline(always)]
    fn new_vec_zeroed(len: usize) -> Result<Vec<Self>, AllocError>
    where
        Self: Sized,
    {
        <[Self]>::new_box_zeroed_with_elems(len).map(Into::into)
    }

    /// Extends a `Vec<Self>` by pushing `additional` new items onto the end of
    /// the vector. The new items are initialized with zeros.
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "extend_vec_zeroed",
        format = "coco_static_size",
    )]
    #[cfg(not(no_zerocopy_panic_in_const_and_vec_try_reserve_1_57_0))]
    #[cfg(feature = "alloc")]
    #[cfg_attr(doc_cfg, doc(cfg(all(rust = "1.57.0", feature = "alloc"))))]
    #[inline(always)]
    fn extend_vec_zeroed(v: &mut Vec<Self>, additional: usize) -> Result<(), AllocError>
    where
        Self: Sized,
    {
        // PANICS: We pass `v.len()` for `position`, so the `position > v.len()`
        // panic condition is not satisfied.
        <Self as FromZeros>::insert_vec_zeroed(v, v.len(), additional)
    }

    /// Inserts `additional` new items into `Vec<Self>` at `position`. The new
    /// items are initialized with zeros.
    ///
    /// # Panics
    ///
    /// Panics if `position > v.len()`.
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "insert_vec_zeroed",
        format = "coco_static_size",
    )]
    #[cfg(not(no_zerocopy_panic_in_const_and_vec_try_reserve_1_57_0))]
    #[cfg(feature = "alloc")]
    #[cfg_attr(doc_cfg, doc(cfg(all(rust = "1.57.0", feature = "alloc"))))]
    #[inline]
    fn insert_vec_zeroed(
        v: &mut Vec<Self>,
        position: usize,
        additional: usize,
    ) -> Result<(), AllocError>
    where
        Self: Sized,
    {
        assert!(position <= v.len());
        // We only conditionally compile on versions on which `try_reserve` is
        // stable; the Clippy lint is a false positive.
        v.try_reserve(additional).map_err(|_| AllocError)?;
        // SAFETY: The `try_reserve` call guarantees that these cannot overflow:
        // * `ptr.add(position)`
        // * `position + additional`
        // * `v.len() + additional`
        //
        // `v.len() - position` cannot overflow because we asserted that
        // `position <= v.len()`.
        #[allow(clippy::multiple_unsafe_ops_per_block)]
        unsafe {
            // This is a potentially overlapping copy.
            let ptr = v.as_mut_ptr();
            #[allow(clippy::arithmetic_side_effects)]
            ptr.add(position).copy_to(ptr.add(position + additional), v.len() - position);
            ptr.add(position).write_bytes(0, additional);
            #[allow(clippy::arithmetic_side_effects)]
            v.set_len(v.len() + additional);
        }

        Ok(())
    }
}

/// Analyzes whether a type is [`FromBytes`].
///
/// This derive analyzes, at compile time, whether the annotated type satisfies
/// the [safety conditions] of `FromBytes` and implements `FromBytes` and its
/// supertraits if it is sound to do so. This derive can be applied to structs,
/// enums, and unions;
/// e.g.:
///
/// ```
/// # use zerocopy_derive::{FromBytes, FromZeros, Immutable};
/// #[derive(FromBytes)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(FromBytes)]
/// #[repr(u8)]
/// enum MyEnum {
/// #   V00, V01, V02, V03, V04, V05, V06, V07, V08, V09, V0A, V0B, V0C, V0D, V0E,
/// #   V0F, V10, V11, V12, V13, V14, V15, V16, V17, V18, V19, V1A, V1B, V1C, V1D,
/// #   V1E, V1F, V20, V21, V22, V23, V24, V25, V26, V27, V28, V29, V2A, V2B, V2C,
/// #   V2D, V2E, V2F, V30, V31, V32, V33, V34, V35, V36, V37, V38, V39, V3A, V3B,
/// #   V3C, V3D, V3E, V3F, V40, V41, V42, V43, V44, V45, V46, V47, V48, V49, V4A,
/// #   V4B, V4C, V4D, V4E, V4F, V50, V51, V52, V53, V54, V55, V56, V57, V58, V59,
/// #   V5A, V5B, V5C, V5D, V5E, V5F, V60, V61, V62, V63, V64, V65, V66, V67, V68,
/// #   V69, V6A, V6B, V6C, V6D, V6E, V6F, V70, V71, V72, V73, V74, V75, V76, V77,
/// #   V78, V79, V7A, V7B, V7C, V7D, V7E, V7F, V80, V81, V82, V83, V84, V85, V86,
/// #   V87, V88, V89, V8A, V8B, V8C, V8D, V8E, V8F, V90, V91, V92, V93, V94, V95,
/// #   V96, V97, V98, V99, V9A, V9B, V9C, V9D, V9E, V9F, VA0, VA1, VA2, VA3, VA4,
/// #   VA5, VA6, VA7, VA8, VA9, VAA, VAB, VAC, VAD, VAE, VAF, VB0, VB1, VB2, VB3,
/// #   VB4, VB5, VB6, VB7, VB8, VB9, VBA, VBB, VBC, VBD, VBE, VBF, VC0, VC1, VC2,
/// #   VC3, VC4, VC5, VC6, VC7, VC8, VC9, VCA, VCB, VCC, VCD, VCE, VCF, VD0, VD1,
/// #   VD2, VD3, VD4, VD5, VD6, VD7, VD8, VD9, VDA, VDB, VDC, VDD, VDE, VDF, VE0,
/// #   VE1, VE2, VE3, VE4, VE5, VE6, VE7, VE8, VE9, VEA, VEB, VEC, VED, VEE, VEF,
/// #   VF0, VF1, VF2, VF3, VF4, VF5, VF6, VF7, VF8, VF9, VFA, VFB, VFC, VFD, VFE,
/// #   VFF,
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(FromBytes, Immutable)]
/// union MyUnion {
/// #   variant: u8,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// [safety conditions]: trait@FromBytes#safety
///
/// # Analysis
///
/// *This section describes, roughly, the analysis performed by this derive to
/// determine whether it is sound to implement `FromBytes` for a given type.
/// Unless you are modifying the implementation of this derive, or attempting to
/// manually implement `FromBytes` for a type yourself, you don't need to read
/// this section.*
///
/// If a type has the following properties, then this derive can implement
/// `FromBytes` for that type:
///
/// - If the type is a struct, all of its fields must be `FromBytes`.
/// - If the type is an enum:
///   - It must have a defined representation which is one of `u8`, `u16`, `i8`,
///     or `i16`.
///   - The maximum number of discriminants must be used (so that every possible
///     bit pattern is a valid one).
///   - Its fields must be `FromBytes`.
///
/// This analysis is subject to change. Unsafe code may *only* rely on the
/// documented [safety conditions] of `FromBytes`, and must *not* rely on the
/// implementation details of this derive.
///
/// ## Why isn't an explicit representation required for structs?
///
/// Neither this derive, nor the [safety conditions] of `FromBytes`, requires
/// that structs are marked with `#[repr(C)]`.
///
/// Per the [Rust reference](reference),
///
/// > The representation of a type can change the padding between fields, but
/// > does not change the layout of the fields themselves.
///
/// [reference]: https://doc.rust-lang.org/reference/type-layout.html#representations
///
/// Since the layout of structs only consists of padding bytes and field bytes,
/// a struct is soundly `FromBytes` if:
/// 1. its padding is soundly `FromBytes`, and
/// 2. its fields are soundly `FromBytes`.
///
/// The answer to the first question is always yes: padding bytes do not have
/// any validity constraints. A [discussion] of this question in the Unsafe Code
/// Guidelines Working Group concluded that it would be virtually unimaginable
/// for future versions of rustc to add validity constraints to padding bytes.
///
/// [discussion]: https://github.com/rust-lang/unsafe-code-guidelines/issues/174
///
/// Whether a struct is soundly `FromBytes` therefore solely depends on whether
/// its fields are `FromBytes`.
#[cfg(any(feature = "derive", test))]
#[cfg_attr(doc_cfg, doc(cfg(feature = "derive")))]
pub use zerocopy_derive::FromBytes;

/// Types for which any bit pattern is valid.
///
/// Any memory region of the appropriate length which contains initialized bytes
/// can be viewed as any `FromBytes` type with no runtime overhead. This is
/// useful for efficiently parsing bytes as structured data.
///
/// # Warning: Padding bytes
///
/// Note that, when a value is moved or copied, only the non-padding bytes of
/// that value are guaranteed to be preserved. It is unsound to assume that
/// values written to padding bytes are preserved after a move or copy. For
/// example, the following is unsound:
///
/// ```rust,no_run
/// use core::mem::{size_of, transmute};
/// use zerocopy::FromZeros;
/// # use zerocopy_derive::*;
///
/// // Assume `Foo` is a type with padding bytes.
/// #[derive(FromZeros, Default)]
/// struct Foo {
/// # /*
///     ...
/// # */
/// }
///
/// let mut foo: Foo = Foo::default();
/// FromZeros::zero(&mut foo);
/// // UNSOUND: Although `FromZeros::zero` writes zeros to all bytes of `foo`,
/// // those writes are not guaranteed to be preserved in padding bytes when
/// // `foo` is moved, so this may expose padding bytes as `u8`s.
/// let foo_bytes: [u8; size_of::<Foo>()] = unsafe { transmute(foo) };
/// ```
///
/// # Implementation
///
/// **Do not implement this trait yourself!** Instead, use
/// [`#[derive(FromBytes)]`][derive]; e.g.:
///
/// ```
/// # use zerocopy_derive::{FromBytes, Immutable};
/// #[derive(FromBytes)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(FromBytes)]
/// #[repr(u8)]
/// enum MyEnum {
/// #   V00, V01, V02, V03, V04, V05, V06, V07, V08, V09, V0A, V0B, V0C, V0D, V0E,
/// #   V0F, V10, V11, V12, V13, V14, V15, V16, V17, V18, V19, V1A, V1B, V1C, V1D,
/// #   V1E, V1F, V20, V21, V22, V23, V24, V25, V26, V27, V28, V29, V2A, V2B, V2C,
/// #   V2D, V2E, V2F, V30, V31, V32, V33, V34, V35, V36, V37, V38, V39, V3A, V3B,
/// #   V3C, V3D, V3E, V3F, V40, V41, V42, V43, V44, V45, V46, V47, V48, V49, V4A,
/// #   V4B, V4C, V4D, V4E, V4F, V50, V51, V52, V53, V54, V55, V56, V57, V58, V59,
/// #   V5A, V5B, V5C, V5D, V5E, V5F, V60, V61, V62, V63, V64, V65, V66, V67, V68,
/// #   V69, V6A, V6B, V6C, V6D, V6E, V6F, V70, V71, V72, V73, V74, V75, V76, V77,
/// #   V78, V79, V7A, V7B, V7C, V7D, V7E, V7F, V80, V81, V82, V83, V84, V85, V86,
/// #   V87, V88, V89, V8A, V8B, V8C, V8D, V8E, V8F, V90, V91, V92, V93, V94, V95,
/// #   V96, V97, V98, V99, V9A, V9B, V9C, V9D, V9E, V9F, VA0, VA1, VA2, VA3, VA4,
/// #   VA5, VA6, VA7, VA8, VA9, VAA, VAB, VAC, VAD, VAE, VAF, VB0, VB1, VB2, VB3,
/// #   VB4, VB5, VB6, VB7, VB8, VB9, VBA, VBB, VBC, VBD, VBE, VBF, VC0, VC1, VC2,
/// #   VC3, VC4, VC5, VC6, VC7, VC8, VC9, VCA, VCB, VCC, VCD, VCE, VCF, VD0, VD1,
/// #   VD2, VD3, VD4, VD5, VD6, VD7, VD8, VD9, VDA, VDB, VDC, VDD, VDE, VDF, VE0,
/// #   VE1, VE2, VE3, VE4, VE5, VE6, VE7, VE8, VE9, VEA, VEB, VEC, VED, VEE, VEF,
/// #   VF0, VF1, VF2, VF3, VF4, VF5, VF6, VF7, VF8, VF9, VFA, VFB, VFC, VFD, VFE,
/// #   VFF,
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(FromBytes, Immutable)]
/// union MyUnion {
/// #   variant: u8,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// This derive performs a sophisticated, compile-time safety analysis to
/// determine whether a type is `FromBytes`.
///
/// # Safety
///
/// *This section describes what is required in order for `T: FromBytes`, and
/// what unsafe code may assume of such types. If you don't plan on implementing
/// `FromBytes` manually, and you don't plan on writing unsafe code that
/// operates on `FromBytes` types, then you don't need to read this section.*
///
/// If `T: FromBytes`, then unsafe code may assume that it is sound to produce a
/// `T` whose bytes are initialized to any sequence of valid `u8`s (in other
/// words, any byte value which is not uninitialized). If a type is marked as
/// `FromBytes` which violates this contract, it may cause undefined behavior.
///
/// `#[derive(FromBytes)]` only permits [types which satisfy these
/// requirements][derive-analysis].
///
#[cfg_attr(
    feature = "derive",
    doc = "[derive]: zerocopy_derive::FromBytes",
    doc = "[derive-analysis]: zerocopy_derive::FromBytes#analysis"
)]
#[cfg_attr(
    not(feature = "derive"),
    doc = concat!("[derive]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.FromBytes.html"),
    doc = concat!("[derive-analysis]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.FromBytes.html#analysis"),
)]
#[cfg_attr(
    not(no_zerocopy_diagnostic_on_unimplemented_1_78_0),
    diagnostic::on_unimplemented(note = "Consider adding `#[derive(FromBytes)]` to `{Self}`")
)]
pub unsafe trait FromBytes: FromZeros {
    // The `Self: Sized` bound makes it so that `FromBytes` is still object
    // safe.
    #[doc(hidden)]
    fn only_derive_is_allowed_to_implement_this_trait()
    where
        Self: Sized;

    /// Interprets the given `source` as a `&Self`.
    ///
    /// This method attempts to return a reference to `source` interpreted as a
    /// `Self`. If the length of `source` is not a [valid size of
    /// `Self`][valid-size], or if `source` is not appropriately aligned, this
    /// returns `Err`. If [`Self: Unaligned`][self-unaligned], you can
    /// [infallibly discard the alignment error][size-error-from].
    ///
    /// `Self` may be a sized type, a slice, or a [slice DST][slice-dst].
    ///
    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
    /// [self-unaligned]: Unaligned
    /// [size-error-from]: error/struct.SizeError.html#method.from-1
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Compile-Time Assertions
    ///
    /// This method cannot yet be used on unsized types whose dynamically-sized
    /// component is zero-sized. Attempting to use this method on such types
    /// results in a compile-time assertion error; e.g.:
    ///
    /// ```compile_fail,E0080
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, Immutable, KnownLayout)]
    /// #[repr(C)]
    /// struct ZSTy {
    ///     leading_sized: u16,
    ///     trailing_dst: [()],
    /// }
    ///
    /// let _ = ZSTy::ref_from_bytes(0u16.as_bytes()); // ⚠ Compile Error!
    /// ```
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct PacketHeader {
    ///     src_port: [u8; 2],
    ///     dst_port: [u8; 2],
    ///     length: [u8; 2],
    ///     checksum: [u8; 2],
    /// }
    ///
    /// #[derive(FromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct Packet {
    ///     header: PacketHeader,
    ///     body: [u8],
    /// }
    ///
    /// // These bytes encode a `Packet`.
    /// let bytes = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11][..];
    ///
    /// let packet = Packet::ref_from_bytes(bytes).unwrap();
    ///
    /// assert_eq!(packet.header.src_port, [0, 1]);
    /// assert_eq!(packet.header.dst_port, [2, 3]);
    /// assert_eq!(packet.header.length, [4, 5]);
    /// assert_eq!(packet.header.checksum, [6, 7]);
    /// assert_eq!(packet.body, [8, 9, 10, 11]);
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "ref_from_bytes",
        format = "coco",
        arity = 3,
        [
            open
            @index 1
            @title "Sized"
            @variant "static_size"
        ],
        [
            @index 2
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 3
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn ref_from_bytes(source: &[u8]) -> Result<&Self, CastError<&[u8], Self>>
    where
        Self: KnownLayout + Immutable,
    {
        static_assert_dst_is_not_zst!(Self);
        match Ptr::from_ref(source).try_cast_into_no_leftover::<_, BecauseImmutable>(None) {
            Ok(ptr) => Ok(ptr.recall_validity().as_ref()),
            Err(err) => Err(err.map_src(|src| src.as_ref())),
        }
    }

    /// Interprets the prefix of the given `source` as a `&Self` without
    /// copying.
    ///
    /// This method computes the [largest possible size of `Self`][valid-size]
    /// that can fit in the leading bytes of `source`, then attempts to return
    /// both a reference to those bytes interpreted as a `Self`, and a reference
    /// to the remaining bytes. If there are insufficient bytes, or if `source`
    /// is not appropriately aligned, this returns `Err`. If [`Self:
    /// Unaligned`][self-unaligned], you can [infallibly discard the alignment
    /// error][size-error-from].
    ///
    /// `Self` may be a sized type, a slice, or a [slice DST][slice-dst].
    ///
    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
    /// [self-unaligned]: Unaligned
    /// [size-error-from]: error/struct.SizeError.html#method.from-1
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Compile-Time Assertions
    ///
    /// This method cannot yet be used on unsized types whose dynamically-sized
    /// component is zero-sized. See [`ref_from_prefix_with_elems`], which does
    /// support such types. Attempting to use this method on such types results
    /// in a compile-time assertion error; e.g.:
    ///
    /// ```compile_fail,E0080
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, Immutable, KnownLayout)]
    /// #[repr(C)]
    /// struct ZSTy {
    ///     leading_sized: u16,
    ///     trailing_dst: [()],
    /// }
    ///
    /// let _ = ZSTy::ref_from_prefix(0u16.as_bytes()); // ⚠ Compile Error!
    /// ```
    ///
    /// [`ref_from_prefix_with_elems`]: FromBytes::ref_from_prefix_with_elems
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct PacketHeader {
    ///     src_port: [u8; 2],
    ///     dst_port: [u8; 2],
    ///     length: [u8; 2],
    ///     checksum: [u8; 2],
    /// }
    ///
    /// #[derive(FromBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct Packet {
    ///     header: PacketHeader,
    ///     body: [[u8; 2]],
    /// }
    ///
    /// // These are more bytes than are needed to encode a `Packet`.
    /// let bytes = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14][..];
    ///
    /// let (packet, suffix) = Packet::ref_from_prefix(bytes).unwrap();
    ///
    /// assert_eq!(packet.header.src_port, [0, 1]);
    /// assert_eq!(packet.header.dst_port, [2, 3]);
    /// assert_eq!(packet.header.length, [4, 5]);
    /// assert_eq!(packet.header.checksum, [6, 7]);
    /// assert_eq!(packet.body, [[8, 9], [10, 11], [12, 13]]);
    /// assert_eq!(suffix, &[14u8][..]);
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "ref_from_prefix",
        format = "coco",
        arity = 3,
        [
            open
            @index 1
            @title "Sized"
            @variant "static_size"
        ],
        [
            @index 2
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 3
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn ref_from_prefix(source: &[u8]) -> Result<(&Self, &[u8]), CastError<&[u8], Self>>
    where
        Self: KnownLayout + Immutable,
    {
        static_assert_dst_is_not_zst!(Self);
        ref_from_prefix_suffix(source, None, CastType::Prefix)
    }

    /// Interprets the suffix of the given bytes as a `&Self`.
    ///
    /// This method computes the [largest possible size of `Self`][valid-size]
    /// that can fit in the trailing bytes of `source`, then attempts to return
    /// both a reference to those bytes interpreted as a `Self`, and a reference
    /// to the preceding bytes. If there are insufficient bytes, or if that
    /// suffix of `source` is not appropriately aligned, this returns `Err`. If
    /// [`Self: Unaligned`][self-unaligned], you can [infallibly discard the
    /// alignment error][size-error-from].
    ///
    /// `Self` may be a sized type, a slice, or a [slice DST][slice-dst].
    ///
    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
    /// [self-unaligned]: Unaligned
    /// [size-error-from]: error/struct.SizeError.html#method.from-1
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Compile-Time Assertions
    ///
    /// This method cannot yet be used on unsized types whose dynamically-sized
    /// component is zero-sized. See [`ref_from_suffix_with_elems`], which does
    /// support such types. Attempting to use this method on such types results
    /// in a compile-time assertion error; e.g.:
    ///
    /// ```compile_fail,E0080
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, Immutable, KnownLayout)]
    /// #[repr(C)]
    /// struct ZSTy {
    ///     leading_sized: u16,
    ///     trailing_dst: [()],
    /// }
    ///
    /// let _ = ZSTy::ref_from_suffix(0u16.as_bytes()); // ⚠ Compile Error!
    /// ```
    ///
    /// [`ref_from_suffix_with_elems`]: FromBytes::ref_from_suffix_with_elems
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, Immutable, KnownLayout)]
    /// #[repr(C)]
    /// struct PacketTrailer {
    ///     frame_check_sequence: [u8; 4],
    /// }
    ///
    /// // These are more bytes than are needed to encode a `PacketTrailer`.
    /// let bytes = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9][..];
    ///
    /// let (prefix, trailer) = PacketTrailer::ref_from_suffix(bytes).unwrap();
    ///
    /// assert_eq!(prefix, &[0, 1, 2, 3, 4, 5][..]);
    /// assert_eq!(trailer.frame_check_sequence, [6, 7, 8, 9]);
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "ref_from_suffix",
        format = "coco",
        arity = 3,
        [
            open
            @index 1
            @title "Sized"
            @variant "static_size"
        ],
        [
            @index 2
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 3
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn ref_from_suffix(source: &[u8]) -> Result<(&[u8], &Self), CastError<&[u8], Self>>
    where
        Self: Immutable + KnownLayout,
    {
        static_assert_dst_is_not_zst!(Self);
        ref_from_prefix_suffix(source, None, CastType::Suffix).map(swap)
    }

    /// Interprets the given `source` as a `&mut Self`.
    ///
    /// This method attempts to return a reference to `source` interpreted as a
    /// `Self`. If the length of `source` is not a [valid size of
    /// `Self`][valid-size], or if `source` is not appropriately aligned, this
    /// returns `Err`. If [`Self: Unaligned`][self-unaligned], you can
    /// [infallibly discard the alignment error][size-error-from].
    ///
    /// `Self` may be a sized type, a slice, or a [slice DST][slice-dst].
    ///
    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
    /// [self-unaligned]: Unaligned
    /// [size-error-from]: error/struct.SizeError.html#method.from-1
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Compile-Time Assertions
    ///
    /// This method cannot yet be used on unsized types whose dynamically-sized
    /// component is zero-sized. See [`mut_from_prefix_with_elems`], which does
    /// support such types. Attempting to use this method on such types results
    /// in a compile-time assertion error; e.g.:
    ///
    /// ```compile_fail,E0080
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, Immutable, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct ZSTy {
    ///     leading_sized: [u8; 2],
    ///     trailing_dst: [()],
    /// }
    ///
    /// let mut source = [85, 85];
    /// let _ = ZSTy::mut_from_bytes(&mut source[..]); // ⚠ Compile Error!
    /// ```
    ///
    /// [`mut_from_prefix_with_elems`]: FromBytes::mut_from_prefix_with_elems
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, IntoBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct PacketHeader {
    ///     src_port: [u8; 2],
    ///     dst_port: [u8; 2],
    ///     length: [u8; 2],
    ///     checksum: [u8; 2],
    /// }
    ///
    /// // These bytes encode a `PacketHeader`.
    /// let bytes = &mut [0, 1, 2, 3, 4, 5, 6, 7][..];
    ///
    /// let header = PacketHeader::mut_from_bytes(bytes).unwrap();
    ///
    /// assert_eq!(header.src_port, [0, 1]);
    /// assert_eq!(header.dst_port, [2, 3]);
    /// assert_eq!(header.length, [4, 5]);
    /// assert_eq!(header.checksum, [6, 7]);
    ///
    /// header.checksum = [0, 0];
    ///
    /// assert_eq!(bytes, [0, 1, 2, 3, 4, 5, 0, 0]);
    ///
    /// ```
    ///
    #[doc = codegen_header!("h5", "mut_from_bytes")]
    ///
    /// See [`FromBytes::ref_from_bytes`](#method.ref_from_bytes.codegen).
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn mut_from_bytes(source: &mut [u8]) -> Result<&mut Self, CastError<&mut [u8], Self>>
    where
        Self: IntoBytes + KnownLayout,
    {
        static_assert_dst_is_not_zst!(Self);
        match Ptr::from_mut(source).try_cast_into_no_leftover::<_, BecauseExclusive>(None) {
            Ok(ptr) => Ok(ptr.recall_validity::<_, (_, (_, _))>().as_mut()),
            Err(err) => Err(err.map_src(|src| src.as_mut())),
        }
    }

    /// Interprets the prefix of the given `source` as a `&mut Self` without
    /// copying.
    ///
    /// This method computes the [largest possible size of `Self`][valid-size]
    /// that can fit in the leading bytes of `source`, then attempts to return
    /// both a reference to those bytes interpreted as a `Self`, and a reference
    /// to the remaining bytes. If there are insufficient bytes, or if `source`
    /// is not appropriately aligned, this returns `Err`. If [`Self:
    /// Unaligned`][self-unaligned], you can [infallibly discard the alignment
    /// error][size-error-from].
    ///
    /// `Self` may be a sized type, a slice, or a [slice DST][slice-dst].
    ///
    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
    /// [self-unaligned]: Unaligned
    /// [size-error-from]: error/struct.SizeError.html#method.from-1
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Compile-Time Assertions
    ///
    /// This method cannot yet be used on unsized types whose dynamically-sized
    /// component is zero-sized. See [`mut_from_suffix_with_elems`], which does
    /// support such types. Attempting to use this method on such types results
    /// in a compile-time assertion error; e.g.:
    ///
    /// ```compile_fail,E0080
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, Immutable, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct ZSTy {
    ///     leading_sized: [u8; 2],
    ///     trailing_dst: [()],
    /// }
    ///
    /// let mut source = [85, 85];
    /// let _ = ZSTy::mut_from_prefix(&mut source[..]); // ⚠ Compile Error!
    /// ```
    ///
    /// [`mut_from_suffix_with_elems`]: FromBytes::mut_from_suffix_with_elems
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, IntoBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct PacketHeader {
    ///     src_port: [u8; 2],
    ///     dst_port: [u8; 2],
    ///     length: [u8; 2],
    ///     checksum: [u8; 2],
    /// }
    ///
    /// // These are more bytes than are needed to encode a `PacketHeader`.
    /// let bytes = &mut [0, 1, 2, 3, 4, 5, 6, 7, 8, 9][..];
    ///
    /// let (header, body) = PacketHeader::mut_from_prefix(bytes).unwrap();
    ///
    /// assert_eq!(header.src_port, [0, 1]);
    /// assert_eq!(header.dst_port, [2, 3]);
    /// assert_eq!(header.length, [4, 5]);
    /// assert_eq!(header.checksum, [6, 7]);
    /// assert_eq!(body, &[8, 9][..]);
    ///
    /// header.checksum = [0, 0];
    /// body.fill(1);
    ///
    /// assert_eq!(bytes, [0, 1, 2, 3, 4, 5, 0, 0, 1, 1]);
    /// ```
    ///
    #[doc = codegen_header!("h5", "mut_from_prefix")]
    ///
    /// See [`FromBytes::ref_from_prefix`](#method.ref_from_prefix.codegen).
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn mut_from_prefix(
        source: &mut [u8],
    ) -> Result<(&mut Self, &mut [u8]), CastError<&mut [u8], Self>>
    where
        Self: IntoBytes + KnownLayout,
    {
        static_assert_dst_is_not_zst!(Self);
        mut_from_prefix_suffix(source, None, CastType::Prefix)
    }

    /// Interprets the suffix of the given `source` as a `&mut Self` without
    /// copying.
    ///
    /// This method computes the [largest possible size of `Self`][valid-size]
    /// that can fit in the trailing bytes of `source`, then attempts to return
    /// both a reference to those bytes interpreted as a `Self`, and a reference
    /// to the preceding bytes. If there are insufficient bytes, or if that
    /// suffix of `source` is not appropriately aligned, this returns `Err`. If
    /// [`Self: Unaligned`][self-unaligned], you can [infallibly discard the
    /// alignment error][size-error-from].
    ///
    /// `Self` may be a sized type, a slice, or a [slice DST][slice-dst].
    ///
    /// [valid-size]: crate::KnownLayout#what-is-a-valid-size
    /// [self-unaligned]: Unaligned
    /// [size-error-from]: error/struct.SizeError.html#method.from-1
    /// [slice-dst]: KnownLayout#dynamically-sized-types
    ///
    /// # Compile-Time Assertions
    ///
    /// This method cannot yet be used on unsized types whose dynamically-sized
    /// component is zero-sized. Attempting to use this method on such types
    /// results in a compile-time assertion error; e.g.:
    ///
    /// ```compile_fail,E0080
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, Immutable, IntoBytes, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct ZSTy {
    ///     leading_sized: [u8; 2],
    ///     trailing_dst: [()],
    /// }
    ///
    /// let mut source = [85, 85];
    /// let _ = ZSTy::mut_from_suffix(&mut source[..]); // ⚠ Compile Error!
    /// ```
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, IntoBytes, KnownLayout, Immutable)]
    /// #[repr(C)]
    /// struct PacketTrailer {
    ///     frame_check_sequence: [u8; 4],
    /// }
    ///
    /// // These are more bytes than are needed to encode a `PacketTrailer`.
    /// let bytes = &mut [0, 1, 2, 3, 4, 5, 6, 7, 8, 9][..];
    ///
    /// let (prefix, trailer) = PacketTrailer::mut_from_suffix(bytes).unwrap();
    ///
    /// assert_eq!(prefix, &[0u8, 1, 2, 3, 4, 5][..]);
    /// assert_eq!(trailer.frame_check_sequence, [6, 7, 8, 9]);
    ///
    /// prefix.fill(0);
    /// trailer.frame_check_sequence.fill(1);
    ///
    /// assert_eq!(bytes, [0, 0, 0, 0, 0, 0, 1, 1, 1, 1]);
    /// ```
    ///
    #[doc = codegen_header!("h5", "mut_from_suffix")]
    ///
    /// See [`FromBytes::ref_from_suffix`](#method.ref_from_suffix.codegen).
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn mut_from_suffix(
        source: &mut [u8],
    ) -> Result<(&mut [u8], &mut Self), CastError<&mut [u8], Self>>
    where
        Self: IntoBytes + KnownLayout,
    {
        static_assert_dst_is_not_zst!(Self);
        mut_from_prefix_suffix(source, None, CastType::Suffix).map(swap)
    }

    /// Interprets the given `source` as a `&Self` with a DST length equal to
    /// `count`.
    ///
    /// This method attempts to return a reference to `source` interpreted as a
    /// `Self` with `count` trailing elements. If the length of `source` is not
    /// equal to the size of `Self` with `count` elements, or if `source` is not
    /// appropriately aligned, this returns `Err`. If [`Self:
    /// Unaligned`][self-unaligned], you can [infallibly discard the alignment
    /// error][size-error-from].
    ///
    /// [self-unaligned]: Unaligned
    /// [size-error-from]: error/struct.SizeError.html#method.from-1
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// # #[derive(Debug, PartialEq, Eq)]
    /// #[derive(FromBytes, Immutable)]
    /// #[repr(C)]
    /// struct Pixel {
    ///     r: u8,
    ///     g: u8,
    ///     b: u8,
    ///     a: u8,
    /// }
    ///
    /// let bytes = &[0, 1, 2, 3, 4, 5, 6, 7][..];
    ///
    /// let pixels = <[Pixel]>::ref_from_bytes_with_elems(bytes, 2).unwrap();
    ///
    /// assert_eq!(pixels, &[
    ///     Pixel { r: 0, g: 1, b: 2, a: 3 },
    ///     Pixel { r: 4, g: 5, b: 6, a: 7 },
    /// ]);
    ///
    /// ```
    ///
    /// Since an explicit `count` is provided, this method supports types with
    /// zero-sized trailing slice elements. Methods such as [`ref_from_bytes`]
    /// which do not take an explicit count do not support such types.
    ///
    /// ```
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, Immutable, KnownLayout)]
    /// #[repr(C)]
    /// struct ZSTy {
    ///     leading_sized: [u8; 2],
    ///     trailing_dst: [()],
    /// }
    ///
    /// let src = &[85, 85][..];
    /// let zsty = ZSTy::ref_from_bytes_with_elems(src, 42).unwrap();
    /// assert_eq!(zsty.trailing_dst.len(), 42);
    /// ```
    ///
    /// [`ref_from_bytes`]: FromBytes::ref_from_bytes
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "ref_from_bytes_with_elems",
        format = "coco",
        arity = 2,
        [
            open
            @index 1
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 2
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn ref_from_bytes_with_elems(
        source: &[u8],
        count: usize,
    ) -> Result<&Self, CastError<&[u8], Self>>
    where
        Self: KnownLayout<PointerMetadata = usize> + Immutable,
    {
        let source = Ptr::from_ref(source);
        let maybe_slf = source.try_cast_into_no_leftover::<_, BecauseImmutable>(Some(count));
        match maybe_slf {
            Ok(slf) => Ok(slf.recall_validity().as_ref()),
            Err(err) => Err(err.map_src(|s| s.as_ref())),
        }
    }

    /// Interprets the prefix of the given `source` as a DST `&Self` with length
    /// equal to `count`.
    ///
    /// This method attempts to return a reference to the prefix of `source`
    /// interpreted as a `Self` with `count` trailing elements, and a reference
    /// to the remaining bytes. If there are insufficient bytes, or if `source`
    /// is not appropriately aligned, this returns `Err`. If [`Self:
    /// Unaligned`][self-unaligned], you can [infallibly discard the alignment
    /// error][size-error-from].
    ///
    /// [self-unaligned]: Unaligned
    /// [size-error-from]: error/struct.SizeError.html#method.from-1
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// # #[derive(Debug, PartialEq, Eq)]
    /// #[derive(FromBytes, Immutable)]
    /// #[repr(C)]
    /// struct Pixel {
    ///     r: u8,
    ///     g: u8,
    ///     b: u8,
    ///     a: u8,
    /// }
    ///
    /// // These are more bytes than are needed to encode two `Pixel`s.
    /// let bytes = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9][..];
    ///
    /// let (pixels, suffix) = <[Pixel]>::ref_from_prefix_with_elems(bytes, 2).unwrap();
    ///
    /// assert_eq!(pixels, &[
    ///     Pixel { r: 0, g: 1, b: 2, a: 3 },
    ///     Pixel { r: 4, g: 5, b: 6, a: 7 },
    /// ]);
    ///
    /// assert_eq!(suffix, &[8, 9]);
    /// ```
    ///
    /// Since an explicit `count` is provided, this method supports types with
    /// zero-sized trailing slice elements. Methods such as [`ref_from_prefix`]
    /// which do not take an explicit count do not support such types.
    ///
    /// ```
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, Immutable, KnownLayout)]
    /// #[repr(C)]
    /// struct ZSTy {
    ///     leading_sized: [u8; 2],
    ///     trailing_dst: [()],
    /// }
    ///
    /// let src = &[85, 85][..];
    /// let (zsty, _) = ZSTy::ref_from_prefix_with_elems(src, 42).unwrap();
    /// assert_eq!(zsty.trailing_dst.len(), 42);
    /// ```
    ///
    /// [`ref_from_prefix`]: FromBytes::ref_from_prefix
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "ref_from_prefix_with_elems",
        format = "coco",
        arity = 2,
        [
            open
            @index 1
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 2
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn ref_from_prefix_with_elems(
        source: &[u8],
        count: usize,
    ) -> Result<(&Self, &[u8]), CastError<&[u8], Self>>
    where
        Self: KnownLayout<PointerMetadata = usize> + Immutable,
    {
        ref_from_prefix_suffix(source, Some(count), CastType::Prefix)
    }

    /// Interprets the suffix of the given `source` as a DST `&Self` with length
    /// equal to `count`.
    ///
    /// This method attempts to return a reference to the suffix of `source`
    /// interpreted as a `Self` with `count` trailing elements, and a reference
    /// to the preceding bytes. If there are insufficient bytes, or if that
    /// suffix of `source` is not appropriately aligned, this returns `Err`. If
    /// [`Self: Unaligned`][self-unaligned], you can [infallibly discard the
    /// alignment error][size-error-from].
    ///
    /// [self-unaligned]: Unaligned
    /// [size-error-from]: error/struct.SizeError.html#method.from-1
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// # #[derive(Debug, PartialEq, Eq)]
    /// #[derive(FromBytes, Immutable)]
    /// #[repr(C)]
    /// struct Pixel {
    ///     r: u8,
    ///     g: u8,
    ///     b: u8,
    ///     a: u8,
    /// }
    ///
    /// // These are more bytes than are needed to encode two `Pixel`s.
    /// let bytes = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9][..];
    ///
    /// let (prefix, pixels) = <[Pixel]>::ref_from_suffix_with_elems(bytes, 2).unwrap();
    ///
    /// assert_eq!(prefix, &[0, 1]);
    ///
    /// assert_eq!(pixels, &[
    ///     Pixel { r: 2, g: 3, b: 4, a: 5 },
    ///     Pixel { r: 6, g: 7, b: 8, a: 9 },
    /// ]);
    /// ```
    ///
    /// Since an explicit `count` is provided, this method supports types with
    /// zero-sized trailing slice elements. Methods such as [`ref_from_suffix`]
    /// which do not take an explicit count do not support such types.
    ///
    /// ```
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, Immutable, KnownLayout)]
    /// #[repr(C)]
    /// struct ZSTy {
    ///     leading_sized: [u8; 2],
    ///     trailing_dst: [()],
    /// }
    ///
    /// let src = &[85, 85][..];
    /// let (_, zsty) = ZSTy::ref_from_suffix_with_elems(src, 42).unwrap();
    /// assert_eq!(zsty.trailing_dst.len(), 42);
    /// ```
    ///
    /// [`ref_from_suffix`]: FromBytes::ref_from_suffix
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "ref_from_suffix_with_elems",
        format = "coco",
        arity = 2,
        [
            open
            @index 1
            @title "Unsized"
            @variant "dynamic_size"
        ],
        [
            @index 2
            @title "Dynamically Padded"
            @variant "dynamic_padding"
        ]
    )]
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn ref_from_suffix_with_elems(
        source: &[u8],
        count: usize,
    ) -> Result<(&[u8], &Self), CastError<&[u8], Self>>
    where
        Self: KnownLayout<PointerMetadata = usize> + Immutable,
    {
        ref_from_prefix_suffix(source, Some(count), CastType::Suffix).map(swap)
    }

    /// Interprets the given `source` as a `&mut Self` with a DST length equal
    /// to `count`.
    ///
    /// This method attempts to return a reference to `source` interpreted as a
    /// `Self` with `count` trailing elements. If the length of `source` is not
    /// equal to the size of `Self` with `count` elements, or if `source` is not
    /// appropriately aligned, this returns `Err`. If [`Self:
    /// Unaligned`][self-unaligned], you can [infallibly discard the alignment
    /// error][size-error-from].
    ///
    /// [self-unaligned]: Unaligned
    /// [size-error-from]: error/struct.SizeError.html#method.from-1
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// # #[derive(Debug, PartialEq, Eq)]
    /// #[derive(KnownLayout, FromBytes, IntoBytes, Immutable)]
    /// #[repr(C)]
    /// struct Pixel {
    ///     r: u8,
    ///     g: u8,
    ///     b: u8,
    ///     a: u8,
    /// }
    ///
    /// let bytes = &mut [0, 1, 2, 3, 4, 5, 6, 7][..];
    ///
    /// let pixels = <[Pixel]>::mut_from_bytes_with_elems(bytes, 2).unwrap();
    ///
    /// assert_eq!(pixels, &[
    ///     Pixel { r: 0, g: 1, b: 2, a: 3 },
    ///     Pixel { r: 4, g: 5, b: 6, a: 7 },
    /// ]);
    ///
    /// pixels[1] = Pixel { r: 0, g: 0, b: 0, a: 0 };
    ///
    /// assert_eq!(bytes, [0, 1, 2, 3, 0, 0, 0, 0]);
    /// ```
    ///
    /// Since an explicit `count` is provided, this method supports types with
    /// zero-sized trailing slice elements. Methods such as [`mut_from_bytes`]
    /// which do not take an explicit count do not support such types.
    ///
    /// ```
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, IntoBytes, Immutable, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct ZSTy {
    ///     leading_sized: [u8; 2],
    ///     trailing_dst: [()],
    /// }
    ///
    /// let src = &mut [85, 85][..];
    /// let zsty = ZSTy::mut_from_bytes_with_elems(src, 42).unwrap();
    /// assert_eq!(zsty.trailing_dst.len(), 42);
    /// ```
    ///
    /// [`mut_from_bytes`]: FromBytes::mut_from_bytes
    ///
    #[doc = codegen_header!("h5", "mut_from_bytes_with_elems")]
    ///
    /// See [`TryFromBytes::ref_from_bytes_with_elems`](#method.ref_from_bytes_with_elems.codegen).
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn mut_from_bytes_with_elems(
        source: &mut [u8],
        count: usize,
    ) -> Result<&mut Self, CastError<&mut [u8], Self>>
    where
        Self: IntoBytes + KnownLayout<PointerMetadata = usize> + Immutable,
    {
        let source = Ptr::from_mut(source);
        let maybe_slf = source.try_cast_into_no_leftover::<_, BecauseImmutable>(Some(count));
        match maybe_slf {
            Ok(slf) => Ok(slf.recall_validity::<_, (_, (_, BecauseExclusive))>().as_mut()),
            Err(err) => Err(err.map_src(|s| s.as_mut())),
        }
    }

    /// Interprets the prefix of the given `source` as a `&mut Self` with DST
    /// length equal to `count`.
    ///
    /// This method attempts to return a reference to the prefix of `source`
    /// interpreted as a `Self` with `count` trailing elements, and a reference
    /// to the preceding bytes. If there are insufficient bytes, or if `source`
    /// is not appropriately aligned, this returns `Err`. If [`Self:
    /// Unaligned`][self-unaligned], you can [infallibly discard the alignment
    /// error][size-error-from].
    ///
    /// [self-unaligned]: Unaligned
    /// [size-error-from]: error/struct.SizeError.html#method.from-1
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// # #[derive(Debug, PartialEq, Eq)]
    /// #[derive(KnownLayout, FromBytes, IntoBytes, Immutable)]
    /// #[repr(C)]
    /// struct Pixel {
    ///     r: u8,
    ///     g: u8,
    ///     b: u8,
    ///     a: u8,
    /// }
    ///
    /// // These are more bytes than are needed to encode two `Pixel`s.
    /// let bytes = &mut [0, 1, 2, 3, 4, 5, 6, 7, 8, 9][..];
    ///
    /// let (pixels, suffix) = <[Pixel]>::mut_from_prefix_with_elems(bytes, 2).unwrap();
    ///
    /// assert_eq!(pixels, &[
    ///     Pixel { r: 0, g: 1, b: 2, a: 3 },
    ///     Pixel { r: 4, g: 5, b: 6, a: 7 },
    /// ]);
    ///
    /// assert_eq!(suffix, &[8, 9]);
    ///
    /// pixels[1] = Pixel { r: 0, g: 0, b: 0, a: 0 };
    /// suffix.fill(1);
    ///
    /// assert_eq!(bytes, [0, 1, 2, 3, 0, 0, 0, 0, 1, 1]);
    /// ```
    ///
    /// Since an explicit `count` is provided, this method supports types with
    /// zero-sized trailing slice elements. Methods such as [`mut_from_prefix`]
    /// which do not take an explicit count do not support such types.
    ///
    /// ```
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, IntoBytes, Immutable, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct ZSTy {
    ///     leading_sized: [u8; 2],
    ///     trailing_dst: [()],
    /// }
    ///
    /// let src = &mut [85, 85][..];
    /// let (zsty, _) = ZSTy::mut_from_prefix_with_elems(src, 42).unwrap();
    /// assert_eq!(zsty.trailing_dst.len(), 42);
    /// ```
    ///
    /// [`mut_from_prefix`]: FromBytes::mut_from_prefix
    ///
    #[doc = codegen_header!("h5", "mut_from_prefix_with_elems")]
    ///
    /// See [`TryFromBytes::ref_from_prefix_with_elems`](#method.ref_from_prefix_with_elems.codegen).
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn mut_from_prefix_with_elems(
        source: &mut [u8],
        count: usize,
    ) -> Result<(&mut Self, &mut [u8]), CastError<&mut [u8], Self>>
    where
        Self: IntoBytes + KnownLayout<PointerMetadata = usize>,
    {
        mut_from_prefix_suffix(source, Some(count), CastType::Prefix)
    }

    /// Interprets the suffix of the given `source` as a `&mut Self` with DST
    /// length equal to `count`.
    ///
    /// This method attempts to return a reference to the suffix of `source`
    /// interpreted as a `Self` with `count` trailing elements, and a reference
    /// to the remaining bytes. If there are insufficient bytes, or if that
    /// suffix of `source` is not appropriately aligned, this returns `Err`. If
    /// [`Self: Unaligned`][self-unaligned], you can [infallibly discard the
    /// alignment error][size-error-from].
    ///
    /// [self-unaligned]: Unaligned
    /// [size-error-from]: error/struct.SizeError.html#method.from-1
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// # #[derive(Debug, PartialEq, Eq)]
    /// #[derive(FromBytes, IntoBytes, Immutable)]
    /// #[repr(C)]
    /// struct Pixel {
    ///     r: u8,
    ///     g: u8,
    ///     b: u8,
    ///     a: u8,
    /// }
    ///
    /// // These are more bytes than are needed to encode two `Pixel`s.
    /// let bytes = &mut [0, 1, 2, 3, 4, 5, 6, 7, 8, 9][..];
    ///
    /// let (prefix, pixels) = <[Pixel]>::mut_from_suffix_with_elems(bytes, 2).unwrap();
    ///
    /// assert_eq!(prefix, &[0, 1]);
    ///
    /// assert_eq!(pixels, &[
    ///     Pixel { r: 2, g: 3, b: 4, a: 5 },
    ///     Pixel { r: 6, g: 7, b: 8, a: 9 },
    /// ]);
    ///
    /// prefix.fill(9);
    /// pixels[1] = Pixel { r: 0, g: 0, b: 0, a: 0 };
    ///
    /// assert_eq!(bytes, [9, 9, 2, 3, 4, 5, 0, 0, 0, 0]);
    /// ```
    ///
    /// Since an explicit `count` is provided, this method supports types with
    /// zero-sized trailing slice elements. Methods such as [`mut_from_suffix`]
    /// which do not take an explicit count do not support such types.
    ///
    /// ```
    /// use zerocopy::*;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, IntoBytes, Immutable, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct ZSTy {
    ///     leading_sized: [u8; 2],
    ///     trailing_dst: [()],
    /// }
    ///
    /// let src = &mut [85, 85][..];
    /// let (_, zsty) = ZSTy::mut_from_suffix_with_elems(src, 42).unwrap();
    /// assert_eq!(zsty.trailing_dst.len(), 42);
    /// ```
    ///
    /// [`mut_from_suffix`]: FromBytes::mut_from_suffix
    ///
    #[doc = codegen_header!("h5", "mut_from_suffix_with_elems")]
    ///
    /// See [`TryFromBytes::ref_from_suffix_with_elems`](#method.ref_from_suffix_with_elems.codegen).
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn mut_from_suffix_with_elems(
        source: &mut [u8],
        count: usize,
    ) -> Result<(&mut [u8], &mut Self), CastError<&mut [u8], Self>>
    where
        Self: IntoBytes + KnownLayout<PointerMetadata = usize>,
    {
        mut_from_prefix_suffix(source, Some(count), CastType::Suffix).map(swap)
    }

    /// Reads a copy of `Self` from the given `source`.
    ///
    /// If `source.len() != size_of::<Self>()`, `read_from_bytes` returns `Err`.
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes)]
    /// #[repr(C)]
    /// struct PacketHeader {
    ///     src_port: [u8; 2],
    ///     dst_port: [u8; 2],
    ///     length: [u8; 2],
    ///     checksum: [u8; 2],
    /// }
    ///
    /// // These bytes encode a `PacketHeader`.
    /// let bytes = &[0, 1, 2, 3, 4, 5, 6, 7][..];
    ///
    /// let header = PacketHeader::read_from_bytes(bytes).unwrap();
    ///
    /// assert_eq!(header.src_port, [0, 1]);
    /// assert_eq!(header.dst_port, [2, 3]);
    /// assert_eq!(header.length, [4, 5]);
    /// assert_eq!(header.checksum, [6, 7]);
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "read_from_bytes",
        format = "coco_static_size",
    )]
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn read_from_bytes(source: &[u8]) -> Result<Self, SizeError<&[u8], Self>>
    where
        Self: Sized,
    {
        match Ref::<_, Unalign<Self>>::sized_from(source) {
            Ok(r) => Ok(Ref::read(&r).into_inner()),
            Err(CastError::Size(e)) => Err(e.with_dst()),
            Err(CastError::Alignment(_)) => {
                // SAFETY: `Unalign<Self>` is trivially aligned, so
                // `Ref::sized_from` cannot fail due to unmet alignment
                // requirements.
                unsafe { core::hint::unreachable_unchecked() }
            }
            Err(CastError::Validity(i)) => match i {},
        }
    }

    /// Reads a copy of `Self` from the prefix of the given `source`.
    ///
    /// This attempts to read a `Self` from the first `size_of::<Self>()` bytes
    /// of `source`, returning that `Self` and any remaining bytes. If
    /// `source.len() < size_of::<Self>()`, it returns `Err`.
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes)]
    /// #[repr(C)]
    /// struct PacketHeader {
    ///     src_port: [u8; 2],
    ///     dst_port: [u8; 2],
    ///     length: [u8; 2],
    ///     checksum: [u8; 2],
    /// }
    ///
    /// // These are more bytes than are needed to encode a `PacketHeader`.
    /// let bytes = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9][..];
    ///
    /// let (header, body) = PacketHeader::read_from_prefix(bytes).unwrap();
    ///
    /// assert_eq!(header.src_port, [0, 1]);
    /// assert_eq!(header.dst_port, [2, 3]);
    /// assert_eq!(header.length, [4, 5]);
    /// assert_eq!(header.checksum, [6, 7]);
    /// assert_eq!(body, [8, 9]);
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "read_from_prefix",
        format = "coco_static_size",
    )]
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn read_from_prefix(source: &[u8]) -> Result<(Self, &[u8]), SizeError<&[u8], Self>>
    where
        Self: Sized,
    {
        match Ref::<_, Unalign<Self>>::sized_from_prefix(source) {
            Ok((r, suffix)) => Ok((Ref::read(&r).into_inner(), suffix)),
            Err(CastError::Size(e)) => Err(e.with_dst()),
            Err(CastError::Alignment(_)) => {
                // SAFETY: `Unalign<Self>` is trivially aligned, so
                // `Ref::sized_from_prefix` cannot fail due to unmet alignment
                // requirements.
                unsafe { core::hint::unreachable_unchecked() }
            }
            Err(CastError::Validity(i)) => match i {},
        }
    }

    /// Reads a copy of `Self` from the suffix of the given `source`.
    ///
    /// This attempts to read a `Self` from the last `size_of::<Self>()` bytes
    /// of `source`, returning that `Self` and any preceding bytes. If
    /// `source.len() < size_of::<Self>()`, it returns `Err`.
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::FromBytes;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes)]
    /// #[repr(C)]
    /// struct PacketTrailer {
    ///     frame_check_sequence: [u8; 4],
    /// }
    ///
    /// // These are more bytes than are needed to encode a `PacketTrailer`.
    /// let bytes = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9][..];
    ///
    /// let (prefix, trailer) = PacketTrailer::read_from_suffix(bytes).unwrap();
    ///
    /// assert_eq!(prefix, [0, 1, 2, 3, 4, 5]);
    /// assert_eq!(trailer.frame_check_sequence, [6, 7, 8, 9]);
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "read_from_suffix",
        format = "coco_static_size",
    )]
    #[must_use = "has no side effects"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    fn read_from_suffix(source: &[u8]) -> Result<(&[u8], Self), SizeError<&[u8], Self>>
    where
        Self: Sized,
    {
        match Ref::<_, Unalign<Self>>::sized_from_suffix(source) {
            Ok((prefix, r)) => Ok((prefix, Ref::read(&r).into_inner())),
            Err(CastError::Size(e)) => Err(e.with_dst()),
            Err(CastError::Alignment(_)) => {
                // SAFETY: `Unalign<Self>` is trivially aligned, so
                // `Ref::sized_from_suffix` cannot fail due to unmet alignment
                // requirements.
                unsafe { core::hint::unreachable_unchecked() }
            }
            Err(CastError::Validity(i)) => match i {},
        }
    }

    /// Reads a copy of `self` from an `io::Read`.
    ///
    /// This is useful for interfacing with operating system byte sinks (files,
    /// sockets, etc.).
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use zerocopy::{byteorder::big_endian::*, FromBytes};
    /// use std::fs::File;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes)]
    /// #[repr(C)]
    /// struct BitmapFileHeader {
    ///     signature: [u8; 2],
    ///     size: U32,
    ///     reserved: U64,
    ///     offset: U64,
    /// }
    ///
    /// let mut file = File::open("image.bin").unwrap();
    /// let header = BitmapFileHeader::read_from_io(&mut file).unwrap();
    /// ```
    #[cfg(feature = "std")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
    #[inline(always)]
    fn read_from_io<R>(mut src: R) -> io::Result<Self>
    where
        Self: Sized,
        R: io::Read,
    {
        // NOTE(#2319, #2320): We do `buf.zero()` separately rather than
        // constructing `let buf = CoreMaybeUninit::zeroed()` because, if `Self`
        // contains padding bytes, then a typed copy of `CoreMaybeUninit<Self>`
        // will not necessarily preserve zeros written to those padding byte
        // locations, and so `buf` could contain uninitialized bytes.
        let mut buf = CoreMaybeUninit::<Self>::uninit();
        buf.zero();

        let ptr = Ptr::from_mut(&mut buf);
        // SAFETY: After `buf.zero()`, `buf` consists entirely of initialized,
        // zeroed bytes. Since `MaybeUninit` has no validity requirements, `ptr`
        // cannot be used to write values which will violate `buf`'s bit
        // validity. Since `ptr` has `Exclusive` aliasing, nothing other than
        // `ptr` may be used to mutate `ptr`'s referent, and so its bit validity
        // cannot be violated even though `buf` may have more permissive bit
        // validity than `ptr`.
        let ptr = unsafe { ptr.assume_validity::<invariant::Initialized>() };
        let ptr = ptr.as_bytes();
        src.read_exact(ptr.as_mut())?;
        // SAFETY: `buf` entirely consists of initialized bytes, and `Self` is
        // `FromBytes`.
        Ok(unsafe { buf.assume_init() })
    }

    #[deprecated(since = "0.8.0", note = "renamed to `FromBytes::ref_from_bytes`")]
    #[doc(hidden)]
    #[must_use = "has no side effects"]
    #[inline(always)]
    fn ref_from(source: &[u8]) -> Option<&Self>
    where
        Self: KnownLayout + Immutable,
    {
        Self::ref_from_bytes(source).ok()
    }

    #[deprecated(since = "0.8.0", note = "renamed to `FromBytes::mut_from_bytes`")]
    #[doc(hidden)]
    #[must_use = "has no side effects"]
    #[inline(always)]
    fn mut_from(source: &mut [u8]) -> Option<&mut Self>
    where
        Self: KnownLayout + IntoBytes,
    {
        Self::mut_from_bytes(source).ok()
    }

    #[deprecated(since = "0.8.0", note = "renamed to `FromBytes::ref_from_prefix_with_elems`")]
    #[doc(hidden)]
    #[must_use = "has no side effects"]
    #[inline(always)]
    fn slice_from_prefix(source: &[u8], count: usize) -> Option<(&[Self], &[u8])>
    where
        Self: Sized + Immutable,
    {
        <[Self]>::ref_from_prefix_with_elems(source, count).ok()
    }

    #[deprecated(since = "0.8.0", note = "renamed to `FromBytes::ref_from_suffix_with_elems`")]
    #[doc(hidden)]
    #[must_use = "has no side effects"]
    #[inline(always)]
    fn slice_from_suffix(source: &[u8], count: usize) -> Option<(&[u8], &[Self])>
    where
        Self: Sized + Immutable,
    {
        <[Self]>::ref_from_suffix_with_elems(source, count).ok()
    }

    #[deprecated(since = "0.8.0", note = "renamed to `FromBytes::mut_from_prefix_with_elems`")]
    #[doc(hidden)]
    #[must_use = "has no side effects"]
    #[inline(always)]
    fn mut_slice_from_prefix(source: &mut [u8], count: usize) -> Option<(&mut [Self], &mut [u8])>
    where
        Self: Sized + IntoBytes,
    {
        <[Self]>::mut_from_prefix_with_elems(source, count).ok()
    }

    #[deprecated(since = "0.8.0", note = "renamed to `FromBytes::mut_from_suffix_with_elems`")]
    #[doc(hidden)]
    #[must_use = "has no side effects"]
    #[inline(always)]
    fn mut_slice_from_suffix(source: &mut [u8], count: usize) -> Option<(&mut [u8], &mut [Self])>
    where
        Self: Sized + IntoBytes,
    {
        <[Self]>::mut_from_suffix_with_elems(source, count).ok()
    }

    #[deprecated(since = "0.8.0", note = "renamed to `FromBytes::read_from_bytes`")]
    #[doc(hidden)]
    #[must_use = "has no side effects"]
    #[inline(always)]
    fn read_from(source: &[u8]) -> Option<Self>
    where
        Self: Sized,
    {
        Self::read_from_bytes(source).ok()
    }
}

/// Interprets the given affix of `source`'s bytes as a `&T`.
///
/// This method uses `meta` if provided; otherwise, it computes the largest
/// possible size of `T` that can fit in the prefix or suffix bytes of `source`.
/// It returns both a reference to those bytes interpreted as a `T`, and a
/// reference to the excess bytes. If there are insufficient bytes, or if that
/// affix of `source` is not appropriately aligned, this returns `Err` containing
/// the original `&S`.
#[inline(always)]
fn ref_from_prefix_suffix<S, T>(
    source: &S,
    meta: Option<T::PointerMetadata>,
    cast_type: CastType,
) -> Result<(&T, &[u8]), CastError<&S, T>>
where
    S: IntoBytes + Immutable + ?Sized,
    T: FromBytes + KnownLayout + Immutable + ?Sized,
{
    let (slf, prefix_suffix) = Ptr::from_ref(source.as_bytes())
        .try_cast_into::<_, BecauseImmutable>(cast_type, meta)
        .map_err(|err| {
            err.map_src(
                #[inline(always)]
                |_| source,
            )
        })?;
    Ok((slf.recall_validity().as_ref(), prefix_suffix.as_ref()))
}

/// Interprets the given affix of the given bytes as a `&mut Self` without
/// copying.
///
/// This method computes the largest possible size of `Self` that can fit in the
/// prefix or suffix bytes of `source`, then attempts to return both a reference
/// to those bytes interpreted as a `Self`, and a reference to the excess bytes.
/// If there are insufficient bytes, or if that affix of `source` is not
/// appropriately aligned, this returns `Err`.
#[inline(always)]
fn mut_from_prefix_suffix<T: FromBytes + IntoBytes + KnownLayout + ?Sized>(
    source: &mut [u8],
    meta: Option<T::PointerMetadata>,
    cast_type: CastType,
) -> Result<(&mut T, &mut [u8]), CastError<&mut [u8], T>> {
    let (slf, prefix_suffix) = Ptr::from_mut(source)
        .try_cast_into::<_, BecauseExclusive>(cast_type, meta)
        .map_err(|err| err.map_src(|s| s.as_mut()))?;
    Ok((slf.recall_validity::<_, (_, (_, _))>().as_mut(), prefix_suffix.as_mut()))
}

/// Analyzes whether a type is [`IntoBytes`].
///
/// This derive analyzes, at compile time, whether the annotated type satisfies
/// the [safety conditions] of `IntoBytes` and implements `IntoBytes` if it is
/// sound to do so. This derive can be applied to structs and enums (see below
/// for union support); e.g.:
///
/// ```
/// # use zerocopy_derive::{IntoBytes};
/// #[derive(IntoBytes)]
/// #[repr(C)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(IntoBytes)]
/// #[repr(u8)]
/// enum MyEnum {
/// #   Variant,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// [safety conditions]: trait@IntoBytes#safety
///
/// # Error Messages
///
/// On Rust toolchains prior to 1.78.0, due to the way that the custom derive
/// for `IntoBytes` is implemented, you may get an error like this:
///
/// ```text
/// error[E0277]: the trait bound `(): PaddingFree<Foo, true>` is not satisfied
///   --> lib.rs:23:10
///    |
///  1 | #[derive(IntoBytes)]
///    |          ^^^^^^^^^ the trait `PaddingFree<Foo, true>` is not implemented for `()`
///    |
///    = help: the following implementations were found:
///                   <() as PaddingFree<T, false>>
/// ```
///
/// This error indicates that the type being annotated has padding bytes, which
/// is illegal for `IntoBytes` types. Consider reducing the alignment of some
/// fields by using types in the [`byteorder`] module, wrapping field types in
/// [`Unalign`], adding explicit struct fields where those padding bytes would
/// be, or using `#[repr(packed)]`. See the Rust Reference's page on [type
/// layout] for more information about type layout and padding.
///
/// [type layout]: https://doc.rust-lang.org/reference/type-layout.html
///
/// # Unions
///
/// Currently, union bit validity is [up in the air][union-validity], and so
/// zerocopy does not support `#[derive(IntoBytes)]` on unions by default.
/// However, implementing `IntoBytes` on a union type is likely sound on all
/// existing Rust toolchains - it's just that it may become unsound in the
/// future. You can opt-in to `#[derive(IntoBytes)]` support on unions by
/// passing the unstable `zerocopy_derive_union_into_bytes` cfg:
///
/// ```shell
/// $ RUSTFLAGS='--cfg zerocopy_derive_union_into_bytes' cargo build
/// ```
///
/// However, it is your responsibility to ensure that this derive is sound on
/// the specific versions of the Rust toolchain you are using! We make no
/// stability or soundness guarantees regarding this cfg, and may remove it at
/// any point.
///
/// We are actively working with Rust to stabilize the necessary language
/// guarantees to support this in a forwards-compatible way, which will enable
/// us to remove the cfg gate. As part of this effort, we need to know how much
/// demand there is for this feature. If you would like to use `IntoBytes` on
/// unions, [please let us know][discussion].
///
/// [union-validity]: https://github.com/rust-lang/unsafe-code-guidelines/issues/438
/// [discussion]: https://github.com/google/zerocopy/discussions/1802
///
/// # Analysis
///
/// *This section describes, roughly, the analysis performed by this derive to
/// determine whether it is sound to implement `IntoBytes` for a given type.
/// Unless you are modifying the implementation of this derive, or attempting to
/// manually implement `IntoBytes` for a type yourself, you don't need to read
/// this section.*
///
/// If a type has the following properties, then this derive can implement
/// `IntoBytes` for that type:
///
/// - If the type is a struct, its fields must be [`IntoBytes`]. Additionally:
///     - if the type is `repr(transparent)` or `repr(packed)`, it is
///       [`IntoBytes`] if its fields are [`IntoBytes`]; else,
///     - if the type is `repr(C)` with at most one field, it is [`IntoBytes`]
///       if its field is [`IntoBytes`]; else,
///     - if the type has no generic parameters, it is [`IntoBytes`] if the type
///       is sized and has no padding bytes; else,
///     - if the type is `repr(C)` without an `align(N)` modifier for `N > 1`
///       (it may have `align(1)` or `packed(N)`), and every field is `T`,
///       `[T; N]`, or a final `[T]` for the same type parameter `T`, it is
///       [`IntoBytes`]; else,
///     - if the type is `repr(C)`, its fields must be [`Unaligned`].
/// - If the type is an enum:
///   - It must have a defined representation (`repr`s `C`, `u8`, `u16`, `u32`,
///     `u64`, `usize`, `i8`, `i16`, `i32`, `i64`, or `isize`).
///   - It must have no padding bytes.
///   - Its fields must be [`IntoBytes`].
///
/// This analysis is subject to change. Unsafe code may *only* rely on the
/// documented [safety conditions] of `FromBytes`, and must *not* rely on the
/// implementation details of this derive.
///
/// [Rust Reference]: https://doc.rust-lang.org/reference/type-layout.html
#[cfg(any(feature = "derive", test))]
#[cfg_attr(doc_cfg, doc(cfg(feature = "derive")))]
pub use zerocopy_derive::IntoBytes;

/// Types that can be converted to an immutable slice of initialized bytes.
///
/// Any `IntoBytes` type can be converted to a slice of initialized bytes of the
/// same size. This is useful for efficiently serializing structured data as raw
/// bytes.
///
/// # Implementation
///
/// **Do not implement this trait yourself!** Instead, use
/// [`#[derive(IntoBytes)]`][derive]; e.g.:
///
/// ```
/// # use zerocopy_derive::IntoBytes;
/// #[derive(IntoBytes)]
/// #[repr(C)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(IntoBytes)]
/// #[repr(u8)]
/// enum MyEnum {
/// #   Variant0,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// This derive performs a sophisticated, compile-time safety analysis to
/// determine whether a type is `IntoBytes`. See the [derive
/// documentation][derive] for guidance on how to interpret error messages
/// produced by the derive's analysis.
///
/// # Safety
///
/// *This section describes what is required in order for `T: IntoBytes`, and
/// what unsafe code may assume of such types. If you don't plan on implementing
/// `IntoBytes` manually, and you don't plan on writing unsafe code that
/// operates on `IntoBytes` types, then you don't need to read this section.*
///
/// If `T: IntoBytes`, then unsafe code may assume that it is sound to treat any
/// `t: T` as an immutable `[u8]` of length `size_of_val(t)`. If a type is
/// marked as `IntoBytes` which violates this contract, it may cause undefined
/// behavior.
///
/// `#[derive(IntoBytes)]` only permits [types which satisfy these
/// requirements][derive-analysis].
///
#[cfg_attr(
    feature = "derive",
    doc = "[derive]: zerocopy_derive::IntoBytes",
    doc = "[derive-analysis]: zerocopy_derive::IntoBytes#analysis"
)]
#[cfg_attr(
    not(feature = "derive"),
    doc = concat!("[derive]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.IntoBytes.html"),
    doc = concat!("[derive-analysis]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.IntoBytes.html#analysis"),
)]
#[cfg_attr(
    not(no_zerocopy_diagnostic_on_unimplemented_1_78_0),
    diagnostic::on_unimplemented(note = "Consider adding `#[derive(IntoBytes)]` to `{Self}`")
)]
pub unsafe trait IntoBytes {
    // The `Self: Sized` bound makes it so that this function doesn't prevent
    // `IntoBytes` from being object safe. Note that other `IntoBytes` methods
    // prevent object safety, but those provide a benefit in exchange for object
    // safety. If at some point we remove those methods, change their type
    // signatures, or move them out of this trait so that `IntoBytes` is object
    // safe again, it's important that this function not prevent object safety.
    #[doc(hidden)]
    fn only_derive_is_allowed_to_implement_this_trait()
    where
        Self: Sized;

    /// Gets the bytes of this value.
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::IntoBytes;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(IntoBytes, Immutable)]
    /// #[repr(C)]
    /// struct PacketHeader {
    ///     src_port: [u8; 2],
    ///     dst_port: [u8; 2],
    ///     length: [u8; 2],
    ///     checksum: [u8; 2],
    /// }
    ///
    /// let header = PacketHeader {
    ///     src_port: [0, 1],
    ///     dst_port: [2, 3],
    ///     length: [4, 5],
    ///     checksum: [6, 7],
    /// };
    ///
    /// let bytes = header.as_bytes();
    ///
    /// assert_eq!(bytes, [0, 1, 2, 3, 4, 5, 6, 7]);
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "as_bytes",
        format = "coco",
        arity = 2,
        [
            open
            @index 1
            @title "Sized"
            @variant "static_size"
        ],
        [
            @index 2
            @title "Unsized"
            @variant "dynamic_size"
        ]
    )]
    #[must_use = "has no side effects"]
    #[inline(always)]
    fn as_bytes(&self) -> &[u8]
    where
        Self: Immutable,
    {
        // Note that this method does not have a `Self: Sized` bound;
        // `size_of_val` works for unsized values too.
        let len = mem::size_of_val(self);
        let slf: *const Self = self;

        // SAFETY:
        // - `slf.cast::<u8>()` is valid for reads for `len * size_of::<u8>()`
        //   many bytes because...
        //   - `slf` is the same pointer as `self`, and `self` is a reference
        //     which points to an object whose size is `len`. Thus...
        //     - The entire region of `len` bytes starting at `slf` is contained
        //       within a single allocation.
        //     - `slf` is non-null.
        //   - `slf` is trivially aligned to `align_of::<u8>() == 1`.
        // - `Self: IntoBytes` ensures that all of the bytes of `slf` are
        //   initialized.
        // - Since `slf` is derived from `self`, and `self` is an immutable
        //   reference, the only other references to this memory region that
        //   could exist are other immutable references, which by `Self:
        //   Immutable` don't permit mutation.
        // - The total size of the resulting slice is no larger than
        //   `isize::MAX` because no allocation produced by safe code can be
        //   larger than `isize::MAX`.
        //
        // FIXME(#429): Add references to docs and quotes.
        unsafe { slice::from_raw_parts(slf.cast::<u8>(), len) }
    }

    /// Gets the bytes of this value mutably.
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::IntoBytes;
    /// # use zerocopy_derive::*;
    ///
    /// # #[derive(Eq, PartialEq, Debug)]
    /// #[derive(FromBytes, IntoBytes, Immutable)]
    /// #[repr(C)]
    /// struct PacketHeader {
    ///     src_port: [u8; 2],
    ///     dst_port: [u8; 2],
    ///     length: [u8; 2],
    ///     checksum: [u8; 2],
    /// }
    ///
    /// let mut header = PacketHeader {
    ///     src_port: [0, 1],
    ///     dst_port: [2, 3],
    ///     length: [4, 5],
    ///     checksum: [6, 7],
    /// };
    ///
    /// let bytes = header.as_mut_bytes();
    ///
    /// assert_eq!(bytes, [0, 1, 2, 3, 4, 5, 6, 7]);
    ///
    /// bytes.reverse();
    ///
    /// assert_eq!(header, PacketHeader {
    ///     src_port: [7, 6],
    ///     dst_port: [5, 4],
    ///     length: [3, 2],
    ///     checksum: [1, 0],
    /// });
    /// ```
    ///
    #[doc = codegen_header!("h5", "as_mut_bytes")]
    ///
    /// See [`IntoBytes::as_bytes`](#method.as_bytes.codegen).
    #[must_use = "has no side effects"]
    #[inline(always)]
    fn as_mut_bytes(&mut self) -> &mut [u8]
    where
        Self: FromBytes,
    {
        // Note that this method does not have a `Self: Sized` bound;
        // `size_of_val` works for unsized values too.
        let len = mem::size_of_val(self);
        let slf: *mut Self = self;

        // SAFETY:
        // - `slf.cast::<u8>()` is valid for reads and writes for `len *
        //   size_of::<u8>()` many bytes because...
        //   - `slf` is the same pointer as `self`, and `self` is a reference
        //     which points to an object whose size is `len`. Thus...
        //     - The entire region of `len` bytes starting at `slf` is contained
        //       within a single allocation.
        //     - `slf` is non-null.
        //   - `slf` is trivially aligned to `align_of::<u8>() == 1`.
        // - `Self: IntoBytes` ensures that all of the bytes of `slf` are
        //   initialized.
        // - `Self: FromBytes` ensures that no write to this memory region
        //   could result in it containing an invalid `Self`.
        // - Since `slf` is derived from `self`, and `self` is a mutable
        //   reference, no other references to this memory region can exist.
        // - The total size of the resulting slice is no larger than
        //   `isize::MAX` because no allocation produced by safe code can be
        //   larger than `isize::MAX`.
        //
        // FIXME(#429): Add references to docs and quotes.
        unsafe { slice::from_raw_parts_mut(slf.cast::<u8>(), len) }
    }

    /// Writes a copy of `self` to `dst`.
    ///
    /// If `dst.len() != size_of_val(self)`, `write_to` returns `Err`.
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::IntoBytes;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(IntoBytes, Immutable)]
    /// #[repr(C)]
    /// struct PacketHeader {
    ///     src_port: [u8; 2],
    ///     dst_port: [u8; 2],
    ///     length: [u8; 2],
    ///     checksum: [u8; 2],
    /// }
    ///
    /// let header = PacketHeader {
    ///     src_port: [0, 1],
    ///     dst_port: [2, 3],
    ///     length: [4, 5],
    ///     checksum: [6, 7],
    /// };
    ///
    /// let mut bytes = [0, 0, 0, 0, 0, 0, 0, 0];
    ///
    /// header.write_to(&mut bytes[..]);
    ///
    /// assert_eq!(bytes, [0, 1, 2, 3, 4, 5, 6, 7]);
    /// ```
    ///
    /// If too many or too few target bytes are provided, `write_to` returns
    /// `Err` and leaves the target bytes unmodified:
    ///
    /// ```
    /// # use zerocopy::IntoBytes;
    /// # let header = u128::MAX;
    /// let mut excessive_bytes = &mut [0u8; 128][..];
    ///
    /// let write_result = header.write_to(excessive_bytes);
    ///
    /// assert!(write_result.is_err());
    /// assert_eq!(excessive_bytes, [0u8; 128]);
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "write_to",
        format = "coco",
        arity = 2,
        [
            open
            @index 1
            @title "Sized"
            @variant "static_size"
        ],
        [
            @index 2
            @title "Unsized"
            @variant "dynamic_size"
        ]
    )]
    #[must_use = "callers should check the return value to see if the operation succeeded"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    #[allow(clippy::mut_from_ref)] // False positive: `&self -> &mut [u8]`
    fn write_to(&self, dst: &mut [u8]) -> Result<(), SizeError<&Self, &mut [u8]>>
    where
        Self: Immutable,
    {
        let src = self.as_bytes();
        if dst.len() == src.len() {
            // SAFETY: Within this branch of the conditional, we have ensured
            // that `dst.len()` is equal to `src.len()`. Neither the size of the
            // source nor the size of the destination change between the above
            // size check and the invocation of `copy_unchecked`.
            unsafe { util::copy_unchecked(src, dst) }
            Ok(())
        } else {
            Err(SizeError::new(self))
        }
    }

    /// Writes a copy of `self` to the prefix of `dst`.
    ///
    /// `write_to_prefix` writes `self` to the first `size_of_val(self)` bytes
    /// of `dst`. If `dst.len() < size_of_val(self)`, it returns `Err`.
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::IntoBytes;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(IntoBytes, Immutable)]
    /// #[repr(C)]
    /// struct PacketHeader {
    ///     src_port: [u8; 2],
    ///     dst_port: [u8; 2],
    ///     length: [u8; 2],
    ///     checksum: [u8; 2],
    /// }
    ///
    /// let header = PacketHeader {
    ///     src_port: [0, 1],
    ///     dst_port: [2, 3],
    ///     length: [4, 5],
    ///     checksum: [6, 7],
    /// };
    ///
    /// let mut bytes = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
    ///
    /// header.write_to_prefix(&mut bytes[..]);
    ///
    /// assert_eq!(bytes, [0, 1, 2, 3, 4, 5, 6, 7, 0, 0]);
    /// ```
    ///
    /// If insufficient target bytes are provided, `write_to_prefix` returns
    /// `Err` and leaves the target bytes unmodified:
    ///
    /// ```
    /// # use zerocopy::IntoBytes;
    /// # let header = u128::MAX;
    /// let mut insufficient_bytes = &mut [0, 0][..];
    ///
    /// let write_result = header.write_to_suffix(insufficient_bytes);
    ///
    /// assert!(write_result.is_err());
    /// assert_eq!(insufficient_bytes, [0, 0]);
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "write_to_prefix",
        format = "coco",
        arity = 2,
        [
            open
            @index 1
            @title "Sized"
            @variant "static_size"
        ],
        [
            @index 2
            @title "Unsized"
            @variant "dynamic_size"
        ]
    )]
    #[must_use = "callers should check the return value to see if the operation succeeded"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    #[allow(clippy::mut_from_ref)] // False positive: `&self -> &mut [u8]`
    fn write_to_prefix(&self, dst: &mut [u8]) -> Result<(), SizeError<&Self, &mut [u8]>>
    where
        Self: Immutable,
    {
        let src = self.as_bytes();
        match dst.get_mut(..src.len()) {
            Some(dst) => {
                // SAFETY: Within this branch of the `match`, we have ensured
                // through fallible subslicing that `dst.len()` is equal to
                // `src.len()`. Neither the size of the source nor the size of
                // the destination change between the above subslicing operation
                // and the invocation of `copy_unchecked`.
                unsafe { util::copy_unchecked(src, dst) }
                Ok(())
            }
            None => Err(SizeError::new(self)),
        }
    }

    /// Writes a copy of `self` to the suffix of `dst`.
    ///
    /// `write_to_suffix` writes `self` to the last `size_of_val(self)` bytes of
    /// `dst`. If `dst.len() < size_of_val(self)`, it returns `Err`.
    ///
    /// # Examples
    ///
    /// ```
    /// use zerocopy::IntoBytes;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(IntoBytes, Immutable)]
    /// #[repr(C)]
    /// struct PacketHeader {
    ///     src_port: [u8; 2],
    ///     dst_port: [u8; 2],
    ///     length: [u8; 2],
    ///     checksum: [u8; 2],
    /// }
    ///
    /// let header = PacketHeader {
    ///     src_port: [0, 1],
    ///     dst_port: [2, 3],
    ///     length: [4, 5],
    ///     checksum: [6, 7],
    /// };
    ///
    /// let mut bytes = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
    ///
    /// header.write_to_suffix(&mut bytes[..]);
    ///
    /// assert_eq!(bytes, [0, 0, 0, 1, 2, 3, 4, 5, 6, 7]);
    ///
    /// let mut insufficient_bytes = &mut [0, 0][..];
    ///
    /// let write_result = header.write_to_suffix(insufficient_bytes);
    ///
    /// assert!(write_result.is_err());
    /// assert_eq!(insufficient_bytes, [0, 0]);
    /// ```
    ///
    /// If insufficient target bytes are provided, `write_to_suffix` returns
    /// `Err` and leaves the target bytes unmodified:
    ///
    /// ```
    /// # use zerocopy::IntoBytes;
    /// # let header = u128::MAX;
    /// let mut insufficient_bytes = &mut [0, 0][..];
    ///
    /// let write_result = header.write_to_suffix(insufficient_bytes);
    ///
    /// assert!(write_result.is_err());
    /// assert_eq!(insufficient_bytes, [0, 0]);
    /// ```
    ///
    #[doc = codegen_section!(
        header = "h5",
        bench = "write_to_suffix",
        format = "coco",
        arity = 2,
        [
            open
            @index 1
            @title "Sized"
            @variant "static_size"
        ],
        [
            @index 2
            @title "Unsized"
            @variant "dynamic_size"
        ]
    )]
    #[must_use = "callers should check the return value to see if the operation succeeded"]
    #[cfg_attr(zerocopy_inline_always, inline(always))]
    #[cfg_attr(not(zerocopy_inline_always), inline)]
    #[allow(clippy::mut_from_ref)] // False positive: `&self -> &mut [u8]`
    fn write_to_suffix(&self, dst: &mut [u8]) -> Result<(), SizeError<&Self, &mut [u8]>>
    where
        Self: Immutable,
    {
        let src = self.as_bytes();
        let start = if let Some(start) = dst.len().checked_sub(src.len()) {
            start
        } else {
            return Err(SizeError::new(self));
        };
        let dst = if let Some(dst) = dst.get_mut(start..) {
            dst
        } else {
            // get_mut() should never return None here. We return a `SizeError`
            // rather than .unwrap() because in the event the branch is not
            // optimized away, returning a value is generally lighter-weight
            // than panicking.
            return Err(SizeError::new(self));
        };
        // SAFETY: Through fallible subslicing of `dst`, we have ensured that
        // `dst.len()` is equal to `src.len()`. Neither the size of the source
        // nor the size of the destination change between the above subslicing
        // operation and the invocation of `copy_unchecked`.
        unsafe {
            util::copy_unchecked(src, dst);
        }
        Ok(())
    }

    /// Writes a copy of `self` to an `io::Write`.
    ///
    /// This is a shorthand for `dst.write_all(self.as_bytes())`, and is useful
    /// for interfacing with operating system byte sinks (files, sockets, etc.).
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use zerocopy::{byteorder::big_endian::U16, FromBytes, IntoBytes};
    /// use std::fs::File;
    /// # use zerocopy_derive::*;
    ///
    /// #[derive(FromBytes, IntoBytes, Immutable, KnownLayout)]
    /// #[repr(C, packed)]
    /// struct GrayscaleImage {
    ///     height: U16,
    ///     width: U16,
    ///     pixels: [U16],
    /// }
    ///
    /// let image = GrayscaleImage::ref_from_bytes(&[0, 0, 0, 0][..]).unwrap();
    /// let mut file = File::create("image.bin").unwrap();
    /// image.write_to_io(&mut file).unwrap();
    /// ```
    ///
    /// If the write fails, `write_to_io` returns `Err` and a partial write may
    /// have occurred; e.g.:
    ///
    /// ```
    /// # use zerocopy::IntoBytes;
    ///
    /// let src = u128::MAX;
    /// let mut dst = [0u8; 2];
    ///
    /// let write_result = src.write_to_io(&mut dst[..]);
    ///
    /// assert!(write_result.is_err());
    /// assert_eq!(dst, [255, 255]);
    /// ```
    #[cfg(feature = "std")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
    #[inline(always)]
    fn write_to_io<W>(&self, mut dst: W) -> io::Result<()>
    where
        Self: Immutable,
        W: io::Write,
    {
        dst.write_all(self.as_bytes())
    }

    #[deprecated(since = "0.8.0", note = "`IntoBytes::as_bytes_mut` was renamed to `as_mut_bytes`")]
    #[doc(hidden)]
    #[inline]
    fn as_bytes_mut(&mut self) -> &mut [u8]
    where
        Self: FromBytes,
    {
        self.as_mut_bytes()
    }
}

/// Analyzes whether a type is [`Unaligned`].
///
/// This derive analyzes, at compile time, whether the annotated type satisfies
/// the [safety conditions] of `Unaligned` and implements `Unaligned` if it is
/// sound to do so. This derive can be applied to structs, enums, and unions;
/// e.g.:
///
/// ```
/// # use zerocopy_derive::Unaligned;
/// #[derive(Unaligned)]
/// #[repr(C)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(Unaligned)]
/// #[repr(u8)]
/// enum MyEnum {
/// #   Variant0,
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(Unaligned)]
/// #[repr(packed)]
/// union MyUnion {
/// #   variant: u8,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// # Analysis
///
/// *This section describes, roughly, the analysis performed by this derive to
/// determine whether it is sound to implement `Unaligned` for a given type.
/// Unless you are modifying the implementation of this derive, or attempting to
/// manually implement `Unaligned` for a type yourself, you don't need to read
/// this section.*
///
/// If a type has the following properties, then this derive can implement
/// `Unaligned` for that type:
///
/// - If the type is a struct or union:
///   - If `repr(align(N))` is provided, `N` must equal 1.
///   - If the type is `repr(C)` or `repr(transparent)`, all fields must be
///     [`Unaligned`].
///   - If the type is not `repr(C)` or `repr(transparent)`, it must be
///     `repr(packed)` or `repr(packed(1))`.
/// - If the type is an enum:
///   - If `repr(align(N))` is provided, `N` must equal 1.
///   - It must be a field-less enum (meaning that all variants have no fields).
///   - It must be `repr(i8)` or `repr(u8)`.
///
/// [safety conditions]: trait@Unaligned#safety
#[cfg(any(feature = "derive", test))]
#[cfg_attr(doc_cfg, doc(cfg(feature = "derive")))]
pub use zerocopy_derive::Unaligned;

/// Types with no alignment requirement.
///
/// If `T: Unaligned`, then `align_of::<T>() == 1`.
///
/// # Implementation
///
/// **Do not implement this trait yourself!** Instead, use
/// [`#[derive(Unaligned)]`][derive]; e.g.:
///
/// ```
/// # use zerocopy_derive::Unaligned;
/// #[derive(Unaligned)]
/// #[repr(C)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(Unaligned)]
/// #[repr(u8)]
/// enum MyEnum {
/// #   Variant0,
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(Unaligned)]
/// #[repr(packed)]
/// union MyUnion {
/// #   variant: u8,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// This derive performs a sophisticated, compile-time safety analysis to
/// determine whether a type is `Unaligned`.
///
/// # Safety
///
/// *This section describes what is required in order for `T: Unaligned`, and
/// what unsafe code may assume of such types. If you don't plan on implementing
/// `Unaligned` manually, and you don't plan on writing unsafe code that
/// operates on `Unaligned` types, then you don't need to read this section.*
///
/// If `T: Unaligned`, then unsafe code may assume that it is sound to produce a
/// reference to `T` at any memory location regardless of alignment. If a type
/// is marked as `Unaligned` which violates this contract, it may cause
/// undefined behavior.
///
/// `#[derive(Unaligned)]` only permits [types which satisfy these
/// requirements][derive-analysis].
///
#[cfg_attr(
    feature = "derive",
    doc = "[derive]: zerocopy_derive::Unaligned",
    doc = "[derive-analysis]: zerocopy_derive::Unaligned#analysis"
)]
#[cfg_attr(
    not(feature = "derive"),
    doc = concat!("[derive]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.Unaligned.html"),
    doc = concat!("[derive-analysis]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.Unaligned.html#analysis"),
)]
#[cfg_attr(
    not(no_zerocopy_diagnostic_on_unimplemented_1_78_0),
    diagnostic::on_unimplemented(note = "Consider adding `#[derive(Unaligned)]` to `{Self}`")
)]
pub unsafe trait Unaligned {
    // The `Self: Sized` bound makes it so that `Unaligned` is still object
    // safe.
    #[doc(hidden)]
    fn only_derive_is_allowed_to_implement_this_trait()
    where
        Self: Sized;
}

/// Derives optimized [`PartialEq`] and [`Eq`] implementations.
///
/// This derive can be applied to structs and enums implementing both
/// [`Immutable`] and [`IntoBytes`]; e.g.:
///
/// ```
/// # use zerocopy_derive::{ByteEq, Immutable, IntoBytes};
/// #[derive(ByteEq, Immutable, IntoBytes)]
/// #[repr(C)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(ByteEq, Immutable, IntoBytes)]
/// #[repr(u8)]
/// enum MyEnum {
/// #   Variant,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// The standard library's [`derive(Eq, PartialEq)`][derive@PartialEq] computes
/// equality by individually comparing each field. Instead, the implementation
/// of [`PartialEq::eq`] emitted by `derive(ByteHash)` converts the entirety of
/// `self` and `other` to byte slices and compares those slices for equality.
/// This may have performance advantages.
#[cfg(any(feature = "derive", test))]
#[cfg_attr(doc_cfg, doc(cfg(feature = "derive")))]
pub use zerocopy_derive::ByteEq;
/// Derives an optimized [`Hash`] implementation.
///
/// This derive can be applied to structs and enums implementing both
/// [`Immutable`] and [`IntoBytes`]; e.g.:
///
/// ```
/// # use zerocopy_derive::{ByteHash, Immutable, IntoBytes};
/// #[derive(ByteHash, Immutable, IntoBytes)]
/// #[repr(C)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
///
/// #[derive(ByteHash, Immutable, IntoBytes)]
/// #[repr(u8)]
/// enum MyEnum {
/// #   Variant,
/// # /*
///     ...
/// # */
/// }
/// ```
///
/// The standard library's [`derive(Hash)`][derive@Hash] produces hashes by
/// individually hashing each field and combining the results. Instead, the
/// implementations of [`Hash::hash()`] and [`Hash::hash_slice()`] generated by
/// `derive(ByteHash)` convert the entirety of `self` to a byte slice and hashes
/// it in a single call to [`Hasher::write()`]. This may have performance
/// advantages.
///
/// [`Hash`]: core::hash::Hash
/// [`Hash::hash()`]: core::hash::Hash::hash()
/// [`Hash::hash_slice()`]: core::hash::Hash::hash_slice()
#[cfg(any(feature = "derive", test))]
#[cfg_attr(doc_cfg, doc(cfg(feature = "derive")))]
pub use zerocopy_derive::ByteHash;
/// Implements [`SplitAt`].
///
/// This derive can be applied to structs; e.g.:
///
/// ```
/// # use zerocopy_derive::{ByteEq, Immutable, IntoBytes};
/// #[derive(ByteEq, Immutable, IntoBytes)]
/// #[repr(C)]
/// struct MyStruct {
/// # /*
///     ...
/// # */
/// }
/// ```
#[cfg(any(feature = "derive", test))]
#[cfg_attr(doc_cfg, doc(cfg(feature = "derive")))]
pub use zerocopy_derive::SplitAt;

#[cfg(feature = "alloc")]
#[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
#[cfg(not(no_zerocopy_panic_in_const_and_vec_try_reserve_1_57_0))]
mod alloc_support {
    use super::*;

    /// Extends a `Vec<T>` by pushing `additional` new items onto the end of the
    /// vector. The new items are initialized with zeros.
    #[cfg(not(no_zerocopy_panic_in_const_and_vec_try_reserve_1_57_0))]
    #[doc(hidden)]
    #[deprecated(since = "0.8.0", note = "moved to `FromZeros`")]
    #[inline(always)]
    pub fn extend_vec_zeroed<T: FromZeros>(
        v: &mut Vec<T>,
        additional: usize,
    ) -> Result<(), AllocError> {
        <T as FromZeros>::extend_vec_zeroed(v, additional)
    }

    /// Inserts `additional` new items into `Vec<T>` at `position`. The new
    /// items are initialized with zeros.
    ///
    /// # Panics
    ///
    /// Panics if `position > v.len()`.
    #[cfg(not(no_zerocopy_panic_in_const_and_vec_try_reserve_1_57_0))]
    #[doc(hidden)]
    #[deprecated(since = "0.8.0", note = "moved to `FromZeros`")]
    #[inline(always)]
    pub fn insert_vec_zeroed<T: FromZeros>(
        v: &mut Vec<T>,
        position: usize,
        additional: usize,
    ) -> Result<(), AllocError> {
        <T as FromZeros>::insert_vec_zeroed(v, position, additional)
    }
}

#[cfg(feature = "alloc")]
#[cfg(not(no_zerocopy_panic_in_const_and_vec_try_reserve_1_57_0))]
#[doc(hidden)]
pub use alloc_support::*;

#[cfg(test)]
#[allow(clippy::assertions_on_result_states, clippy::unreadable_literal)]
mod tests {
    use static_assertions::assert_impl_all;

    use super::*;
    use crate::util::testutil::*;

    // An unsized type.
    //
    // This is used to test the custom derives of our traits. The `[u8]` type
    // gets a hand-rolled impl, so it doesn't exercise our custom derives.
    #[derive(Debug, Eq, PartialEq, FromBytes, IntoBytes, Unaligned, Immutable)]
    #[repr(transparent)]
    struct Unsized([u8]);

    impl Unsized {
        fn from_mut_slice(slc: &mut [u8]) -> &mut Unsized {
            // SAFETY: This *probably* sound - since the layouts of `[u8]` and
            // `Unsized` are the same, so are the layouts of `&mut [u8]` and
            // `&mut Unsized`. [1] Even if it turns out that this isn't actually
            // guaranteed by the language spec, we can just change this since
            // it's in test code.
            //
            // [1] https://github.com/rust-lang/unsafe-code-guidelines/issues/375
            unsafe { mem::transmute(slc) }
        }
    }

    // These types exercise a nested slice DST whose inner trailing-padding
    // operation cannot be flattened into the alignment of the packed outer
    // type. For `NestedPackedDstOuter<[u8]>`, the trailing slice begins at
    // offset 7, but the size for `elems` elements is
    // `2 + round_up(5 + elems, 4)`.
    //
    // The Reference rejects directly placing an alignment-4 field in a
    // `repr(packed(2))` type. Rust currently accepts this construction through
    // generic indirection. So long as Rust accepts it, this test deliberately
    // treats its layout as following the otherwise-applicable `repr(C)` and
    // `repr(packed)` rules.
    #[derive(FromBytes, Immutable, KnownLayout)]
    #[repr(C, align(4))]
    struct NestedPackedDstWord([u8; 4]);

    #[derive(FromBytes, Immutable, KnownLayout)]
    #[repr(C)]
    struct NestedPackedDstInner<T: ?Sized> {
        a: NestedPackedDstWord,
        b: u8,
        tail: T,
    }

    #[derive(FromBytes, Immutable, KnownLayout)]
    #[repr(C, packed(2))]
    struct NestedPackedDstOuter<T: ?Sized> {
        z: u8,
        inner: ManuallyDrop<NestedPackedDstInner<T>>,
    }

    #[repr(C, align(2))]
    struct Align2<T>(T);

    #[test]
    fn test_known_layout() {
        // Test that `$ty` and `ManuallyDrop<$ty>` have the expected layout.
        // Test that `PhantomData<$ty>` has the same layout as `()` regardless
        // of `$ty`.
        macro_rules! test {
            ($ty:ty, $expect:expr) => {
                let expect = $expect;
                assert_eq!(<$ty as KnownLayout>::LAYOUT, expect);
                assert_eq!(<ManuallyDrop<$ty> as KnownLayout>::LAYOUT, expect);
                assert_eq!(<PhantomData<$ty> as KnownLayout>::LAYOUT, <() as KnownLayout>::LAYOUT);
            };
        }

        let layout = |offset, align, trailing_slice_elem_size, statically_shallow_unpadded| {
            let align = NonZeroUsize::new(align).unwrap();
            let size_phase = offset % align;
            DstLayout {
                align,
                size_info: match trailing_slice_elem_size {
                    None => SizeInfo::Sized { size: offset },
                    Some(elem_size) => SizeInfo::SliceDst(TrailingSliceLayout {
                        offset,
                        elem_size,
                        size_base: offset - size_phase,
                        size_rounding_align_and_phase: RoundingAlignAndPhase::new(
                            align, size_phase,
                        ),
                    }),
                },
                statically_shallow_unpadded,
            }
        };

        test!((), layout(0, 1, None, false));
        test!(u8, layout(1, 1, None, false));
        // Use `align_of` because `u64` alignment may be smaller than 8 on some
        // platforms.
        test!(u64, layout(8, mem::align_of::<u64>(), None, false));
        test!(AU64, layout(8, 8, None, false));

        test!(Option<&'static ()>, usize::LAYOUT);

        test!([()], layout(0, 1, Some(0), true));
        test!([u8], layout(0, 1, Some(1), true));
        test!(str, layout(0, 1, Some(1), true));
    }

    #[test]
    fn test_known_layout_pointer_to_metadata() {
        fn test<T>(data: *mut T, elems: usize) {
            let ptr = ptr::slice_from_raw_parts_mut(data, elems);
            assert_eq!(<[T] as KnownLayout>::pointer_to_metadata(ptr), elems);
        }

        for elems in [0, 1, usize::MAX] {
            test(ptr::null_mut::<u8>(), elems);
        }

        test(NonNull::<u8>::dangling().as_ptr(), 0);

        let mut bytes = [1, 2, 3];
        test(bytes.as_mut_ptr(), bytes.len());
        assert_eq!(bytes, [1, 2, 3]);

        test(NonNull::<()>::dangling().as_ptr(), usize::MAX);

        // Retaining and inspecting a raw pointer after freeing its allocation
        // is safe so long as the pointer is not dereferenced.
        let deallocated = {
            let mut byte = Box::new(0u8);
            let ptr: *mut u8 = &mut *byte;
            drop(byte);
            ptr
        };
        test(deallocated, 3);

        // Metadata extraction requires neither element storage nor alignment
        // for `T`; it does not access the pointer's referent.
        test(NonNull::<u8>::dangling().as_ptr().cast::<u64>(), usize::MAX);
    }

    #[cfg(feature = "derive")]
    #[test]
    fn test_known_layout_pointer_to_metadata_derive() {
        #[derive(KnownLayout)]
        #[repr(C)]
        struct Dst {
            prefix: u8,
            trailing: [u8],
        }

        for elems in [0, 1, usize::MAX] {
            let ptr = ptr::slice_from_raw_parts_mut(ptr::null_mut::<u8>(), elems);
            #[allow(clippy::as_conversions)]
            let ptr = ptr as *mut Dst;
            assert_eq!(Dst::pointer_to_metadata(ptr), elems);
        }
    }

    #[test]
    fn test_nested_packed_dst_layout() {
        fn assert_native_size<const N: usize>(expected: usize) {
            let value = NestedPackedDstOuter {
                z: 0,
                inner: ManuallyDrop::new(NestedPackedDstInner {
                    a: NestedPackedDstWord([0; 4]),
                    b: 0,
                    tail: [0; N],
                }),
            };
            let value: &NestedPackedDstOuter<[u8]> = &value;
            assert_eq!(mem::size_of_val(value), expected);
            assert_eq!(NestedPackedDstOuter::<[u8]>::size_for_metadata(N), Some(expected));
        }

        assert_native_size::<0>(10);
        assert_native_size::<1>(10);
        assert_native_size::<2>(10);
        assert_native_size::<3>(10);
        assert_native_size::<4>(14);
        assert_native_size::<7>(14);
        assert_native_size::<8>(18);

        let layout = NestedPackedDstOuter::<[u8]>::LAYOUT;
        assert_eq!(layout.align.get(), 2);
        assert!(layout.requires_dynamic_padding());
        assert_eq!(
            layout.size_info,
            SizeInfo::SliceDst(TrailingSliceLayout {
                offset: 7,
                elem_size: 1,
                size_base: 6,
                size_rounding_align_and_phase: RoundingAlignAndPhase::new(
                    NonZeroUsize::new(4).unwrap(),
                    1
                ),
            })
        );

        // Eight bytes used to be incorrectly accepted for a zero-element
        // value. The actual Rust layout requires ten bytes.
        let short = Align2([0u8; 8]);
        assert!(NestedPackedDstOuter::<[u8]>::ref_from_bytes_with_elems(&short.0, 0).is_err());
        assert!(pointer::Ptr::from_ref(&short.0[..])
            .try_cast_into::<NestedPackedDstOuter<[u8]>, pointer::BecauseImmutable>(
                CastType::Prefix,
                Some(0),
            )
            .is_err());

        let full = Align2([0, 0, 0, 0, 0, 0, 0, 0, b'S', b'!']);
        let value = NestedPackedDstOuter::<[u8]>::ref_from_bytes_with_elems(&full.0, 0).unwrap();
        assert_eq!(mem::size_of_val(value), 10);

        // When metadata is inferred, lengths 0 through 3 all produce a
        // ten-byte object. Parsing chooses the largest trailing-slice length
        // which fits exactly.
        let inferred = NestedPackedDstOuter::<[u8]>::ref_from_bytes(&full.0).unwrap();
        assert_eq!(pointer::Ptr::from_ref(inferred).len(), 3);

        // Trailing padding includes the inner type's padding, even when the
        // outer type's alignment is reduced by packing.
        let trailing_layout = trailing_slice_layout::<NestedPackedDstOuter<[u8]>>();
        for (elems, expected) in [(0, 3), (1, 2), (2, 1), (3, 0), (4, 3)] {
            assert_eq!(trailing_layout.padding_for_elems(elems), expected);
        }
    }

    // Before Rust 1.76, rust-lang/rust#118537 caused `addr_of_mut!` to
    // miscompute the offset of an unsized field in a packed struct. The fix is
    // rust-lang/rust#118540. Keep the layout and parsing regression above on
    // all supported compilers, and gate only the projection regression which
    // necessarily exercises that independent compiler bug.
    #[rustversion::since(1.76.0)]
    #[test]
    fn test_nested_packed_dst_projection() {
        let full = Align2([0, 0, 0, 0, 0, 0, 0, 0, b'S', b'!']);

        // The projected inner value occupies the final eight bytes of the full
        // ten-byte input, including the two sentinel bytes.
        let (value, rest) = pointer::Ptr::from_ref(&full.0[..])
            .try_cast_into::<NestedPackedDstOuter<[u8]>, pointer::BecauseImmutable>(
                CastType::Prefix,
                Some(0),
            )
            .unwrap();
        assert!(rest.as_ref().is_empty());
        let inner = value.project::<project_clients::TryFromBytesDerive, _, { STRUCT_VARIANT_ID }, { ident_id!(inner) }>().unwrap();
        assert_eq!(
            inner.as_bytes::<pointer::BecauseImmutable>().as_ref(),
            &[0, 0, 0, 0, 0, 0, b'S', b'!']
        );
    }

    #[cfg(feature = "derive")]
    #[test]
    fn test_known_layout_derive() {
        // In this and other files (`late_compile_pass.rs`,
        // `mid_compile_pass.rs`, and `struct.rs`), we test success and failure
        // modes of `derive(KnownLayout)` for the following combination of
        // properties:
        //
        // +------------+--------------------------------------+-----------+
        // |            |      trailing field properties       |           |
        // | `repr(C)`? | generic? | `KnownLayout`? | `Sized`? | Type Name |
        // |------------+----------+----------------+----------+-----------|
        // |          N |        N |              N |        N |      KL00 |
        // |          N |        N |              N |        Y |      KL01 |
        // |          N |        N |              Y |        N |      KL02 |
        // |          N |        N |              Y |        Y |      KL03 |
        // |          N |        Y |              N |        N |      KL04 |
        // |          N |        Y |              N |        Y |      KL05 |
        // |          N |        Y |              Y |        N |      KL06 |
        // |          N |        Y |              Y |        Y |      KL07 |
        // |          Y |        N |              N |        N |      KL08 |
        // |          Y |        N |              N |        Y |      KL09 |
        // |          Y |        N |              Y |        N |      KL10 |
        // |          Y |        N |              Y |        Y |      KL11 |
        // |          Y |        Y |              N |        N |      KL12 |
        // |          Y |        Y |              N |        Y |      KL13 |
        // |          Y |        Y |              Y |        N |      KL14 |
        // |          Y |        Y |              Y |        Y |      KL15 |
        // +------------+----------+----------------+----------+-----------+

        struct NotKnownLayout<T = ()> {
            _t: T,
        }

        #[derive(KnownLayout)]
        #[repr(C)]
        struct AlignSize<const ALIGN: usize, const SIZE: usize>
        where
            elain::Align<ALIGN>: elain::Alignment,
        {
            _align: elain::Align<ALIGN>,
            size: [u8; SIZE],
        }

        type AU16 = AlignSize<2, 2>;
        type AU32 = AlignSize<4, 4>;

        fn _assert_kl<T: ?Sized + KnownLayout>(_: &T) {}

        let sized_layout = |align, size| DstLayout {
            align: NonZeroUsize::new(align).unwrap(),
            size_info: SizeInfo::Sized { size },
            statically_shallow_unpadded: false,
        };

        let unsized_layout = |align, elem_size, offset, statically_shallow_unpadded| {
            let align = NonZeroUsize::new(align).unwrap();
            let size_phase = offset % align;
            DstLayout {
                align,
                size_info: SizeInfo::SliceDst(TrailingSliceLayout {
                    offset,
                    elem_size,
                    size_base: offset - size_phase,
                    size_rounding_align_and_phase: RoundingAlignAndPhase::new(align, size_phase),
                }),
                statically_shallow_unpadded,
            }
        };

        // | `repr(C)`? | generic? | `KnownLayout`? | `Sized`? | Type Name |
        // |          N |        N |              N |        Y |      KL01 |
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        struct KL01(NotKnownLayout<AU32>, NotKnownLayout<AU16>);

        let expected = DstLayout::for_type::<KL01>();

        assert_eq!(<KL01 as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL01 as KnownLayout>::LAYOUT, sized_layout(4, 8));

        // ...with `align(N)`:
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(align(64))]
        struct KL01Align(NotKnownLayout<AU32>, NotKnownLayout<AU16>);

        let expected = DstLayout::for_type::<KL01Align>();

        assert_eq!(<KL01Align as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL01Align as KnownLayout>::LAYOUT, sized_layout(64, 64));

        // ...with `packed`:
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(packed)]
        struct KL01Packed(NotKnownLayout<AU32>, NotKnownLayout<AU16>);

        let expected = DstLayout::for_type::<KL01Packed>();

        assert_eq!(<KL01Packed as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL01Packed as KnownLayout>::LAYOUT, sized_layout(1, 6));

        // ...with `packed(N)`:
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(packed(2))]
        struct KL01PackedN(NotKnownLayout<AU32>, NotKnownLayout<AU16>);

        assert_impl_all!(KL01PackedN: KnownLayout);

        let expected = DstLayout::for_type::<KL01PackedN>();

        assert_eq!(<KL01PackedN as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL01PackedN as KnownLayout>::LAYOUT, sized_layout(2, 6));

        // | `repr(C)`? | generic? | `KnownLayout`? | `Sized`? | Type Name |
        // |          N |        N |              Y |        Y |      KL03 |
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        struct KL03(NotKnownLayout, u8);

        let expected = DstLayout::for_type::<KL03>();

        assert_eq!(<KL03 as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL03 as KnownLayout>::LAYOUT, sized_layout(1, 1));

        // ... with `align(N)`
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(align(64))]
        struct KL03Align(NotKnownLayout<AU32>, u8);

        let expected = DstLayout::for_type::<KL03Align>();

        assert_eq!(<KL03Align as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL03Align as KnownLayout>::LAYOUT, sized_layout(64, 64));

        // ... with `packed`:
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(packed)]
        struct KL03Packed(NotKnownLayout<AU32>, u8);

        let expected = DstLayout::for_type::<KL03Packed>();

        assert_eq!(<KL03Packed as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL03Packed as KnownLayout>::LAYOUT, sized_layout(1, 5));

        // ... with `packed(N)`
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(packed(2))]
        struct KL03PackedN(NotKnownLayout<AU32>, u8);

        assert_impl_all!(KL03PackedN: KnownLayout);

        let expected = DstLayout::for_type::<KL03PackedN>();

        assert_eq!(<KL03PackedN as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL03PackedN as KnownLayout>::LAYOUT, sized_layout(2, 6));

        // | `repr(C)`? | generic? | `KnownLayout`? | `Sized`? | Type Name |
        // |          N |        Y |              N |        Y |      KL05 |
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        struct KL05<T>(u8, T);

        fn _test_kl05<T>(t: T) -> impl KnownLayout {
            KL05(0u8, t)
        }

        // | `repr(C)`? | generic? | `KnownLayout`? | `Sized`? | Type Name |
        // |          N |        Y |              Y |        Y |      KL07 |
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        struct KL07<T: KnownLayout>(u8, T);

        fn _test_kl07<T: KnownLayout>(t: T) -> impl KnownLayout {
            let _ = KL07(0u8, t);
        }

        // | `repr(C)`? | generic? | `KnownLayout`? | `Sized`? | Type Name |
        // |          Y |        N |              Y |        N |      KL10 |
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(C)]
        struct KL10(NotKnownLayout<AU32>, [u8]);

        let expected = DstLayout::new_zst(None)
            .extend(DstLayout::for_type::<NotKnownLayout<AU32>>(), None)
            .extend(<[u8] as KnownLayout>::LAYOUT, None)
            .pad_to_align();

        assert_eq!(<KL10 as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL10 as KnownLayout>::LAYOUT, unsized_layout(4, 1, 4, false));

        // ...with `align(N)`:
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(C, align(64))]
        struct KL10Align(NotKnownLayout<AU32>, [u8]);

        let repr_align = NonZeroUsize::new(64);

        let expected = DstLayout::new_zst(repr_align)
            .extend(DstLayout::for_type::<NotKnownLayout<AU32>>(), None)
            .extend(<[u8] as KnownLayout>::LAYOUT, None)
            .pad_to_align();

        assert_eq!(<KL10Align as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL10Align as KnownLayout>::LAYOUT, unsized_layout(64, 1, 4, false));

        // ...with `packed`:
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(C, packed)]
        struct KL10Packed(NotKnownLayout<AU32>, [u8]);

        let repr_packed = NonZeroUsize::new(1);

        let expected = DstLayout::new_zst(None)
            .extend(DstLayout::for_type::<NotKnownLayout<AU32>>(), repr_packed)
            .extend(<[u8] as KnownLayout>::LAYOUT, repr_packed)
            .pad_to_align();

        assert_eq!(<KL10Packed as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL10Packed as KnownLayout>::LAYOUT, unsized_layout(1, 1, 4, false));

        // ...with `packed(N)`:
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(C, packed(2))]
        struct KL10PackedN(NotKnownLayout<AU32>, [u8]);

        let repr_packed = NonZeroUsize::new(2);

        let expected = DstLayout::new_zst(None)
            .extend(DstLayout::for_type::<NotKnownLayout<AU32>>(), repr_packed)
            .extend(<[u8] as KnownLayout>::LAYOUT, repr_packed)
            .pad_to_align();

        assert_eq!(<KL10PackedN as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL10PackedN as KnownLayout>::LAYOUT, unsized_layout(2, 1, 4, false));

        // | `repr(C)`? | generic? | `KnownLayout`? | `Sized`? | Type Name |
        // |          Y |        N |              Y |        Y |      KL11 |
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(C)]
        struct KL11(NotKnownLayout<AU64>, u8);

        let expected = DstLayout::new_zst(None)
            .extend(DstLayout::for_type::<NotKnownLayout<AU64>>(), None)
            .extend(<u8 as KnownLayout>::LAYOUT, None)
            .pad_to_align();

        assert_eq!(<KL11 as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL11 as KnownLayout>::LAYOUT, sized_layout(8, 16));

        // ...with `align(N)`:
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(C, align(64))]
        struct KL11Align(NotKnownLayout<AU64>, u8);

        let repr_align = NonZeroUsize::new(64);

        let expected = DstLayout::new_zst(repr_align)
            .extend(DstLayout::for_type::<NotKnownLayout<AU64>>(), None)
            .extend(<u8 as KnownLayout>::LAYOUT, None)
            .pad_to_align();

        assert_eq!(<KL11Align as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL11Align as KnownLayout>::LAYOUT, sized_layout(64, 64));

        // ...with `packed`:
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(C, packed)]
        struct KL11Packed(NotKnownLayout<AU64>, u8);

        let repr_packed = NonZeroUsize::new(1);

        let expected = DstLayout::new_zst(None)
            .extend(DstLayout::for_type::<NotKnownLayout<AU64>>(), repr_packed)
            .extend(<u8 as KnownLayout>::LAYOUT, repr_packed)
            .pad_to_align();

        assert_eq!(<KL11Packed as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL11Packed as KnownLayout>::LAYOUT, sized_layout(1, 9));

        // ...with `packed(N)`:
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(C, packed(2))]
        struct KL11PackedN(NotKnownLayout<AU64>, u8);

        let repr_packed = NonZeroUsize::new(2);

        let expected = DstLayout::new_zst(None)
            .extend(DstLayout::for_type::<NotKnownLayout<AU64>>(), repr_packed)
            .extend(<u8 as KnownLayout>::LAYOUT, repr_packed)
            .pad_to_align();

        assert_eq!(<KL11PackedN as KnownLayout>::LAYOUT, expected);
        assert_eq!(<KL11PackedN as KnownLayout>::LAYOUT, sized_layout(2, 10));

        // | `repr(C)`? | generic? | `KnownLayout`? | `Sized`? | Type Name |
        // |          Y |        Y |              Y |        N |      KL14 |
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(C)]
        struct KL14<T: ?Sized + KnownLayout>(u8, T);

        fn _test_kl14<T: ?Sized + KnownLayout>(kl: &KL14<T>) {
            _assert_kl(kl)
        }

        // | `repr(C)`? | generic? | `KnownLayout`? | `Sized`? | Type Name |
        // |          Y |        Y |              Y |        Y |      KL15 |
        #[allow(dead_code)]
        #[derive(KnownLayout)]
        #[repr(C)]
        struct KL15<T: KnownLayout>(u8, T);

        fn _test_kl15<T: KnownLayout>(t: T) -> impl KnownLayout {
            let _ = KL15(0u8, t);
        }

        // Test a variety of combinations of field types:
        //  - ()
        //  - u8
        //  - AU16
        //  - [()]
        //  - [u8]
        //  - [AU16]

        #[allow(clippy::upper_case_acronyms, dead_code)]
        #[derive(KnownLayout)]
        #[repr(C)]
        struct KLTU<T, U: ?Sized>(T, U);

        assert_eq!(<KLTU<(), ()> as KnownLayout>::LAYOUT, sized_layout(1, 0));

        assert_eq!(<KLTU<(), u8> as KnownLayout>::LAYOUT, sized_layout(1, 1));

        assert_eq!(<KLTU<(), AU16> as KnownLayout>::LAYOUT, sized_layout(2, 2));

        assert_eq!(<KLTU<(), [()]> as KnownLayout>::LAYOUT, unsized_layout(1, 0, 0, false));

        assert_eq!(<KLTU<(), [u8]> as KnownLayout>::LAYOUT, unsized_layout(1, 1, 0, false));

        assert_eq!(<KLTU<(), [AU16]> as KnownLayout>::LAYOUT, unsized_layout(2, 2, 0, false));

        assert_eq!(<KLTU<u8, ()> as KnownLayout>::LAYOUT, sized_layout(1, 1));

        assert_eq!(<KLTU<u8, u8> as KnownLayout>::LAYOUT, sized_layout(1, 2));

        assert_eq!(<KLTU<u8, AU16> as KnownLayout>::LAYOUT, sized_layout(2, 4));

        assert_eq!(<KLTU<u8, [()]> as KnownLayout>::LAYOUT, unsized_layout(1, 0, 1, false));

        assert_eq!(<KLTU<u8, [u8]> as KnownLayout>::LAYOUT, unsized_layout(1, 1, 1, false));

        assert_eq!(<KLTU<u8, [AU16]> as KnownLayout>::LAYOUT, unsized_layout(2, 2, 2, false));

        assert_eq!(<KLTU<AU16, ()> as KnownLayout>::LAYOUT, sized_layout(2, 2));

        assert_eq!(<KLTU<AU16, u8> as KnownLayout>::LAYOUT, sized_layout(2, 4));

        assert_eq!(<KLTU<AU16, AU16> as KnownLayout>::LAYOUT, sized_layout(2, 4));

        assert_eq!(<KLTU<AU16, [()]> as KnownLayout>::LAYOUT, unsized_layout(2, 0, 2, false));

        assert_eq!(<KLTU<AU16, [u8]> as KnownLayout>::LAYOUT, unsized_layout(2, 1, 2, false));

        assert_eq!(<KLTU<AU16, [AU16]> as KnownLayout>::LAYOUT, unsized_layout(2, 2, 2, false));

        // Test a variety of field counts.

        #[derive(KnownLayout)]
        #[repr(C)]
        struct KLF0;

        assert_eq!(<KLF0 as KnownLayout>::LAYOUT, sized_layout(1, 0));

        #[derive(KnownLayout)]
        #[repr(C)]
        struct KLF1([u8]);

        assert_eq!(<KLF1 as KnownLayout>::LAYOUT, unsized_layout(1, 1, 0, true));

        #[derive(KnownLayout)]
        #[repr(C)]
        struct KLF2(NotKnownLayout<u8>, [u8]);

        assert_eq!(<KLF2 as KnownLayout>::LAYOUT, unsized_layout(1, 1, 1, false));

        #[derive(KnownLayout)]
        #[repr(C)]
        struct KLF3(NotKnownLayout<u8>, NotKnownLayout<AU16>, [u8]);

        assert_eq!(<KLF3 as KnownLayout>::LAYOUT, unsized_layout(2, 1, 4, false));

        #[derive(KnownLayout)]
        #[repr(C)]
        struct KLF4(NotKnownLayout<u8>, NotKnownLayout<AU16>, NotKnownLayout<AU32>, [u8]);

        assert_eq!(<KLF4 as KnownLayout>::LAYOUT, unsized_layout(4, 1, 8, false));
    }

    #[test]
    fn test_object_safety() {
        fn _takes_immutable(_: &dyn Immutable) {}
        fn _takes_unaligned(_: &dyn Unaligned) {}
    }

    #[test]
    fn test_from_zeros_only() {
        // Test types that implement `FromZeros` but not `FromBytes`.

        assert!(!bool::new_zeroed());
        assert_eq!(char::new_zeroed(), '\0');

        #[cfg(feature = "alloc")]
        {
            assert_eq!(bool::new_box_zeroed(), Ok(Box::new(false)));
            assert_eq!(char::new_box_zeroed(), Ok(Box::new('\0')));

            assert_eq!(
                <[bool]>::new_box_zeroed_with_elems(3).unwrap().as_ref(),
                [false, false, false]
            );
            assert_eq!(
                <[char]>::new_box_zeroed_with_elems(3).unwrap().as_ref(),
                ['\0', '\0', '\0']
            );

            assert_eq!(bool::new_vec_zeroed(3).unwrap().as_ref(), [false, false, false]);
            assert_eq!(char::new_vec_zeroed(3).unwrap().as_ref(), ['\0', '\0', '\0']);
        }

        let mut string = "hello".to_string();
        let s: &mut str = string.as_mut();
        assert_eq!(s, "hello");
        s.zero();
        assert_eq!(s, "\0\0\0\0\0");
    }

    #[test]
    fn test_zst_count_preserved() {
        // Test that, when an explicit count is provided to for a type with a
        // ZST trailing slice element, that count is preserved. This is
        // important since, for such types, all element counts result in objects
        // of the same size, and so the correct behavior is ambiguous. However,
        // preserving the count as requested by the user is the behavior that we
        // document publicly.

        // FromZeros methods
        #[cfg(feature = "alloc")]
        assert_eq!(<[()]>::new_box_zeroed_with_elems(3).unwrap().len(), 3);
        #[cfg(feature = "alloc")]
        assert_eq!(<()>::new_vec_zeroed(3).unwrap().len(), 3);

        // FromBytes methods
        assert_eq!(<[()]>::ref_from_bytes_with_elems(&[][..], 3).unwrap().len(), 3);
        assert_eq!(<[()]>::ref_from_prefix_with_elems(&[][..], 3).unwrap().0.len(), 3);
        assert_eq!(<[()]>::ref_from_suffix_with_elems(&[][..], 3).unwrap().1.len(), 3);
        assert_eq!(<[()]>::mut_from_bytes_with_elems(&mut [][..], 3).unwrap().len(), 3);
        assert_eq!(<[()]>::mut_from_prefix_with_elems(&mut [][..], 3).unwrap().0.len(), 3);
        assert_eq!(<[()]>::mut_from_suffix_with_elems(&mut [][..], 3).unwrap().1.len(), 3);
    }

    #[test]
    fn test_read_write() {
        const VAL: u64 = 0x12345678;
        #[cfg(target_endian = "big")]
        const VAL_BYTES: [u8; 8] = VAL.to_be_bytes();
        #[cfg(target_endian = "little")]
        const VAL_BYTES: [u8; 8] = VAL.to_le_bytes();
        const ZEROS: [u8; 8] = [0u8; 8];

        // Test `FromBytes::{read_from, read_from_prefix, read_from_suffix}`.

        assert_eq!(u64::read_from_bytes(&VAL_BYTES[..]), Ok(VAL));
        // The first 8 bytes are from `VAL_BYTES` and the second 8 bytes are all
        // zeros.
        let bytes_with_prefix: [u8; 16] = transmute!([VAL_BYTES, [0; 8]]);
        assert_eq!(u64::read_from_prefix(&bytes_with_prefix[..]), Ok((VAL, &ZEROS[..])));
        assert_eq!(u64::read_from_suffix(&bytes_with_prefix[..]), Ok((&VAL_BYTES[..], 0)));
        // The first 8 bytes are all zeros and the second 8 bytes are from
        // `VAL_BYTES`
        let bytes_with_suffix: [u8; 16] = transmute!([[0; 8], VAL_BYTES]);
        assert_eq!(u64::read_from_prefix(&bytes_with_suffix[..]), Ok((0, &VAL_BYTES[..])));
        assert_eq!(u64::read_from_suffix(&bytes_with_suffix[..]), Ok((&ZEROS[..], VAL)));

        // Test `IntoBytes::{write_to, write_to_prefix, write_to_suffix}`.

        let mut bytes = [0u8; 8];
        assert_eq!(VAL.write_to(&mut bytes[..]), Ok(()));
        assert_eq!(bytes, VAL_BYTES);
        let mut bytes = [0u8; 16];
        assert_eq!(VAL.write_to_prefix(&mut bytes[..]), Ok(()));
        let want: [u8; 16] = transmute!([VAL_BYTES, [0; 8]]);
        assert_eq!(bytes, want);
        let mut bytes = [0u8; 16];
        assert_eq!(VAL.write_to_suffix(&mut bytes[..]), Ok(()));
        let want: [u8; 16] = transmute!([[0; 8], VAL_BYTES]);
        assert_eq!(bytes, want);
    }

    #[test]
    #[cfg(feature = "std")]
    fn test_read_io_with_padding_soundness() {
        // This test is designed to exhibit potential UB in
        // `FromBytes::read_from_io`. (see #2319, #2320).

        // On most platforms (where `align_of::<u16>() == 2`), `WithPadding`
        // will have inter-field padding between `x` and `y`.
        #[derive(FromBytes)]
        #[repr(C)]
        struct WithPadding {
            x: u8,
            y: u16,
        }
        struct ReadsInRead;
        impl std::io::Read for ReadsInRead {
            fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
                // This body branches on every byte of `buf`, ensuring that it
                // exhibits UB if any byte of `buf` is uninitialized.
                if buf.iter().all(|&x| x == 0) {
                    Ok(buf.len())
                } else {
                    buf.iter_mut().for_each(|x| *x = 0);
                    Ok(buf.len())
                }
            }
        }
        assert!(matches!(WithPadding::read_from_io(ReadsInRead), Ok(WithPadding { x: 0, y: 0 })));
    }

    #[test]
    #[cfg(feature = "std")]
    fn test_read_write_io() {
        let mut long_buffer = [0, 0, 0, 0];
        assert!(matches!(u16::MAX.write_to_io(&mut long_buffer[..]), Ok(())));
        assert_eq!(long_buffer, [255, 255, 0, 0]);
        assert!(matches!(u16::read_from_io(&long_buffer[..]), Ok(u16::MAX)));

        let mut short_buffer = [0, 0];
        assert!(u32::MAX.write_to_io(&mut short_buffer[..]).is_err());
        assert_eq!(short_buffer, [255, 255]);
        assert!(u32::read_from_io(&short_buffer[..]).is_err());
    }

    #[test]
    fn test_try_from_bytes_try_read_from() {
        assert_eq!(<bool as TryFromBytes>::try_read_from_bytes(&[0]), Ok(false));
        assert_eq!(<bool as TryFromBytes>::try_read_from_bytes(&[1]), Ok(true));

        assert_eq!(<bool as TryFromBytes>::try_read_from_prefix(&[0, 2]), Ok((false, &[2][..])));
        assert_eq!(<bool as TryFromBytes>::try_read_from_prefix(&[1, 2]), Ok((true, &[2][..])));

        assert_eq!(<bool as TryFromBytes>::try_read_from_suffix(&[2, 0]), Ok((&[2][..], false)));
        assert_eq!(<bool as TryFromBytes>::try_read_from_suffix(&[2, 1]), Ok((&[2][..], true)));

        // If we don't pass enough bytes, it fails.
        assert!(matches!(
            <u8 as TryFromBytes>::try_read_from_bytes(&[]),
            Err(TryReadError::Size(_))
        ));
        assert!(matches!(
            <u8 as TryFromBytes>::try_read_from_prefix(&[]),
            Err(TryReadError::Size(_))
        ));
        assert!(matches!(
            <u8 as TryFromBytes>::try_read_from_suffix(&[]),
            Err(TryReadError::Size(_))
        ));

        // If we pass too many bytes, it fails.
        assert!(matches!(
            <u8 as TryFromBytes>::try_read_from_bytes(&[0, 0]),
            Err(TryReadError::Size(_))
        ));

        // If we pass an invalid value, it fails.
        assert!(matches!(
            <bool as TryFromBytes>::try_read_from_bytes(&[2]),
            Err(TryReadError::Validity(_))
        ));
        assert!(matches!(
            <bool as TryFromBytes>::try_read_from_prefix(&[2, 0]),
            Err(TryReadError::Validity(_))
        ));
        assert!(matches!(
            <bool as TryFromBytes>::try_read_from_suffix(&[0, 2]),
            Err(TryReadError::Validity(_))
        ));

        // Reading from a misaligned buffer should still succeed. Since `AU64`'s
        // alignment is 8, and since we read from two adjacent addresses one
        // byte apart, it is guaranteed that at least one of them (though
        // possibly both) will be misaligned.
        let bytes: [u8; 9] = [0, 0, 0, 0, 0, 0, 0, 0, 0];
        assert_eq!(<AU64 as TryFromBytes>::try_read_from_bytes(&bytes[..8]), Ok(AU64(0)));
        assert_eq!(<AU64 as TryFromBytes>::try_read_from_bytes(&bytes[1..9]), Ok(AU64(0)));

        assert_eq!(
            <AU64 as TryFromBytes>::try_read_from_prefix(&bytes[..8]),
            Ok((AU64(0), &[][..]))
        );
        assert_eq!(
            <AU64 as TryFromBytes>::try_read_from_prefix(&bytes[1..9]),
            Ok((AU64(0), &[][..]))
        );

        assert_eq!(
            <AU64 as TryFromBytes>::try_read_from_suffix(&bytes[..8]),
            Ok((&[][..], AU64(0)))
        );
        assert_eq!(
            <AU64 as TryFromBytes>::try_read_from_suffix(&bytes[1..9]),
            Ok((&[][..], AU64(0)))
        );
    }

    #[test]
    fn test_try_read_from_typed_source() {
        #[derive(IntoBytes, Immutable)]
        #[repr(transparent)]
        struct Source<T: ?Sized>(T);

        fn check<S: IntoBytes + Immutable + ?Sized>(source: &S, valid: bool) {
            match try_read_from::<_, [bool; 4]>(source) {
                Ok(value) => {
                    assert!(valid);
                    assert_eq!(value, [false, true, false, true]);
                }
                Err(error) => {
                    assert!(!valid);
                    assert!(matches!(error, TryReadError::Validity(_)));
                    assert!(ptr::eq(error.into_src(), source));
                }
            }

            let error = try_read_from::<_, [bool; 5]>(source).unwrap_err();
            assert!(matches!(error, TryReadError::Size(_)));
            assert!(ptr::eq(error.into_src(), source));
        }

        for (value, valid) in [(u16::from_ne_bytes([0, 1]), true), (0x0202, false)] {
            let source = Source([value; 2]);
            check(&source, valid);
            let source: &Source<[u16]> = &source;
            check(source, valid);
        }

        let source = Source([(); 2]);
        assert!(try_read_from::<_, ()>(&source).is_ok());
        let source: &Source<[()]> = &source;
        assert!(try_read_from::<_, ()>(source).is_ok());
    }

    #[test]
    fn test_try_read_initialized_padding() {
        #[derive(KnownLayout, Immutable, Debug, PartialEq, Eq)]
        #[repr(C, align(8))]
        struct Padded(u8);

        // SAFETY: `Padded` has only a `u8` field and padding, so all
        // initialized byte sequences are valid. Per
        // https://doc.rust-lang.org/1.93.1/reference/behavior-considered-undefined.html#invalid-values:
        //
        //   An integer (`i*`/`u*`), floating point value (`f*`), or raw pointer
        //   must be initialized, i.e., must not be obtained from uninitialized
        //   memory.
        //
        //   A `struct`, tuple, and array requires all fields/elements to be
        //   valid at their respective type.
        const _: () = unsafe {
            unsafe_impl!(=> TryFromBytes for Padded; |candidate| {
                // Observe every byte while validation is running, including
                // padding that a typed move of `MaybeUninit<Padded>` could
                // discard. The returned `Padded` need not retain its padding.
                assert_eq!(candidate.as_bytes::<BecauseImmutable>().as_ref(), &[0xA5; 8]);
                true
            })
        };

        let source = [0xA5; 8];
        assert_eq!(Padded::try_read_from_bytes(&source), Ok(Padded(0xA5)));

        let prefix_source = [0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0];
        let (value, suffix) = Padded::try_read_from_prefix(&prefix_source).unwrap();
        assert_eq!(value, Padded(0xA5));
        assert!(ptr::eq(suffix, &prefix_source[8..]));

        let suffix_source = [0, 0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0xA5, 0xA5];
        let (prefix, value) = Padded::try_read_from_suffix(&suffix_source).unwrap();
        assert_eq!(value, Padded(0xA5));
        assert!(ptr::eq(prefix, &suffix_source[..1]));
    }

    #[test]
    fn test_try_read_error_sources_and_zero_sized() {
        let invalid = [2, 3];
        for (error, source) in [
            (bool::try_read_from_bytes(&invalid[..1]).unwrap_err(), &invalid[..1]),
            (bool::try_read_from_prefix(&invalid).unwrap_err(), &invalid[..]),
            (bool::try_read_from_suffix(&invalid).unwrap_err(), &invalid[..]),
        ] {
            assert!(matches!(error, TryReadError::Validity(_)));
            assert!(ptr::eq(error.into_src(), source));
        }
        for source in [&[][..], &invalid[..]] {
            let error = bool::try_read_from_bytes(source).unwrap_err();
            assert!(matches!(error, TryReadError::Size(_)));
            assert!(ptr::eq(error.into_src(), source));
        }
        for error in [
            bool::try_read_from_prefix(&invalid[..0]).unwrap_err(),
            bool::try_read_from_suffix(&invalid[..0]).unwrap_err(),
        ] {
            assert!(matches!(error, TryReadError::Size(_)));
            assert!(ptr::eq(error.into_src(), &invalid[..0]));
        }

        assert_eq!(<()>::try_read_from_bytes(&[]), Ok(()));
        assert_eq!(<()>::try_read_from_prefix(&invalid), Ok(((), &invalid[..])));
        assert_eq!(<()>::try_read_from_suffix(&invalid), Ok((&invalid[..], ())));
    }

    #[test]
    fn test_ref_from_mut_from_bytes() {
        // Test `FromBytes::{ref_from_bytes, mut_from_bytes}{,_prefix,Suffix}`
        // success cases. Exhaustive coverage for these methods is covered by
        // the `Ref` tests above, which these helper methods defer to.

        let mut buf =
            Align::<[u8; 16], AU64>::new([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]);

        assert_eq!(
            AU64::ref_from_bytes(&buf.t[8..]).unwrap().0.to_ne_bytes(),
            [8, 9, 10, 11, 12, 13, 14, 15]
        );
        let suffix = AU64::mut_from_bytes(&mut buf.t[8..]).unwrap();
        suffix.0 = 0x0101010101010101;
        // The `[u8:9]` is a non-half size of the full buffer, which would catch
        // `from_prefix` having the same implementation as `from_suffix` (issues #506, #511).
        assert_eq!(
            <[u8; 9]>::ref_from_suffix(&buf.t[..]).unwrap(),
            (&[0, 1, 2, 3, 4, 5, 6][..], &[7u8, 1, 1, 1, 1, 1, 1, 1, 1])
        );
        let (prefix, suffix) = AU64::mut_from_suffix(&mut buf.t[1..]).unwrap();
        assert_eq!(prefix, &mut [1u8, 2, 3, 4, 5, 6, 7][..]);
        suffix.0 = 0x0202020202020202;
        let (prefix, suffix) = <[u8; 10]>::mut_from_suffix(&mut buf.t[..]).unwrap();
        assert_eq!(prefix, &mut [0u8, 1, 2, 3, 4, 5][..]);
        suffix[0] = 42;
        assert_eq!(
            <[u8; 9]>::ref_from_prefix(&buf.t[..]).unwrap(),
            (&[0u8, 1, 2, 3, 4, 5, 42, 7, 2], &[2u8, 2, 2, 2, 2, 2, 2][..])
        );
        <[u8; 2]>::mut_from_prefix(&mut buf.t[..]).unwrap().0[1] = 30;
        assert_eq!(buf.t, [0, 30, 2, 3, 4, 5, 42, 7, 2, 2, 2, 2, 2, 2, 2, 2]);
    }

    #[test]
    fn test_ref_from_mut_from_bytes_error() {
        // Test `FromBytes::{ref_from_bytes, mut_from_bytes}{,_prefix,Suffix}`
        // error cases.

        // Fail because the buffer is too large.
        let mut buf = Align::<[u8; 16], AU64>::default();
        // `buf.t` should be aligned to 8, so only the length check should fail.
        assert!(AU64::ref_from_bytes(&buf.t[..]).is_err());
        assert!(AU64::mut_from_bytes(&mut buf.t[..]).is_err());
        assert!(<[u8; 8]>::ref_from_bytes(&buf.t[..]).is_err());
        assert!(<[u8; 8]>::mut_from_bytes(&mut buf.t[..]).is_err());

        // Fail because the buffer is too small.
        let mut buf = Align::<[u8; 4], AU64>::default();
        assert!(AU64::ref_from_bytes(&buf.t[..]).is_err());
        assert!(AU64::mut_from_bytes(&mut buf.t[..]).is_err());
        assert!(<[u8; 8]>::ref_from_bytes(&buf.t[..]).is_err());
        assert!(<[u8; 8]>::mut_from_bytes(&mut buf.t[..]).is_err());
        assert!(AU64::ref_from_prefix(&buf.t[..]).is_err());
        assert!(AU64::mut_from_prefix(&mut buf.t[..]).is_err());
        assert!(AU64::ref_from_suffix(&buf.t[..]).is_err());
        assert!(AU64::mut_from_suffix(&mut buf.t[..]).is_err());
        assert!(<[u8; 8]>::ref_from_prefix(&buf.t[..]).is_err());
        assert!(<[u8; 8]>::mut_from_prefix(&mut buf.t[..]).is_err());
        assert!(<[u8; 8]>::ref_from_suffix(&buf.t[..]).is_err());
        assert!(<[u8; 8]>::mut_from_suffix(&mut buf.t[..]).is_err());

        // Fail because the alignment is insufficient.
        let mut buf = Align::<[u8; 13], AU64>::default();
        assert!(AU64::ref_from_bytes(&buf.t[1..]).is_err());
        assert!(AU64::mut_from_bytes(&mut buf.t[1..]).is_err());
        assert!(AU64::ref_from_bytes(&buf.t[1..]).is_err());
        assert!(AU64::mut_from_bytes(&mut buf.t[1..]).is_err());
        assert!(AU64::ref_from_prefix(&buf.t[1..]).is_err());
        assert!(AU64::mut_from_prefix(&mut buf.t[1..]).is_err());
        assert!(AU64::ref_from_suffix(&buf.t[..]).is_err());
        assert!(AU64::mut_from_suffix(&mut buf.t[..]).is_err());
    }

    #[test]
    fn test_to_methods() {
        /// Run a series of tests by calling `IntoBytes` methods on `t`.
        ///
        /// `bytes` is the expected byte sequence returned from `t.as_bytes()`
        /// before `t` has been modified. `post_mutation` is the expected
        /// sequence returned from `t.as_bytes()` after `t.as_mut_bytes()[0]`
        /// has had its bits flipped (by applying `^= 0xFF`).
        ///
        /// `N` is the size of `t` in bytes.
        fn test<T: FromBytes + IntoBytes + Immutable + Debug + Eq + ?Sized, const N: usize>(
            t: &mut T,
            bytes: &[u8],
            post_mutation: &T,
        ) {
            // Test that we can access the underlying bytes, and that we get the
            // right bytes and the right number of bytes.
            assert_eq!(t.as_bytes(), bytes);

            // Test that changes to the underlying byte slices are reflected in
            // the original object.
            t.as_mut_bytes()[0] ^= 0xFF;
            assert_eq!(t, post_mutation);
            t.as_mut_bytes()[0] ^= 0xFF;

            // `write_to` rejects slices that are too small or too large.
            assert!(t.write_to(&mut vec![0; N - 1][..]).is_err());
            assert!(t.write_to(&mut vec![0; N + 1][..]).is_err());

            // `write_to` works as expected.
            let mut bytes = [0; N];
            assert_eq!(t.write_to(&mut bytes[..]), Ok(()));
            assert_eq!(bytes, t.as_bytes());

            // `write_to_prefix` rejects slices that are too small.
            assert!(t.write_to_prefix(&mut vec![0; N - 1][..]).is_err());

            // `write_to_prefix` works with exact-sized slices.
            let mut bytes = [0; N];
            assert_eq!(t.write_to_prefix(&mut bytes[..]), Ok(()));
            assert_eq!(bytes, t.as_bytes());

            // `write_to_prefix` works with too-large slices, and any bytes past
            // the prefix aren't modified.
            let mut too_many_bytes = vec![0; N + 1];
            too_many_bytes[N] = 123;
            assert_eq!(t.write_to_prefix(&mut too_many_bytes[..]), Ok(()));
            assert_eq!(&too_many_bytes[..N], t.as_bytes());
            assert_eq!(too_many_bytes[N], 123);

            // `write_to_suffix` rejects slices that are too small.
            assert!(t.write_to_suffix(&mut vec![0; N - 1][..]).is_err());

            // `write_to_suffix` works with exact-sized slices.
            let mut bytes = [0; N];
            assert_eq!(t.write_to_suffix(&mut bytes[..]), Ok(()));
            assert_eq!(bytes, t.as_bytes());

            // `write_to_suffix` works with too-large slices, and any bytes
            // before the suffix aren't modified.
            let mut too_many_bytes = vec![0; N + 1];
            too_many_bytes[0] = 123;
            assert_eq!(t.write_to_suffix(&mut too_many_bytes[..]), Ok(()));
            assert_eq!(&too_many_bytes[1..], t.as_bytes());
            assert_eq!(too_many_bytes[0], 123);
        }

        #[derive(Debug, Eq, PartialEq, FromBytes, IntoBytes, Immutable)]
        #[repr(C)]
        struct Foo {
            a: u32,
            b: Wrapping<u32>,
            c: Option<NonZeroU32>,
        }

        let expected_bytes: Vec<u8> = if cfg!(target_endian = "little") {
            vec![1, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0]
        } else {
            vec![0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 0]
        };
        let post_mutation_expected_a =
            if cfg!(target_endian = "little") { 0x00_00_00_FE } else { 0xFF_00_00_01 };
        test::<_, 12>(
            &mut Foo { a: 1, b: Wrapping(2), c: None },
            expected_bytes.as_bytes(),
            &Foo { a: post_mutation_expected_a, b: Wrapping(2), c: None },
        );
        test::<_, 3>(
            Unsized::from_mut_slice(&mut [1, 2, 3]),
            &[1, 2, 3],
            Unsized::from_mut_slice(&mut [0xFE, 2, 3]),
        );
    }

    #[test]
    fn test_array() {
        #[derive(FromBytes, IntoBytes, Immutable)]
        #[repr(C)]
        struct Foo {
            a: [u16; 33],
        }

        let foo = Foo { a: [0xFFFF; 33] };
        let expected = [0xFFu8; 66];
        assert_eq!(foo.as_bytes(), &expected[..]);
    }

    #[test]
    fn test_new_zeroed() {
        assert!(!bool::new_zeroed());
        assert_eq!(u64::new_zeroed(), 0);
        // This test exists in order to exercise unsafe code, especially when
        // running under Miri.
        #[allow(clippy::unit_cmp)]
        {
            assert_eq!(<()>::new_zeroed(), ());
        }
    }

    #[test]
    fn test_transparent_packed_generic_struct() {
        #[derive(IntoBytes, FromBytes, Unaligned)]
        #[repr(transparent)]
        #[allow(dead_code)] // We never construct this type
        struct Foo<T> {
            _t: T,
            _phantom: PhantomData<()>,
        }

        assert_impl_all!(Foo<u32>: FromZeros, FromBytes, IntoBytes);
        assert_impl_all!(Foo<u8>: Unaligned);

        #[derive(IntoBytes, FromBytes, Unaligned)]
        #[repr(C, packed)]
        #[allow(dead_code)] // We never construct this type
        struct Bar<T, U> {
            _t: T,
            _u: U,
        }

        assert_impl_all!(Bar<u8, AU64>: FromZeros, FromBytes, IntoBytes, Unaligned);
    }

    #[cfg(feature = "alloc")]
    mod alloc {
        use super::*;

        #[cfg(not(no_zerocopy_panic_in_const_and_vec_try_reserve_1_57_0))]
        #[test]
        fn test_extend_vec_zeroed() {
            // Test extending when there is an existing allocation.
            let mut v = vec![100u16, 200, 300];
            FromZeros::extend_vec_zeroed(&mut v, 3).unwrap();
            assert_eq!(v.len(), 6);
            assert_eq!(&*v, &[100, 200, 300, 0, 0, 0]);
            drop(v);

            // Test extending when there is no existing allocation.
            let mut v: Vec<u64> = Vec::new();
            FromZeros::extend_vec_zeroed(&mut v, 3).unwrap();
            assert_eq!(v.len(), 3);
            assert_eq!(&*v, &[0, 0, 0]);
            drop(v);
        }

        #[cfg(not(no_zerocopy_panic_in_const_and_vec_try_reserve_1_57_0))]
        #[test]
        fn test_extend_vec_zeroed_zst() {
            // Test extending when there is an existing (fake) allocation.
            let mut v = vec![(), (), ()];
            <()>::extend_vec_zeroed(&mut v, 3).unwrap();
            assert_eq!(v.len(), 6);
            assert_eq!(&*v, &[(), (), (), (), (), ()]);
            drop(v);

            // Test extending when there is no existing (fake) allocation.
            let mut v: Vec<()> = Vec::new();
            <()>::extend_vec_zeroed(&mut v, 3).unwrap();
            assert_eq!(&*v, &[(), (), ()]);
            drop(v);
        }

        #[cfg(not(no_zerocopy_panic_in_const_and_vec_try_reserve_1_57_0))]
        #[test]
        fn test_insert_vec_zeroed() {
            // Insert at start (no existing allocation).
            let mut v: Vec<u64> = Vec::new();
            u64::insert_vec_zeroed(&mut v, 0, 2).unwrap();
            assert_eq!(v.len(), 2);
            assert_eq!(&*v, &[0, 0]);
            drop(v);

            // Insert at start.
            let mut v = vec![100u64, 200, 300];
            u64::insert_vec_zeroed(&mut v, 0, 2).unwrap();
            assert_eq!(v.len(), 5);
            assert_eq!(&*v, &[0, 0, 100, 200, 300]);
            drop(v);

            // Insert at middle.
            let mut v = vec![100u64, 200, 300];
            u64::insert_vec_zeroed(&mut v, 1, 1).unwrap();
            assert_eq!(v.len(), 4);
            assert_eq!(&*v, &[100, 0, 200, 300]);
            drop(v);

            // Insert at end.
            let mut v = vec![100u64, 200, 300];
            u64::insert_vec_zeroed(&mut v, 3, 1).unwrap();
            assert_eq!(v.len(), 4);
            assert_eq!(&*v, &[100, 200, 300, 0]);
            drop(v);
        }

        #[cfg(not(no_zerocopy_panic_in_const_and_vec_try_reserve_1_57_0))]
        #[test]
        fn test_insert_vec_zeroed_zst() {
            // Insert at start (no existing fake allocation).
            let mut v: Vec<()> = Vec::new();
            <()>::insert_vec_zeroed(&mut v, 0, 2).unwrap();
            assert_eq!(v.len(), 2);
            assert_eq!(&*v, &[(), ()]);
            drop(v);

            // Insert at start.
            let mut v = vec![(), (), ()];
            <()>::insert_vec_zeroed(&mut v, 0, 2).unwrap();
            assert_eq!(v.len(), 5);
            assert_eq!(&*v, &[(), (), (), (), ()]);
            drop(v);

            // Insert at middle.
            let mut v = vec![(), (), ()];
            <()>::insert_vec_zeroed(&mut v, 1, 1).unwrap();
            assert_eq!(v.len(), 4);
            assert_eq!(&*v, &[(), (), (), ()]);
            drop(v);

            // Insert at end.
            let mut v = vec![(), (), ()];
            <()>::insert_vec_zeroed(&mut v, 3, 1).unwrap();
            assert_eq!(v.len(), 4);
            assert_eq!(&*v, &[(), (), (), ()]);
            drop(v);
        }

        #[test]
        fn test_new_box_zeroed() {
            assert_eq!(u64::new_box_zeroed(), Ok(Box::new(0)));
        }

        #[test]
        fn test_new_box_zeroed_array() {
            drop(<[u32; 0x1000]>::new_box_zeroed());
        }

        #[test]
        fn test_new_box_zeroed_zst() {
            // This test exists in order to exercise unsafe code, especially
            // when running under Miri.
            #[allow(clippy::unit_cmp)]
            {
                assert_eq!(<()>::new_box_zeroed(), Ok(Box::new(())));
            }
        }

        #[test]
        fn test_new_box_zeroed_with_elems() {
            let mut s: Box<[u64]> = <[u64]>::new_box_zeroed_with_elems(3).unwrap();
            assert_eq!(s.len(), 3);
            assert_eq!(&*s, &[0, 0, 0]);
            s[1] = 3;
            assert_eq!(&*s, &[0, 3, 0]);
        }

        #[test]
        fn test_new_box_zeroed_with_elems_nested_packed_dst() {
            let mut value = NestedPackedDstOuter::<[u8]>::new_box_zeroed_with_elems(0).unwrap();
            assert_eq!(mem::size_of_val(&*value), 10);

            // Exercise the whole-object write that exposed the former
            // under-allocation under Miri and AddressSanitizer.
            value.zero();
        }

        #[test]
        fn test_new_box_zeroed_with_elems_empty() {
            let s: Box<[u64]> = <[u64]>::new_box_zeroed_with_elems(0).unwrap();
            assert_eq!(s.len(), 0);
        }

        #[test]
        fn test_new_box_zeroed_with_elems_zst() {
            let mut s: Box<[()]> = <[()]>::new_box_zeroed_with_elems(3).unwrap();
            assert_eq!(s.len(), 3);
            assert!(s.get(10).is_none());
            // This test exists in order to exercise unsafe code, especially
            // when running under Miri.
            #[allow(clippy::unit_cmp)]
            {
                assert_eq!(s[1], ());
            }
            s[2] = ();
        }

        #[test]
        fn test_new_box_zeroed_with_elems_zst_empty() {
            let s: Box<[()]> = <[()]>::new_box_zeroed_with_elems(0).unwrap();
            assert_eq!(s.len(), 0);
        }

        #[test]
        fn new_box_zeroed_with_elems_errors() {
            assert_eq!(<[u16]>::new_box_zeroed_with_elems(usize::MAX), Err(AllocError));

            let max = <usize as core::convert::TryFrom<_>>::try_from(isize::MAX).unwrap();
            assert_eq!(
                <[u16]>::new_box_zeroed_with_elems((max / mem::size_of::<u16>()) + 1),
                Err(AllocError)
            );
        }
    }

    #[test]
    #[allow(deprecated)]
    fn test_deprecated_from_bytes() {
        let val = 0u32;
        let bytes = val.as_bytes();

        assert!(u32::ref_from(bytes).is_some());
        // mut_from needs mut bytes
        let mut val = 0u32;
        let mut_bytes = val.as_mut_bytes();
        assert!(u32::mut_from(mut_bytes).is_some());

        assert!(u32::read_from(bytes).is_some());

        let (slc, rest) = <u32>::slice_from_prefix(bytes, 0).unwrap();
        assert!(slc.is_empty());
        assert_eq!(rest.len(), 4);

        let (rest, slc) = <u32>::slice_from_suffix(bytes, 0).unwrap();
        assert!(slc.is_empty());
        assert_eq!(rest.len(), 4);

        let (slc, rest) = <u32>::mut_slice_from_prefix(mut_bytes, 0).unwrap();
        assert!(slc.is_empty());
        assert_eq!(rest.len(), 4);

        let (rest, slc) = <u32>::mut_slice_from_suffix(mut_bytes, 0).unwrap();
        assert!(slc.is_empty());
        assert_eq!(rest.len(), 4);
    }

    #[test]
    fn test_ref_from_prefix_suffix_typed_source() {
        #[derive(IntoBytes, Immutable)]
        #[repr(transparent)]
        struct Source<T: ?Sized>(T);

        fn check<S: IntoBytes + Immutable + ?Sized>(source: &S) {
            let bytes = source.as_bytes();
            assert_eq!(bytes.len(), 4);

            for cast_type in [CastType::Prefix, CastType::Suffix] {
                let (expected, expected_rest) = match cast_type {
                    CastType::Prefix => (&bytes[..3], &bytes[3..]),
                    CastType::Suffix => (&bytes[1..], &bytes[..1]),
                };
                for meta in [None, Some(1)] {
                    let (value, rest) =
                        ref_from_prefix_suffix::<_, [[u8; 3]]>(source, meta, cast_type).unwrap();
                    assert_eq!(value, &[expected]);
                    assert!(ptr::eq(value.as_bytes(), expected));
                    assert!(ptr::eq(rest, expected_rest));
                }

                let err =
                    ref_from_prefix_suffix::<_, [u8; 5]>(source, None, cast_type).unwrap_err();
                assert!(matches!(err, CastError::Size(_)));
                assert!(ptr::eq(err.into_src(), source));

                let err =
                    ref_from_prefix_suffix::<_, [u8]>(source, Some(5), cast_type).unwrap_err();
                assert!(matches!(err, CastError::Size(_)));
                assert!(ptr::eq(err.into_src(), source));
            }
        }

        check(&Source([0x0102u16, 0x0304]));
        let source = Source([false, true, false, true]);
        let source: &Source<[bool]> = &source;
        check(source);
    }

    #[test]
    fn test_ref_from_prefix_suffix_typed_source_alignment_error() {
        let storage = Align::<[bool; 9], AU64>::new([false; 9]);
        for cast_type in [CastType::Prefix, CastType::Suffix] {
            let source = match cast_type {
                CastType::Prefix => &storage.t[1..],
                CastType::Suffix => &storage.t[..],
            };
            let err = ref_from_prefix_suffix::<_, AU64>(source, None, cast_type).unwrap_err();
            assert!(matches!(err, CastError::Alignment(_)));
            let recovered: &[bool] = err.into_src();
            assert!(ptr::eq(recovered, source));

            let err = try_ref_from_prefix_suffix::<_, AU64>(source, cast_type, None).unwrap_err();
            assert!(matches!(err, TryCastError::Alignment(_)));
            let recovered: &[bool] = err.into_src();
            assert!(ptr::eq(recovered, source));
        }
    }

    #[test]
    fn test_try_ref_from_prefix_suffix_typed_source() {
        #[derive(IntoBytes, Immutable)]
        #[repr(transparent)]
        struct Source<T: ?Sized>(T);

        fn check<S: IntoBytes + Immutable + ?Sized>(source: &S) {
            let bytes = source.as_bytes();
            assert_eq!(bytes, [0, 1, 0, 1]);

            for cast_type in [CastType::Prefix, CastType::Suffix] {
                let (expected, expected_rest) = match cast_type {
                    CastType::Prefix => (&bytes[..3], &bytes[3..]),
                    CastType::Suffix => (&bytes[1..], &bytes[..1]),
                };
                for meta in [None, Some(1)] {
                    let (value, rest) =
                        try_ref_from_prefix_suffix::<_, [[bool; 3]]>(source, cast_type, meta)
                            .unwrap();
                    assert_eq!(value.as_bytes(), expected);
                    assert!(ptr::eq(value.as_bytes(), expected));
                    assert!(ptr::eq(rest, expected_rest));
                }

                let err = try_ref_from_prefix_suffix::<_, [bool; 5]>(source, cast_type, None)
                    .unwrap_err();
                assert!(matches!(err, TryCastError::Size(_)));
                assert!(ptr::eq(err.into_src(), source));

                let err = try_ref_from_prefix_suffix::<_, [bool]>(source, cast_type, Some(5))
                    .unwrap_err();
                assert!(matches!(err, TryCastError::Size(_)));
                assert!(ptr::eq(err.into_src(), source));
            }
        }

        check(&Source([u16::from_ne_bytes([0, 1]); 2]));
        let source = Source([false, true, false, true]);
        let source: &Source<[bool]> = &source;
        check(source);
    }

    #[test]
    fn test_try_ref_from_prefix_suffix_typed_source_validity_error() {
        fn check<S: IntoBytes + Immutable + ?Sized>(source: &S) {
            for cast_type in [CastType::Prefix, CastType::Suffix] {
                let err =
                    try_ref_from_prefix_suffix::<_, bool>(source, cast_type, None).unwrap_err();
                assert!(matches!(err, TryCastError::Validity(_)));
                assert!(ptr::eq(err.into_src(), source));

                for meta in [None, Some(1)] {
                    let err = try_ref_from_prefix_suffix::<_, [bool]>(source, cast_type, meta)
                        .unwrap_err();
                    assert!(matches!(err, TryCastError::Validity(_)));
                    assert!(ptr::eq(err.into_src(), source));
                }
            }
        }

        let source = [u16::from_ne_bytes([2, 2]); 2];
        check(&source);
        check(&source[..]);
        check(source.as_bytes());
    }

    #[test]
    fn test_try_ref_from_prefix_suffix() {
        use crate::util::testutil::Align;
        let bytes = &Align::<[u8; 4], u32>::new([0u8; 4]).t[..];
        let (r, rest): (&u32, &[u8]) = u32::try_ref_from_prefix(bytes).unwrap();
        assert_eq!(*r, 0);
        assert_eq!(rest.len(), 0);

        let (rest, r): (&[u8], &u32) = u32::try_ref_from_suffix(bytes).unwrap();
        assert_eq!(*r, 0);
        assert_eq!(rest.len(), 0);
    }

    #[test]
    fn test_raw_dangling() {
        use crate::util::AsAddress;
        let ptr: NonNull<u32> = u32::raw_dangling();
        assert_eq!(AsAddress::addr(ptr), 1);

        let ptr: NonNull<[u32]> = <[u32]>::raw_dangling();
        assert_eq!(AsAddress::addr(ptr), 1);
    }

    #[test]
    fn test_try_ref_from_prefix_with_elems() {
        use crate::util::testutil::Align;
        let bytes = &Align::<[u8; 8], u32>::new([0u8; 8]).t[..];
        let (r, rest): (&[u32], &[u8]) = <[u32]>::try_ref_from_prefix_with_elems(bytes, 2).unwrap();
        assert_eq!(r.len(), 2);
        assert_eq!(rest.len(), 0);
    }

    #[test]
    fn test_try_ref_from_suffix_with_elems() {
        use crate::util::testutil::Align;
        let bytes = &Align::<[u8; 8], u32>::new([0u8; 8]).t[..];
        let (rest, r): (&[u8], &[u32]) = <[u32]>::try_ref_from_suffix_with_elems(bytes, 2).unwrap();
        assert_eq!(r.len(), 2);
        assert_eq!(rest.len(), 0);
    }
}