zerocopy/util/

macro_util.rs

// SPDX-License-Identifier: (BSD-2-Clause OR Apache-2.0) OR MIT
//
// Copyright 2022 The Fuchsia Authors
//
// Licensed under a BSD-style license <LICENSE-BSD>, 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.

//! Utilities used by macros and by `zerocopy-derive`.
//!
//! These are defined here `zerocopy` rather than in code generated by macros or
//! by `zerocopy-derive` so that they can be compiled once rather than
//! recompiled for every invocation (e.g., if they were defined in generated
//! code, then deriving `IntoBytes` and `FromBytes` on three different types
//! would result in the code in question being emitted and compiled six
//! different times).

#![allow(missing_debug_implementations)]

// FIXME(#29), FIXME(https://github.com/rust-lang/rust/issues/69835): Remove
// this `cfg` when `size_of_val_raw` is stabilized.
#[cfg(__ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS)]
#[cfg(not(target_pointer_width = "16"))]
use core::ptr::{self, NonNull};
use core::{marker::PhantomData, mem, num::Wrapping};

use crate::{
    pointer::{
        cast::CastSizedExact,
        invariant::{Aligned, Initialized, Safe},
        BecauseImmutable,
    },
    FromBytes, Immutable, IntoBytes, KnownLayout, Ptr, ReadOnly, TryFromBytes, ValidityError,
};

/// Projects the type of the field at `Index` in `Self` without regard for field
/// privacy.
///
/// The `Index` parameter is any sort of handle that identifies the field; its
/// definition is the obligation of the implementer.
///
/// # Safety
///
/// Unsafe code may assume that this accurately reflects the definition of
/// `Self`.
pub unsafe trait Field<Index> {
    /// The type of the field at `Index`.
    type Type: ?Sized;
}

#[cfg_attr(
    not(no_zerocopy_diagnostic_on_unimplemented_1_78_0),
    diagnostic::on_unimplemented(
        message = "`{T}` has {PADDING_BYTES} total byte(s) of padding",
        label = "types with padding cannot implement `IntoBytes`",
        note = "consider using `zerocopy::Unalign` to lower the alignment of individual fields",
        note = "consider adding explicit fields where padding would be",
        note = "consider using `#[repr(packed)]` to remove padding"
    )
)]
pub trait PaddingFree<T: ?Sized, const PADDING_BYTES: usize> {}
impl<T: ?Sized> PaddingFree<T, 0> for () {}

// FIXME(#1112): In the slice DST case, we should delegate to *both*
// `PaddingFree` *and* `DynamicPaddingFree` (and probably rename `PaddingFree`
// to `StaticPaddingFree` or something - or introduce a third trait with that
// name) so that we can have more clear error messages.

#[cfg_attr(
    not(no_zerocopy_diagnostic_on_unimplemented_1_78_0),
    diagnostic::on_unimplemented(
        message = "`{T}` has one or more padding bytes",
        label = "types with padding cannot implement `IntoBytes`",
        note = "consider using `zerocopy::Unalign` to lower the alignment of individual fields",
        note = "consider adding explicit fields where padding would be",
        note = "consider using `#[repr(packed)]` to remove padding"
    )
)]
pub trait DynamicPaddingFree<T: ?Sized, const HAS_PADDING: bool> {}
impl<T: ?Sized> DynamicPaddingFree<T, false> for () {}

#[cfg(__ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS)]
#[cfg(not(target_pointer_width = "16"))]
const _64K: usize = 1 << 16;

// FIXME(#29), FIXME(https://github.com/rust-lang/rust/issues/69835): Remove
// this `cfg` when `size_of_val_raw` is stabilized.
#[cfg(__ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS)]
#[cfg(not(target_pointer_width = "16"))]
#[repr(C, align(65536))]
struct Aligned64kAllocation([u8; _64K]);

/// A pointer to an aligned allocation of size 2^16.
///
/// # Safety
///
/// `ALIGNED_64K_ALLOCATION` is guaranteed to point to the entirety of an
/// allocation with size and alignment 2^16, and to have valid provenance.
// FIXME(#29), FIXME(https://github.com/rust-lang/rust/issues/69835): Remove
// this `cfg` when `size_of_val_raw` is stabilized.
#[cfg(__ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS)]
#[cfg(not(target_pointer_width = "16"))]
pub const ALIGNED_64K_ALLOCATION: NonNull<[u8]> = {
    const REF: &Aligned64kAllocation = &Aligned64kAllocation([0; _64K]);
    let ptr: *const Aligned64kAllocation = REF;
    let ptr: *const [u8] = ptr::slice_from_raw_parts(ptr.cast(), _64K);
    // SAFETY:
    // - `ptr` is derived from a Rust reference, which is guaranteed to be
    //   non-null.
    // - `ptr` is derived from an `&Aligned64kAllocation`, which has size and
    //   alignment `_64K` as promised. Its length is initialized to `_64K`,
    //   which means that it refers to the entire allocation.
    // - `ptr` is derived from a Rust reference, which is guaranteed to have
    //   valid provenance.
    //
    // FIXME(#429): Once `NonNull::new_unchecked` docs document that it
    // preserves provenance, cite those docs.
    // FIXME: Replace this `as` with `ptr.cast_mut()` once our MSRV >= 1.65
    #[allow(clippy::as_conversions)]
    unsafe {
        NonNull::new_unchecked(ptr as *mut _)
    }
};

/// Computes the offset of the base of the field `$trailing_field_name` within
/// the type `$ty`.
///
/// `trailing_field_offset!` produces code which is valid in a `const` context.
// FIXME(#29), FIXME(https://github.com/rust-lang/rust/issues/69835): Remove
// this `cfg` when `size_of_val_raw` is stabilized.
#[cfg(__ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS)]
#[doc(hidden)] // `#[macro_export]` bypasses this module's `#[doc(hidden)]`.
#[macro_export]
macro_rules! trailing_field_offset {
    ($ty:ty, $trailing_field_name:tt) => {{
        let min_size = {
            let zero_elems: *const [()] =
                $crate::util::macro_util::core_reexport::ptr::slice_from_raw_parts(
                    $crate::util::macro_util::core_reexport::ptr::NonNull::<()>::dangling()
                        .as_ptr()
                        .cast_const(),
                    0,
                );
            // SAFETY:
            // - If `$ty` is `Sized`, `size_of_val_raw` is always safe to call.
            // - Otherwise:
            //   - If `$ty` is not a slice DST, this pointer conversion will
            //     fail due to "mismatched vtable kinds", and compilation will
            //     fail.
            //   - If `$ty` is a slice DST, we have constructed `zero_elems` to
            //     have zero trailing slice elements. Per the `size_of_val_raw`
            //     docs, "For the special case where the dynamic tail length is
            //     0, this function is safe to call." [1]
            //
            // [1] https://doc.rust-lang.org/nightly/std/mem/fn.size_of_val_raw.html
            unsafe {
                #[allow(clippy::as_conversions)]
                $crate::util::macro_util::core_reexport::mem::size_of_val_raw(
                    zero_elems as *const $ty,
                )
            }
        };

        assert!(min_size <= _64K);

        #[allow(clippy::as_conversions)]
        let ptr = ALIGNED_64K_ALLOCATION.as_ptr() as *const $ty;

        // SAFETY:
        // - Thanks to the preceding `assert!`, we know that the value with zero
        //   elements fits in `_64K` bytes, and thus in the allocation addressed
        //   by `ALIGNED_64K_ALLOCATION`. The offset of the trailing field is
        //   guaranteed to be no larger than this size, so this field projection
        //   is guaranteed to remain in-bounds of its allocation.
        // - Because the minimum size is no larger than `_64K` bytes, and
        //   because an object's size must always be a multiple of its alignment
        //   [1], we know that `$ty`'s alignment is no larger than `_64K`. The
        //   allocation addressed by `ALIGNED_64K_ALLOCATION` is guaranteed to
        //   be aligned to `_64K`, so `ptr` is guaranteed to satisfy `$ty`'s
        //   alignment.
        // - As required by `addr_of!`, we do not write through `field`.
        //
        //   Note that, as of [2], this requirement is technically unnecessary
        //   for Rust versions >= 1.75.0, but no harm in guaranteeing it anyway
        //   until we bump our MSRV.
        //
        // [1] Per https://doc.rust-lang.org/reference/type-layout.html:
        //
        //   The size of a value is always a multiple of its alignment.
        //
        // [2] https://github.com/rust-lang/reference/pull/1387
        let field = unsafe {
            $crate::util::macro_util::core_reexport::ptr::addr_of!((*ptr).$trailing_field_name)
        };
        // SAFETY:
        // - Both `ptr` and `field` are derived from the same allocated object.
        // - By the preceding safety comment, `field` is in bounds of that
        //   allocated object.
        // - The distance, in bytes, between `ptr` and `field` is required to be
        //   a multiple of the size of `u8`, which is trivially true because
        //   `u8`'s size is 1.
        // - The distance, in bytes, cannot overflow `isize`. This is guaranteed
        //   because no allocated object can have a size larger than can fit in
        //   `isize`. [1]
        // - The distance being in-bounds cannot rely on wrapping around the
        //   address space. This is guaranteed because the same is guaranteed of
        //   allocated objects. [1]
        //
        // [1] FIXME(#429), FIXME(https://github.com/rust-lang/rust/pull/116675):
        //     Once these are guaranteed in the Reference, cite it.
        let offset = unsafe { field.cast::<u8>().offset_from(ptr.cast::<u8>()) };
        // Guaranteed not to be lossy: `field` comes after `ptr`, so the offset
        // from `ptr` to `field` is guaranteed to be positive.
        assert!(offset >= 0);
        Some(
            #[allow(clippy::as_conversions)]
            {
                offset as usize
            },
        )
    }};
}

/// Computes alignment of `$ty: ?Sized`.
///
/// `align_of!` produces code which is valid in a `const` context.
// FIXME(#29), FIXME(https://github.com/rust-lang/rust/issues/69835): Remove
// this `cfg` when `size_of_val_raw` is stabilized.
#[cfg(__ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS)]
#[doc(hidden)] // `#[macro_export]` bypasses this module's `#[doc(hidden)]`.
#[macro_export]
macro_rules! align_of {
    ($ty:ty) => {{
        // SAFETY: `OffsetOfTrailingIsAlignment` is `repr(C)`, and its layout is
        // guaranteed [1] to begin with the single-byte layout for `_byte`,
        // followed by the padding needed to align `_trailing`, then the layout
        // for `_trailing`, and finally any trailing padding bytes needed to
        // correctly-align the entire struct.
        //
        // This macro computes the alignment of `$ty` by counting the number of
        // bytes preceding `_trailing`. For instance, if the alignment of `$ty`
        // is `1`, then no padding is required align `_trailing` and it will be
        // located immediately after `_byte` at offset 1. If the alignment of
        // `$ty` is 2, then a single padding byte is required before
        // `_trailing`, and `_trailing` will be located at offset 2.

        // This correspondence between offset and alignment holds for all valid
        // Rust alignments, and we confirm this exhaustively (or, at least up to
        // the maximum alignment supported by `trailing_field_offset!`) in
        // `test_align_of_dst`.
        //
        // [1]: https://doc.rust-lang.org/nomicon/other-reprs.html#reprc

        #[repr(C)]
        struct OffsetOfTrailingIsAlignment {
            _byte: u8,
            _trailing: $ty,
        }

        trailing_field_offset!(OffsetOfTrailingIsAlignment, _trailing)
    }};
}

mod size_to_tag {
    pub trait SizeToTag<const SIZE: usize> {
        type Tag;
    }

    impl SizeToTag<1> for () {
        type Tag = u8;
    }
    impl SizeToTag<2> for () {
        type Tag = u16;
    }
    impl SizeToTag<4> for () {
        type Tag = u32;
    }
    impl SizeToTag<8> for () {
        type Tag = u64;
    }
    impl SizeToTag<16> for () {
        type Tag = u128;
    }
}

/// An alias for the unsigned integer of the given size in bytes.
#[doc(hidden)]
pub type SizeToTag<const SIZE: usize> = <() as size_to_tag::SizeToTag<SIZE>>::Tag;

// We put `Sized` in its own module so it can have the same name as the standard
// library `Sized` without shadowing it in the parent module.
#[cfg(not(no_zerocopy_diagnostic_on_unimplemented_1_78_0))]
mod __size_of {
    #[diagnostic::on_unimplemented(
        message = "`{Self}` is unsized",
        label = "`IntoBytes` needs all field types to be `Sized` in order to determine whether there is padding",
        note = "consider using `#[repr(packed)]` to remove padding",
        note = "`IntoBytes` does not require the fields of `#[repr(packed)]` types to be `Sized`"
    )]
    pub trait Sized: core::marker::Sized {}
    impl<T: core::marker::Sized> Sized for T {}

    #[inline(always)]
    #[must_use]
    #[allow(clippy::needless_maybe_sized)]
    pub const fn size_of<T: Sized + ?core::marker::Sized>() -> usize {
        core::mem::size_of::<T>()
    }
}

#[cfg(no_zerocopy_diagnostic_on_unimplemented_1_78_0)]
pub use core::mem::size_of;

#[cfg(not(no_zerocopy_diagnostic_on_unimplemented_1_78_0))]
pub use __size_of::size_of;

/// How many padding bytes does the struct type `$t` have?
///
/// `$ts` is the list of the type of every field in `$t`. `$t` must be a struct
/// type, or else `struct_padding!`'s result may be meaningless.
///
/// Note that `struct_padding!`'s results are independent of `repcr` since they
/// only consider the size of the type and the sizes of the fields. Whatever the
/// repr, the size of the type already takes into account any padding that the
/// compiler has decided to add. Structs with well-defined representations (such
/// as `repr(C)`) can use this macro to check for padding. Note that while this
/// may yield some consistent value for some `repr(Rust)` structs, it is not
/// guaranteed across platforms or compilations.
#[doc(hidden)] // `#[macro_export]` bypasses this module's `#[doc(hidden)]`.
#[macro_export]
macro_rules! struct_padding {
    ($t:ty, $_align:expr, $_packed:expr, [$($ts:ty),*]) => {{
        // The `align` and `packed` directives can be ignored here. Regardless
        // of if and how they are set, comparing the size of `$t` to the sum of
        // its field sizes is a reliable indicator of the presence of padding.
        $crate::util::macro_util::size_of::<$t>() - (0 $(+ $crate::util::macro_util::size_of::<$ts>())*)
    }};
}

/// Does the `repr(C)` struct type `$t` have padding?
///
/// `$ts` is the list of the type of every field in `$t`. `$t` must be a
/// `repr(C)` struct type, or else `struct_has_padding!`'s result may be
/// meaningless.
#[doc(hidden)] // `#[macro_export]` bypasses this module's `#[doc(hidden)]`.
#[macro_export]
macro_rules! repr_c_struct_has_padding {
    ($t:ty, $align:expr, $packed:expr, [$($ts:tt),*]) => {{
        let layout = $crate::DstLayout::for_repr_c_struct(
            $align,
            $packed,
            &[$($crate::repr_c_struct_has_padding!(@field $ts),)*]
        );
        layout.requires_static_padding() || layout.requires_dynamic_padding()
    }};
    (@field ([$t:ty])) => {
        <[$t] as $crate::KnownLayout>::LAYOUT
    };
    (@field ($t:ty)) => {
        $crate::DstLayout::for_unpadded_type::<$t>()
    };
    (@field [$t:ty]) => {
        <[$t] as $crate::KnownLayout>::LAYOUT
    };
    (@field $t:ty) => {
        $crate::DstLayout::for_unpadded_type::<$t>()
    };
}

/// Does the union type `$t` have padding?
///
/// `$ts` is the list of the type of every field in `$t`. `$t` must be a union
/// type, or else `union_padding!`'s result may be meaningless.
///
/// Note that `union_padding!`'s results are independent of `repr` since they
/// only consider the size of the type and the sizes of the fields. Whatever the
/// repr, the size of the type already takes into account any padding that the
/// compiler has decided to add. Unions with well-defined representations (such
/// as `repr(C)`) can use this macro to check for padding. Note that while this
/// may yield some consistent value for some `repr(Rust)` unions, it is not
/// guaranteed across platforms or compilations.
#[doc(hidden)] // `#[macro_export]` bypasses this module's `#[doc(hidden)]`.
#[macro_export]
macro_rules! union_padding {
    ($t:ty, $_align:expr, $_packed:expr, [$($ts:ty),*]) => {{
        // The `align` and `packed` directives can be ignored here. Regardless
        // of if and how they are set, comparing the size of `$t` to each of its
        // field sizes is a reliable indicator of the presence of padding.
        let mut max = 0;
        $({
            let padding = $crate::util::macro_util::size_of::<$t>() - $crate::util::macro_util::size_of::<$ts>();
            if padding > max {
                max = padding;
            }
        })*
        max
    }};
}

/// How many padding bytes does the enum type `$t` have?
///
/// `$disc` is the type of the enum tag, and `$ts` is a list of fields in each
/// square-bracket-delimited variant. `$t` must be an enum, or else
/// `enum_padding!`'s result may be meaningless. An enum has padding if any of
/// its variant structs [1][2] contain padding, and so all of the variants of an
/// enum must be "full" in order for the enum to not have padding.
///
/// The results of `enum_padding!` require that the enum is not `repr(Rust)`, as
/// `repr(Rust)` enums may niche the enum's tag and reduce the total number of
/// bytes required to represent the enum as a result. As long as the enum is
/// `repr(C)`, `repr(int)`, or `repr(C, int)`, this will consistently return
/// whether the enum contains any padding bytes.
///
/// [1]: https://doc.rust-lang.org/1.81.0/reference/type-layout.html#reprc-enums-with-fields
/// [2]: https://doc.rust-lang.org/1.81.0/reference/type-layout.html#primitive-representation-of-enums-with-fields
#[doc(hidden)] // `#[macro_export]` bypasses this module's `#[doc(hidden)]`.
#[macro_export]
macro_rules! enum_padding {
    ($t:ty, $_align:expr, $packed:expr, $disc:ty, $([$($ts:ty),*]),*) => {{
        // The `align` and `packed` directives are irrelevant. `$align` can be
        // ignored because regardless of if and how it is set, comparing the
        // size of `$t` to each of its field sizes is a reliable indicator of
        // the presence of padding. `$packed` is irrelevant because it is
        // forbidden on enums.
        #[allow(clippy::as_conversions)]
        const _: [(); 1] = [(); $packed.is_none() as usize];
        let mut max = 0;
        $({
            let padding = $crate::util::macro_util::size_of::<$t>()
                - (
                    $crate::util::macro_util::size_of::<$disc>()
                    $(+ $crate::util::macro_util::size_of::<$ts>())*
                );
            if padding > max {
                max = padding;
            }
        })*
        max
    }};
}

/// Unwraps an infallible `Result`.
#[doc(hidden)]
#[macro_export]
macro_rules! into_inner {
    ($e:expr) => {
        match $e {
            $crate::util::macro_util::core_reexport::result::Result::Ok(e) => e,
            $crate::util::macro_util::core_reexport::result::Result::Err(i) => match i {},
        }
    };
}

/// Translates an identifier or tuple index into a numeric identifier.
#[doc(hidden)] // `#[macro_export]` bypasses this module's `#[doc(hidden)]`.
#[macro_export]
macro_rules! ident_id {
    ($field:ident) => {
        $crate::util::macro_util::hash_name(stringify!($field))
    };
    ($field:literal) => {
        $field
    };
}

/// Computes the hash of a string.
///
/// NOTE(#2749) on hash collisions: This function's output only needs to be
/// deterministic within a particular compilation. Thus, if a user ever reports
/// a hash collision (very unlikely given the <= 16-byte special case), we can
/// strengthen the hash function at that point and publish a new version. Since
/// this is computed at compile time on small strings, we can easily use more
/// expensive and higher-quality hash functions if need be.
#[inline(always)]
#[must_use]
#[allow(clippy::as_conversions, clippy::indexing_slicing, clippy::arithmetic_side_effects)]
pub const fn hash_name(name: &str) -> i128 {
    let name = name.as_bytes();

    // We guarantee freedom from hash collisions between any two strings of
    // length 16 or less by having the hashes of such strings be equal to
    // their value. There is still a possibility that such strings will have
    // the same value as the hash of a string of length > 16.
    if name.len() <= size_of::<u128>() {
        let mut bytes = [0u8; 16];

        let mut i = 0;
        while i < name.len() {
            bytes[i] = name[i];
            i += 1;
        }

        return i128::from_ne_bytes(bytes);
    };

    // An implementation of FxHasher, although returning a u128. Probably
    // not as strong as it could be, but probably more collision resistant
    // than normal 64-bit FxHasher.
    let mut hash = 0u128;
    let mut i = 0;
    while i < name.len() {
        // This is just FxHasher's `0x517cc1b727220a95` constant
        // concatenated back-to-back.
        const K: u128 = 0x517cc1b727220a95517cc1b727220a95;
        hash = (hash.rotate_left(5) ^ (name[i] as u128)).wrapping_mul(K);
        i += 1;
    }
    i128::from_ne_bytes(hash.to_ne_bytes())
}

/// Attempts to transmute `Src` into `Dst`.
///
/// A helper for `try_transmute!`.
///
/// # Panics
///
/// `try_transmute` may either produce a post-monomorphization error or a panic
/// if `Dst` is bigger than `Src`. Otherwise, `try_transmute` panics under the
/// same circumstances as [`is_safe`].
///
/// [`is_safe`]: TryFromBytes::is_safe
#[inline(always)]
pub fn try_transmute<Src, Dst>(src: Src) -> Result<Dst, ValidityError<Src, Dst>>
where
    Src: IntoBytes,
    Dst: TryFromBytes,
{
    static_assert!(Src, Dst => mem::size_of::<Dst>() == mem::size_of::<Src>());

    let src = mem::ManuallyDrop::new(ReadOnly::new(src));
    let mut ptr =
        Ptr::from_ref(&*src).transmute_with::<ReadOnly<Dst>, Initialized, CastSizedExact, _>();
    if Dst::is_safe(ptr.reborrow_shared()) {
        // SAFETY: `read_unaligned` requires that `ptr` be valid for reads and
        // point to a properly initialized `ReadOnly<Dst>` [1]. `ptr` was
        // derived from a live reference to `src`; `CastSizedExact` preserves
        // that reference's address, provenance, and exact byte extent. Thus,
        // the entire range remains live and valid for reads. The reference is
        // non-null even when `ReadOnly<Dst>` is zero-sized, and the cast
        // preserves that address, satisfying [1]'s explicit ZST requirement.
        //
        // `Src: IntoBytes` and the `ReadOnly<Src>` bridge let `transmute_with`
        // establish that every byte in that exact range is initialized.
        // `Dst::is_safe` then established that the same bytes are bit-valid for
        // `Dst`, and thus for `ReadOnly<Dst>`. The candidate was a shared
        // `ReadOnly` pointer, so it could not mutate the bytes; no operation
        // occurs between validation and this read which could do so. Alignment
        // is not an additional precondition because [1] expressly permits
        // unaligned pointers.
        //
        // `read_unaligned` leaves the source memory unchanged and makes a
        // bitwise copy [1]. The source is wrapped in `ManuallyDrop`, which
        // inhibits its destructor [2]. After this read, neither `src` nor
        // `ptr` is used to access the inner value, and the wrapper's eventual
        // drop does not drop it. Thus, only the returned `Dst` is subsequently
        // used or dropped; there is no duplicate ownership use.
        //
        // [1] Per https://doc.rust-lang.org/1.56.0/std/ptr/fn.read_unaligned.html:
        //
        //     Reads the value from `src` without moving it. This leaves the
        //     memory in `src` unchanged.
        //
        //     Unlike `read`, `read_unaligned` works with unaligned pointers.
        //
        //     Behavior is undefined if any of the following conditions are
        //     violated:
        //
        //     * `src` must be valid for reads.
        //
        //     * `src` must point to a properly initialized value of type `T`.
        //
        //     Like `read`, `read_unaligned` creates a bitwise copy of `T`,
        //     regardless of whether `T` is `Copy`. If `T` is not `Copy`, using
        //     both the returned value and the value at `*src` can violate
        //     memory safety.
        //
        //     Note that even if `T` has size `0`, the pointer must be non-null.
        //
        // [2] Per https://doc.rust-lang.org/1.56.0/std/mem/struct.ManuallyDrop.html:
        //
        //     A wrapper to inhibit compiler from automatically calling `T`'s
        //     destructor.
        let dst = unsafe { core::ptr::read_unaligned(ptr.as_inner().as_ptr()) };
        Ok(ReadOnly::into_inner(dst))
    } else {
        let src = ReadOnly::into_inner(mem::ManuallyDrop::into_inner(src));
        Err(ValidityError::new(src))
    }
}

/// See `try_transmute_ref!` documentation.
pub trait TryTransmuteRefDst<'a> {
    type Dst: ?Sized;

    /// See `try_transmute_ref!` documentation.
    fn try_transmute_ref(self) -> Result<&'a Self::Dst, ValidityError<&'a Self::Src, Self::Dst>>
    where
        Self: TryTransmuteRefSrc<'a>,
        Self::Src: IntoBytes + Immutable + KnownLayout,
        Self::Dst: TryFromBytes + Immutable + KnownLayout;
}

pub trait TryTransmuteRefSrc<'a> {
    type Src: ?Sized;
}

impl<'a, Src, Dst> TryTransmuteRefSrc<'a> for Wrap<&'a Src, &'a Dst>
where
    Src: ?Sized,
    Dst: ?Sized,
{
    type Src = Src;
}

impl<'a, Src, Dst> TryTransmuteRefDst<'a> for Wrap<&'a Src, &'a Dst>
where
    Src: IntoBytes + Immutable + KnownLayout + ?Sized,
    Dst: TryFromBytes + Immutable + KnownLayout + ?Sized,
{
    type Dst = Dst;

    #[inline(always)]
    fn try_transmute_ref(
        self,
    ) -> Result<
        &'a Dst,
        ValidityError<&'a <Wrap<&'a Src, &'a Dst> as TryTransmuteRefSrc<'a>>::Src, Dst>,
    > {
        let ptr = Ptr::from_ref(self.0);
        #[rustfmt::skip]
        let res = ptr.try_with(#[inline(always)] |ptr| {
            let ptr = ptr.recall_validity::<Initialized, _>();
            let ptr = ptr.cast::<_, crate::layout::CastFrom<Dst>, _>();
            ptr.try_into_safe()
        });
        match res {
            Ok(ptr) => {
                static_assert!(Src: ?Sized + KnownLayout, Dst: ?Sized + KnownLayout => {
                    Src::LAYOUT.align.get() >= Dst::LAYOUT.align.get()
                }, "cannot transmute reference when destination type has higher alignment than source type");
                // SAFETY: We have checked that `Dst` does not have a stricter
                // alignment requirement than `Src`.
                let ptr = unsafe { ptr.assume_alignment::<Aligned>() };
                Ok(ptr.as_ref())
            }
            Err(err) => Err(err.map_src(Ptr::as_ref)),
        }
    }
}

pub trait TryTransmuteMutDst<'a> {
    type Dst: ?Sized;

    /// See `try_transmute_mut!` documentation.
    fn try_transmute_mut(
        self,
    ) -> Result<&'a mut Self::Dst, ValidityError<&'a mut Self::Src, Self::Dst>>
    where
        Self: TryTransmuteMutSrc<'a>,
        Self::Src: IntoBytes,
        Self::Dst: TryFromBytes;
}

pub trait TryTransmuteMutSrc<'a> {
    type Src: ?Sized;
}

impl<'a, Src, Dst> TryTransmuteMutSrc<'a> for Wrap<&'a mut Src, &'a mut Dst>
where
    Src: ?Sized,
    Dst: ?Sized,
{
    type Src = Src;
}

impl<'a, Src, Dst> TryTransmuteMutDst<'a> for Wrap<&'a mut Src, &'a mut Dst>
where
    Src: FromBytes + IntoBytes + KnownLayout + ?Sized,
    Dst: TryFromBytes + IntoBytes + KnownLayout + ?Sized,
{
    type Dst = Dst;

    #[inline(always)]
    fn try_transmute_mut(
        self,
    ) -> Result<
        &'a mut Dst,
        ValidityError<&'a mut <Wrap<&'a mut Src, &'a mut Dst> as TryTransmuteMutSrc<'a>>::Src, Dst>,
    > {
        let ptr = Ptr::from_mut(self.0);
        // SAFETY: The provided closure returns the only copy of `ptr`.
        #[rustfmt::skip]
        let res = unsafe {
            ptr.try_with_unchecked(#[inline(always)] |ptr| {
                let ptr = ptr.recall_validity::<Initialized, (_, (_, _))>();
                let ptr = ptr.cast::<_, crate::layout::CastFrom<Dst>, _>();
                ptr.try_into_safe()
            })
        };
        match res {
            Ok(ptr) => {
                static_assert!(Src: ?Sized + KnownLayout, Dst: ?Sized + KnownLayout => {
                    Src::LAYOUT.align.get() >= Dst::LAYOUT.align.get()
                }, "cannot transmute reference when destination type has higher alignment than source type");
                // SAFETY: We have checked that `Dst` does not have a stricter
                // alignment requirement than `Src`.
                let ptr = unsafe { ptr.assume_alignment::<Aligned>() };
                Ok(ptr.as_mut())
            }
            Err(err) => Err(err.map_src(Ptr::as_mut)),
        }
    }
}

// Used in `transmute_ref!` and friends.
//
// This permits us to use the autoref specialization trick to dispatch to
// associated functions for `transmute_ref` and `transmute_mut` when both `Src`
// and `Dst` are `Sized`, and to trait methods otherwise. The associated
// functions, unlike the trait methods, do not require a `KnownLayout` bound.
// This permits us to add support for transmuting references to unsized types
// without breaking backwards-compatibility (on v0.8.x) with the old
// implementation, which did not require a `KnownLayout` bound to transmute
// sized types.
#[derive(Copy, Clone)]
pub struct Wrap<Src, Dst>(pub Src, pub PhantomData<Dst>);

impl<Src, Dst> Wrap<Src, Dst> {
    #[inline(always)]
    pub const fn new(src: Src) -> Self {
        Wrap(src, PhantomData)
    }
}

impl<'a, Src, Dst> Wrap<&'a Src, &'a Dst>
where
    Src: ?Sized,
    Dst: ?Sized,
{
    #[allow(clippy::must_use_candidate, clippy::missing_inline_in_public_items, clippy::empty_loop)]
    pub const fn transmute_ref_inference_helper(self) -> &'a Dst {
        loop {}
    }
}

impl<'a, Src, Dst> Wrap<&'a Src, &'a Dst> {
    /// # Safety
    /// The caller must guarantee that:
    /// - `Src: IntoBytes + Immutable`
    /// - `Dst: FromBytes + Immutable`
    ///
    /// # PME
    ///
    /// Instantiating this method PMEs unless both:
    /// - `mem::size_of::<Dst>() == mem::size_of::<Src>()`
    /// - `mem::align_of::<Dst>() <= mem::align_of::<Src>()`
    #[inline(always)]
    #[must_use]
    pub const unsafe fn transmute_ref(self) -> &'a Dst {
        static_assert!(Src, Dst => mem::size_of::<Dst>() == mem::size_of::<Src>());
        static_assert!(Src, Dst => mem::align_of::<Dst>() <= mem::align_of::<Src>());

        let src: *const Src = self.0;
        let dst = src.cast::<Dst>();
        // SAFETY:
        // - We know that it is sound to view the target type of the input
        //   reference (`Src`) as the target type of the output reference
        //   (`Dst`) because the caller has guaranteed that `Src: IntoBytes`,
        //   `Dst: FromBytes`, and `size_of::<Src>() == size_of::<Dst>()`.
        // - We know that there are no `UnsafeCell`s, and thus we don't have to
        //   worry about `UnsafeCell` overlap, because `Src: Immutable` and
        //   `Dst: Immutable`.
        // - The caller has guaranteed that alignment is not increased.
        // - We know that the returned lifetime will not outlive the input
        //   lifetime thanks to the lifetime bounds on this function.
        //
        // FIXME(#67): Once our MSRV is 1.58, replace this `transmute` with
        // `&*dst`.
        #[allow(clippy::transmute_ptr_to_ref)]
        unsafe {
            mem::transmute(dst)
        }
    }

    #[inline(always)]
    pub fn try_transmute_ref(self) -> Result<&'a Dst, ValidityError<&'a Src, Dst>>
    where
        Src: IntoBytes + Immutable,
        Dst: TryFromBytes + Immutable,
    {
        static_assert!(Src => mem::align_of::<Src>() == mem::align_of::<Wrapping<Src>>());
        static_assert!(Dst => mem::align_of::<Dst>() == mem::align_of::<Wrapping<Dst>>());

        // SAFETY: By the preceding assert, `Src` and `Wrapping<Src>` have the
        // same alignment.
        let src: &Wrapping<Src> =
            unsafe { crate::util::transmute_ref::<_, _, BecauseImmutable>(self.0) };
        let src = Wrap::new(src);
        <Wrap<&'a Wrapping<Src>, &'a Wrapping<Dst>> as TryTransmuteRefDst<'a>>::try_transmute_ref(
            src,
        )
        .map(
            // SAFETY: By the preceding assert, `Dst` and `Wrapping<Dst>` have
            // the same alignment.
            #[inline(always)]
            |dst| unsafe { crate::util::transmute_ref::<_, _, BecauseImmutable>(dst) },
        )
        .map_err(
            #[inline(always)]
            |err| {
                // SAFETY: By the preceding assert, `Src` and `Wrapping<Src>` have the
                // same alignment.
                ValidityError::new(unsafe {
                    crate::util::transmute_ref::<_, _, BecauseImmutable>(err.into_src())
                })
            },
        )
    }
}

impl<'a, Src, Dst> Wrap<&'a mut Src, &'a mut Dst>
where
    Src: ?Sized,
    Dst: ?Sized,
{
    #[allow(clippy::must_use_candidate, clippy::missing_inline_in_public_items, clippy::empty_loop)]
    pub fn transmute_mut_inference_helper(self) -> &'a mut Dst {
        loop {}
    }
}

impl<'a, Src, Dst> Wrap<&'a mut Src, &'a mut Dst> {
    /// Transmutes a mutable reference of one type to a mutable reference of
    /// another type.
    ///
    /// # PME
    ///
    /// Instantiating this method PMEs unless both:
    /// - `mem::size_of::<Dst>() == mem::size_of::<Src>()`
    /// - `mem::align_of::<Dst>() <= mem::align_of::<Src>()`
    #[inline(always)]
    #[must_use]
    pub fn transmute_mut(self) -> &'a mut Dst
    where
        Src: FromBytes + IntoBytes,
        Dst: FromBytes + IntoBytes,
    {
        static_assert!(Src, Dst => mem::size_of::<Dst>() == mem::size_of::<Src>());
        static_assert!(Src, Dst => mem::align_of::<Dst>() <= mem::align_of::<Src>());

        let src: *mut Src = self.0;
        let dst = src.cast::<Dst>();
        // SAFETY:
        // - We know that it is sound to view the target type of the input
        //   reference (`Src`) as the target type of the output reference
        //   (`Dst`) and vice-versa because `Src: FromBytes + IntoBytes`, `Dst:
        //   FromBytes + IntoBytes`, and (as asserted above) `size_of::<Src>()
        //   == size_of::<Dst>()`.
        // - We asserted above that alignment will not increase.
        // - We know that the returned lifetime will not outlive the input
        //   lifetime thanks to the lifetime bounds on this function.
        unsafe { &mut *dst }
    }

    #[inline(always)]
    pub fn try_transmute_mut(self) -> Result<&'a mut Dst, ValidityError<&'a mut Src, Dst>>
    where
        Src: FromBytes + IntoBytes,
        Dst: TryFromBytes + IntoBytes,
    {
        static_assert!(Src => mem::align_of::<Src>() == mem::align_of::<Wrapping<Src>>());
        static_assert!(Dst => mem::align_of::<Dst>() == mem::align_of::<Wrapping<Dst>>());

        // SAFETY: By the preceding assert, `Src` and `Wrapping<Src>` have the
        // same alignment.
        let src: &mut Wrapping<Src> =
            unsafe { crate::util::transmute_mut::<_, _, (_, (_, _))>(self.0) };
        let src = Wrap::new(src);
        <Wrap<&'a mut Wrapping<Src>, &'a mut Wrapping<Dst>> as TryTransmuteMutDst<'a>>
            ::try_transmute_mut(src)
            // SAFETY: By the preceding assert, `Dst` and `Wrapping<Dst>` have the
            // same alignment.
            .map(|dst| unsafe { crate::util::transmute_mut::<_, _, (_, (_, _))>(dst) })
            .map_err(|err| {
                // SAFETY: By the preceding assert, `Src` and `Wrapping<Src>` have the
                // same alignment.
                ValidityError::new(unsafe {
                    crate::util::transmute_mut::<_, _, (_, (_, _))>(err.into_src())
                })
            })
    }
}

pub trait TransmuteRefDst<'a> {
    type Dst: ?Sized;

    #[must_use]
    fn transmute_ref(self) -> &'a Self::Dst;
}

impl<'a, Src: ?Sized, Dst: ?Sized> TransmuteRefDst<'a> for Wrap<&'a Src, &'a Dst>
where
    Src: KnownLayout + IntoBytes + Immutable,
    Dst: KnownLayout<PointerMetadata = usize> + FromBytes + Immutable,
{
    type Dst = Dst;

    #[inline(always)]
    fn transmute_ref(self) -> &'a Dst {
        let ptr = Ptr::from_ref(self.0)
            .recall_validity::<Initialized, _>()
            .transmute_with::<Dst, Initialized, crate::layout::CastFrom<Dst>, (crate::pointer::BecauseMutationCompatible, _)>()
            .recall_validity::<Safe, _>();

        static_assert!(Src: ?Sized + KnownLayout, Dst: ?Sized + KnownLayout => {
            Src::LAYOUT.align.get() >= Dst::LAYOUT.align.get()
        }, "cannot transmute reference when destination type has higher alignment than source type");

        // SAFETY: The preceding `static_assert!` ensures that
        // `Src::LAYOUT.align >= Dst::LAYOUT.align`. Since `self` is
        // validly-aligned for `Src`, it is also validly-aligned for `Dst`.
        let ptr = unsafe { ptr.assume_alignment() };

        ptr.as_ref()
    }
}

pub trait TransmuteMutDst<'a> {
    type Dst: ?Sized;
    #[must_use]
    fn transmute_mut(self) -> &'a mut Self::Dst;
}

impl<'a, Src: ?Sized, Dst: ?Sized> TransmuteMutDst<'a> for Wrap<&'a mut Src, &'a mut Dst>
where
    Src: KnownLayout + FromBytes + IntoBytes,
    Dst: KnownLayout<PointerMetadata = usize> + FromBytes + IntoBytes,
{
    type Dst = Dst;

    #[inline(always)]
    fn transmute_mut(self) -> &'a mut Dst {
        let ptr = Ptr::from_mut(self.0)
            .recall_validity::<Initialized, (_, (_, _))>()
            .transmute_with::<Dst, Initialized, crate::layout::CastFrom<Dst>, _>()
            .recall_validity::<Safe, (_, (_, _))>();

        static_assert!(Src: ?Sized + KnownLayout, Dst: ?Sized + KnownLayout => {
            Src::LAYOUT.align.get() >= Dst::LAYOUT.align.get()
        }, "cannot transmute reference when destination type has higher alignment than source type");

        // SAFETY: The preceding `static_assert!` ensures that
        // `Src::LAYOUT.align >= Dst::LAYOUT.align`. Since `self` is
        // validly-aligned for `Src`, it is also validly-aligned for `Dst`.
        let ptr = unsafe { ptr.assume_alignment() };

        ptr.as_mut()
    }
}

/// A function which emits a warning if its return value is not used.
#[must_use]
#[inline(always)]
pub const fn must_use<T>(t: T) -> T {
    t
}

// NOTE: We can't change this to a `pub use core as core_reexport` until [1] is
// fixed or we update to a semver-breaking version (as of this writing, 0.8.0)
// on the `main` branch.
//
// [1] https://github.com/obi1kenobi/cargo-semver-checks/issues/573
pub mod core_reexport {
    pub use core::*;

    pub mod mem {
        pub use core::mem::*;
    }
}

/// Projects `Self` to itself.
///
/// This is used by macros to have the compiler verify that two syntactically
/// similar types actually resolve to the same type. The blanket implementation
/// ensures that no other implementation can project a type to anything other
/// than itself.
#[doc(hidden)]
pub trait Identity {
    /// `Self`.
    type Type: ?Sized;
}

impl<T: ?Sized> Identity for T {
    type Type = T;
}

#[cfg(test)]
mod tests {
    use core::num::NonZeroUsize;

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

    #[cfg(__ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS)]
    mod nightly {
        use super::super::*;
        use crate::util::testutil::*;

        // FIXME(#29), FIXME(https://github.com/rust-lang/rust/issues/69835):
        // Remove this `cfg` when `size_of_val_raw` is stabilized.
        #[allow(clippy::decimal_literal_representation)]
        #[test]
        fn test_trailing_field_offset() {
            assert_eq!(mem::align_of::<Aligned64kAllocation>(), _64K);

            macro_rules! test {
                (#[$cfg:meta] ($($ts:ty),* ; $trailing_field_ty:ty) => $expect:expr) => {{
                    #[$cfg]
                    struct Test($(#[allow(dead_code)] $ts,)* #[allow(dead_code)] $trailing_field_ty);
                    assert_eq!(test!(@offset $($ts),* ; $trailing_field_ty), $expect);
                }};
                (#[$cfg:meta] $(#[$cfgs:meta])* ($($ts:ty),* ; $trailing_field_ty:ty) => $expect:expr) => {
                    test!(#[$cfg] ($($ts),* ; $trailing_field_ty) => $expect);
                    test!($(#[$cfgs])* ($($ts),* ; $trailing_field_ty) => $expect);
                };
                (@offset ; $_trailing:ty) => { trailing_field_offset!(Test, 0) };
                (@offset $_t:ty ; $_trailing:ty) => { trailing_field_offset!(Test, 1) };
            }

            test!(#[repr(C)] #[repr(transparent)] #[repr(packed)](; u8) => Some(0));
            test!(#[repr(C)] #[repr(transparent)] #[repr(packed)](; [u8]) => Some(0));
            test!(#[repr(C)] #[repr(C, packed)] (u8; u8) => Some(1));
            test!(#[repr(C)] (; AU64) => Some(0));
            test!(#[repr(C)] (; [AU64]) => Some(0));
            test!(#[repr(C)] (u8; AU64) => Some(8));
            test!(#[repr(C)] (u8; [AU64]) => Some(8));

            #[derive(
                Immutable, FromBytes, Eq, PartialEq, Ord, PartialOrd, Default, Debug, Copy, Clone,
            )]
            #[repr(C)]
            pub(crate) struct Nested<T, U: ?Sized> {
                _t: T,
                _u: U,
            }

            test!(#[repr(C)] (; Nested<u8, AU64>) => Some(0));
            test!(#[repr(C)] (; Nested<u8, [AU64]>) => Some(0));
            test!(#[repr(C)] (u8; Nested<u8, AU64>) => Some(8));
            test!(#[repr(C)] (u8; Nested<u8, [AU64]>) => Some(8));

            // Test that `packed(N)` limits the offset of the trailing field.
            test!(#[repr(C, packed(        1))] (u8; elain::Align<        2>) => Some(        1));
            test!(#[repr(C, packed(        2))] (u8; elain::Align<        4>) => Some(        2));
            test!(#[repr(C, packed(        4))] (u8; elain::Align<        8>) => Some(        4));
            test!(#[repr(C, packed(        8))] (u8; elain::Align<       16>) => Some(        8));
            test!(#[repr(C, packed(       16))] (u8; elain::Align<       32>) => Some(       16));
            test!(#[repr(C, packed(       32))] (u8; elain::Align<       64>) => Some(       32));
            test!(#[repr(C, packed(       64))] (u8; elain::Align<      128>) => Some(       64));
            test!(#[repr(C, packed(      128))] (u8; elain::Align<      256>) => Some(      128));
            test!(#[repr(C, packed(      256))] (u8; elain::Align<      512>) => Some(      256));
            test!(#[repr(C, packed(      512))] (u8; elain::Align<     1024>) => Some(      512));
            test!(#[repr(C, packed(     1024))] (u8; elain::Align<     2048>) => Some(     1024));
            test!(#[repr(C, packed(     2048))] (u8; elain::Align<     4096>) => Some(     2048));
            test!(#[repr(C, packed(     4096))] (u8; elain::Align<     8192>) => Some(     4096));
            test!(#[repr(C, packed(     8192))] (u8; elain::Align<    16384>) => Some(     8192));
            test!(#[repr(C, packed(    16384))] (u8; elain::Align<    32768>) => Some(    16384));
            test!(#[repr(C, packed(    32768))] (u8; elain::Align<    65536>) => Some(    32768));
            test!(#[repr(C, packed(    65536))] (u8; elain::Align<   131072>) => Some(    65536));
            /* Alignments above 65536 are not yet supported.
            test!(#[repr(C, packed(   131072))] (u8; elain::Align<   262144>) => Some(   131072));
            test!(#[repr(C, packed(   262144))] (u8; elain::Align<   524288>) => Some(   262144));
            test!(#[repr(C, packed(   524288))] (u8; elain::Align<  1048576>) => Some(   524288));
            test!(#[repr(C, packed(  1048576))] (u8; elain::Align<  2097152>) => Some(  1048576));
            test!(#[repr(C, packed(  2097152))] (u8; elain::Align<  4194304>) => Some(  2097152));
            test!(#[repr(C, packed(  4194304))] (u8; elain::Align<  8388608>) => Some(  4194304));
            test!(#[repr(C, packed(  8388608))] (u8; elain::Align< 16777216>) => Some(  8388608));
            test!(#[repr(C, packed( 16777216))] (u8; elain::Align< 33554432>) => Some( 16777216));
            test!(#[repr(C, packed( 33554432))] (u8; elain::Align< 67108864>) => Some( 33554432));
            test!(#[repr(C, packed( 67108864))] (u8; elain::Align< 33554432>) => Some( 67108864));
            test!(#[repr(C, packed( 33554432))] (u8; elain::Align<134217728>) => Some( 33554432));
            test!(#[repr(C, packed(134217728))] (u8; elain::Align<268435456>) => Some(134217728));
            test!(#[repr(C, packed(268435456))] (u8; elain::Align<268435456>) => Some(268435456));
            */

            // Test that `align(N)` does not limit the offset of the trailing field.
            test!(#[repr(C, align(        1))] (u8; elain::Align<        2>) => Some(        2));
            test!(#[repr(C, align(        2))] (u8; elain::Align<        4>) => Some(        4));
            test!(#[repr(C, align(        4))] (u8; elain::Align<        8>) => Some(        8));
            test!(#[repr(C, align(        8))] (u8; elain::Align<       16>) => Some(       16));
            test!(#[repr(C, align(       16))] (u8; elain::Align<       32>) => Some(       32));
            test!(#[repr(C, align(       32))] (u8; elain::Align<       64>) => Some(       64));
            test!(#[repr(C, align(       64))] (u8; elain::Align<      128>) => Some(      128));
            test!(#[repr(C, align(      128))] (u8; elain::Align<      256>) => Some(      256));
            test!(#[repr(C, align(      256))] (u8; elain::Align<      512>) => Some(      512));
            test!(#[repr(C, align(      512))] (u8; elain::Align<     1024>) => Some(     1024));
            test!(#[repr(C, align(     1024))] (u8; elain::Align<     2048>) => Some(     2048));
            test!(#[repr(C, align(     2048))] (u8; elain::Align<     4096>) => Some(     4096));
            test!(#[repr(C, align(     4096))] (u8; elain::Align<     8192>) => Some(     8192));
            test!(#[repr(C, align(     8192))] (u8; elain::Align<    16384>) => Some(    16384));
            test!(#[repr(C, align(    16384))] (u8; elain::Align<    32768>) => Some(    32768));
            test!(#[repr(C, align(    32768))] (u8; elain::Align<    65536>) => Some(    65536));
            /* Alignments above 65536 are not yet supported.
            test!(#[repr(C, align(    65536))] (u8; elain::Align<   131072>) => Some(   131072));
            test!(#[repr(C, align(   131072))] (u8; elain::Align<   262144>) => Some(   262144));
            test!(#[repr(C, align(   262144))] (u8; elain::Align<   524288>) => Some(   524288));
            test!(#[repr(C, align(   524288))] (u8; elain::Align<  1048576>) => Some(  1048576));
            test!(#[repr(C, align(  1048576))] (u8; elain::Align<  2097152>) => Some(  2097152));
            test!(#[repr(C, align(  2097152))] (u8; elain::Align<  4194304>) => Some(  4194304));
            test!(#[repr(C, align(  4194304))] (u8; elain::Align<  8388608>) => Some(  8388608));
            test!(#[repr(C, align(  8388608))] (u8; elain::Align< 16777216>) => Some( 16777216));
            test!(#[repr(C, align( 16777216))] (u8; elain::Align< 33554432>) => Some( 33554432));
            test!(#[repr(C, align( 33554432))] (u8; elain::Align< 67108864>) => Some( 67108864));
            test!(#[repr(C, align( 67108864))] (u8; elain::Align< 33554432>) => Some( 33554432));
            test!(#[repr(C, align( 33554432))] (u8; elain::Align<134217728>) => Some(134217728));
            test!(#[repr(C, align(134217728))] (u8; elain::Align<268435456>) => Some(268435456));
            */
        }

        // FIXME(#29), FIXME(https://github.com/rust-lang/rust/issues/69835):
        // Remove this `cfg` when `size_of_val_raw` is stabilized.
        #[allow(clippy::decimal_literal_representation)]
        #[test]
        fn test_align_of_dst() {
            // Test that `align_of!` correctly computes the alignment of DSTs.
            assert_eq!(align_of!([elain::Align<1>]), Some(1));
            assert_eq!(align_of!([elain::Align<2>]), Some(2));
            assert_eq!(align_of!([elain::Align<4>]), Some(4));
            assert_eq!(align_of!([elain::Align<8>]), Some(8));
            assert_eq!(align_of!([elain::Align<16>]), Some(16));
            assert_eq!(align_of!([elain::Align<32>]), Some(32));
            assert_eq!(align_of!([elain::Align<64>]), Some(64));
            assert_eq!(align_of!([elain::Align<128>]), Some(128));
            assert_eq!(align_of!([elain::Align<256>]), Some(256));
            assert_eq!(align_of!([elain::Align<512>]), Some(512));
            assert_eq!(align_of!([elain::Align<1024>]), Some(1024));
            assert_eq!(align_of!([elain::Align<2048>]), Some(2048));
            assert_eq!(align_of!([elain::Align<4096>]), Some(4096));
            assert_eq!(align_of!([elain::Align<8192>]), Some(8192));
            assert_eq!(align_of!([elain::Align<16384>]), Some(16384));
            assert_eq!(align_of!([elain::Align<32768>]), Some(32768));
            assert_eq!(align_of!([elain::Align<65536>]), Some(65536));
            /* Alignments above 65536 are not yet supported.
            assert_eq!(align_of!([elain::Align<131072>]), Some(131072));
            assert_eq!(align_of!([elain::Align<262144>]), Some(262144));
            assert_eq!(align_of!([elain::Align<524288>]), Some(524288));
            assert_eq!(align_of!([elain::Align<1048576>]), Some(1048576));
            assert_eq!(align_of!([elain::Align<2097152>]), Some(2097152));
            assert_eq!(align_of!([elain::Align<4194304>]), Some(4194304));
            assert_eq!(align_of!([elain::Align<8388608>]), Some(8388608));
            assert_eq!(align_of!([elain::Align<16777216>]), Some(16777216));
            assert_eq!(align_of!([elain::Align<33554432>]), Some(33554432));
            assert_eq!(align_of!([elain::Align<67108864>]), Some(67108864));
            assert_eq!(align_of!([elain::Align<33554432>]), Some(33554432));
            assert_eq!(align_of!([elain::Align<134217728>]), Some(134217728));
            assert_eq!(align_of!([elain::Align<268435456>]), Some(268435456));
            */
        }
    }

    #[test]
    fn test_enum_casts() {
        // Test that casting the variants of enums with signed integer reprs to
        // unsigned integers obeys expected signed -> unsigned casting rules.

        #[repr(i8)]
        enum ReprI8 {
            MinusOne = -1,
            Zero = 0,
            Min = i8::MIN,
            Max = i8::MAX,
        }

        #[allow(clippy::as_conversions)]
        let x = ReprI8::MinusOne as u8;
        assert_eq!(x, u8::MAX);

        #[allow(clippy::as_conversions)]
        let x = ReprI8::Zero as u8;
        assert_eq!(x, 0);

        #[allow(clippy::as_conversions)]
        let x = ReprI8::Min as u8;
        assert_eq!(x, 128);

        #[allow(clippy::as_conversions)]
        let x = ReprI8::Max as u8;
        assert_eq!(x, 127);
    }

    #[test]
    fn test_struct_padding() {
        // Test that, for each provided repr, `struct_padding!` reports the
        // expected value.
        macro_rules! test {
            (#[$cfg:meta] ($($ts:ty),*) => $expect:expr) => {{
                #[$cfg]
                #[allow(dead_code)]
                struct Test($($ts),*);
                assert_eq!(struct_padding!(Test, None::<NonZeroUsize>, None::<NonZeroUsize>, [$($ts),*]), $expect);
            }};
            (#[$cfg:meta] $(#[$cfgs:meta])* ($($ts:ty),*) => $expect:expr) => {
                test!(#[$cfg] ($($ts),*) => $expect);
                test!($(#[$cfgs])* ($($ts),*) => $expect);
            };
        }

        test!(#[repr(C)] #[repr(transparent)] #[repr(packed)] () => 0);
        test!(#[repr(C)] #[repr(transparent)] #[repr(packed)] (u8) => 0);
        test!(#[repr(C)] #[repr(transparent)] #[repr(packed)] (u8, ()) => 0);
        test!(#[repr(C)] #[repr(packed)] (u8, u8) => 0);

        test!(#[repr(C)] (u8, AU64) => 7);
        // Rust won't let you put `#[repr(packed)]` on a type which contains a
        // `#[repr(align(n > 1))]` type (`AU64`), so we have to use `u64` here.
        // It's not ideal, but it definitely has align > 1 on /some/ of our CI
        // targets, and this isn't a particularly complex macro we're testing
        // anyway.
        test!(#[repr(packed)] (u8, u64) => 0);
    }

    #[test]
    fn test_repr_c_struct_padding() {
        // Test that, for each provided repr, `repr_c_struct_padding!` reports
        // the expected value.
        macro_rules! test {
            (($($ts:tt),*) => $expect:expr) => {{
                #[repr(C)]
                #[allow(dead_code)]
                struct Test($($ts),*);
                assert_eq!(repr_c_struct_has_padding!(Test, None::<NonZeroUsize>, None::<NonZeroUsize>, [$($ts),*]), $expect);
            }};
        }

        // Test static padding
        test!(() => false);
        test!(([u8]) => false);
        test!((u8) => false);
        test!((u8, [u8]) => false);
        test!((u8, ()) => false);
        test!((u8, (), [u8]) => false);
        test!((u8, u8) => false);
        test!((u8, u8, [u8]) => false);

        test!((u8, AU64) => true);
        test!((u8, AU64, [u8]) => true);

        // Test dynamic padding
        test!((AU64, [AU64]) => false);
        test!((u8, [AU64]) => true);

        #[repr(align(4))]
        struct AU32(#[allow(unused)] u32);
        test!((AU64, [AU64]) => false);
        test!((AU64, [AU32]) => true);
    }

    #[test]
    fn test_union_padding() {
        // Test that, for each provided repr, `union_padding!` reports the
        // expected value.
        macro_rules! test {
            (#[$cfg:meta] {$($fs:ident: $ts:ty),*} => $expect:expr) => {{
                #[$cfg]
                #[allow(unused)] // fields are never read
                union Test{ $($fs: $ts),* }
                assert_eq!(union_padding!(Test, None::<NonZeroUsize>, None::<usize>, [$($ts),*]), $expect);
            }};
            (#[$cfg:meta] $(#[$cfgs:meta])* {$($fs:ident: $ts:ty),*} => $expect:expr) => {
                test!(#[$cfg] {$($fs: $ts),*} => $expect);
                test!($(#[$cfgs])* {$($fs: $ts),*} => $expect);
            };
        }

        test!(#[repr(C)] #[repr(packed)] {a: u8} => 0);
        test!(#[repr(C)] #[repr(packed)] {a: u8, b: u8} => 0);

        // Rust won't let you put `#[repr(packed)]` on a type which contains a
        // `#[repr(align(n > 1))]` type (`AU64`), so we have to use `u64` here.
        // It's not ideal, but it definitely has align > 1 on /some/ of our CI
        // targets, and this isn't a particularly complex macro we're testing
        // anyway.
        test!(#[repr(C)] #[repr(packed)] {a: u8, b: u64} => 7);
    }

    #[test]
    fn test_enum_padding() {
        // Test that, for each provided repr, `enum_has_padding!` reports the
        // expected value.
        macro_rules! test {
            (#[repr($disc:ident $(, $c:ident)?)] { $($vs:ident ($($ts:ty),*),)* } => $expect:expr) => {
                test!(@case #[repr($disc $(, $c)?)] { $($vs ($($ts),*),)* } => $expect);
            };
            (#[repr($disc:ident $(, $c:ident)?)] #[$cfg:meta] $(#[$cfgs:meta])* { $($vs:ident ($($ts:ty),*),)* } => $expect:expr) => {
                test!(@case #[repr($disc $(, $c)?)] #[$cfg] { $($vs ($($ts),*),)* } => $expect);
                test!(#[repr($disc $(, $c)?)] $(#[$cfgs])* { $($vs ($($ts),*),)* } => $expect);
            };
            (@case #[repr($disc:ident $(, $c:ident)?)] $(#[$cfg:meta])? { $($vs:ident ($($ts:ty),*),)* } => $expect:expr) => {{
                #[repr($disc $(, $c)?)]
                $(#[$cfg])?
                #[allow(unused)] // variants and fields are never used
                enum Test {
                    $($vs ($($ts),*),)*
                }
                assert_eq!(
                    enum_padding!(Test, None::<NonZeroUsize>, None::<NonZeroUsize>, $disc, $([$($ts),*]),*),
                    $expect
                );
            }};
        }

        #[allow(unused)]
        #[repr(align(2))]
        struct U16(u16);

        #[allow(unused)]
        #[repr(align(4))]
        struct U32(u32);

        test!(#[repr(u8)] #[repr(C)] {
            A(u8),
        } => 0);
        test!(#[repr(u16)] #[repr(C)] {
            A(u8, u8),
            B(U16),
        } => 0);
        test!(#[repr(u32)] #[repr(C)] {
            A(u8, u8, u8, u8),
            B(U16, u8, u8),
            C(u8, u8, U16),
            D(U16, U16),
            E(U32),
        } => 0);

        // `repr(int)` can pack the discriminant more efficiently
        test!(#[repr(u8)] {
            A(u8, U16),
        } => 0);
        test!(#[repr(u8)] {
            A(u8, U16, U32),
        } => 0);

        // `repr(C)` cannot
        test!(#[repr(u8, C)] {
            A(u8, U16),
        } => 2);
        test!(#[repr(u8, C)] {
            A(u8, u8, u8, U32),
        } => 4);

        // And field ordering can always cause problems
        test!(#[repr(u8)] #[repr(C)] {
            A(U16, u8),
        } => 2);
        test!(#[repr(u8)] #[repr(C)] {
            A(U32, u8, u8, u8),
        } => 4);
    }
}