syn/
parse_macro_input.rs

1// SPDX-License-Identifier: Apache-2.0 OR MIT
2
3/// Parse the input TokenStream of a macro, triggering a compile error if the
4/// tokens fail to parse.
5///
6/// Refer to the [`parse` module] documentation for more details about parsing
7/// in Syn.
8///
9/// [`parse` module]: mod@crate::parse
10///
11/// <br>
12///
13/// # Intended usage
14///
15/// This macro must be called from a function that returns
16/// `proc_macro::TokenStream`. Usually this will be your proc macro entry point,
17/// the function that has the #\[proc_macro\] / #\[proc_macro_derive\] /
18/// #\[proc_macro_attribute\] attribute.
19///
20/// ```
21/// # extern crate proc_macro;
22/// #
23/// use proc_macro::TokenStream;
24/// use syn::{parse_macro_input, Result};
25/// use syn::parse::{Parse, ParseStream};
26///
27/// struct MyMacroInput {
28///     /* ... */
29/// }
30///
31/// impl Parse for MyMacroInput {
32///     fn parse(input: ParseStream) -> Result<Self> {
33///         /* ... */
34/// #       Ok(MyMacroInput {})
35///     }
36/// }
37///
38/// # const IGNORE: &str = stringify! {
39/// #[proc_macro]
40/// # };
41/// pub fn my_macro(tokens: TokenStream) -> TokenStream {
42///     let input = parse_macro_input!(tokens as MyMacroInput);
43///
44///     /* ... */
45/// #   TokenStream::new()
46/// }
47/// ```
48///
49/// <br>
50///
51/// # Usage with Parser
52///
53/// This macro can also be used with the [`Parser` trait] for types that have
54/// multiple ways that they can be parsed.
55///
56/// [`Parser` trait]: crate::parse::Parser
57///
58/// ```
59/// # extern crate proc_macro;
60/// #
61/// # use proc_macro::TokenStream;
62/// # use syn::{parse_macro_input, Result};
63/// # use syn::parse::ParseStream;
64/// #
65/// # struct MyMacroInput {}
66/// #
67/// impl MyMacroInput {
68///     fn parse_alternate(input: ParseStream) -> Result<Self> {
69///         /* ... */
70/// #       Ok(MyMacroInput {})
71///     }
72/// }
73///
74/// # const IGNORE: &str = stringify! {
75/// #[proc_macro]
76/// # };
77/// pub fn my_macro(tokens: TokenStream) -> TokenStream {
78///     let input = parse_macro_input!(tokens with MyMacroInput::parse_alternate);
79///
80///     /* ... */
81/// #   TokenStream::new()
82/// }
83/// ```
84///
85/// <br>
86///
87/// # Expansion
88///
89/// `parse_macro_input!($variable as $Type)` expands to something like:
90///
91/// ```no_run
92/// # extern crate proc_macro;
93/// #
94/// # macro_rules! doc_test {
95/// #     ($variable:ident as $Type:ty) => {
96/// match syn::parse::<$Type>($variable) {
97///     Ok(syntax_tree) => syntax_tree,
98///     Err(err) => return proc_macro::TokenStream::from(err.to_compile_error()),
99/// }
100/// #     };
101/// # }
102/// #
103/// # fn test(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
104/// #     let _ = doc_test!(input as syn::Ident);
105/// #     proc_macro::TokenStream::new()
106/// # }
107/// ```
108#[macro_export]
109#[cfg_attr(docsrs, doc(cfg(all(feature = "parsing", feature = "proc-macro"))))]
110macro_rules! parse_macro_input {
111    ($tokenstream:ident as $ty:ty) => {
112        match $crate::parse::<$ty>($tokenstream) {
113            $crate::__private::Ok(data) => data,
114            $crate::__private::Err(err) => {
115                return $crate::__private::TokenStream::from(err.to_compile_error());
116            }
117        }
118    };
119    ($tokenstream:ident with $parser:path) => {
120        match $crate::parse::Parser::parse($parser, $tokenstream) {
121            $crate::__private::Ok(data) => data,
122            $crate::__private::Err(err) => {
123                return $crate::__private::TokenStream::from(err.to_compile_error());
124            }
125        }
126    };
127    ($tokenstream:ident) => {
128        $crate::parse_macro_input!($tokenstream as _)
129    };
130}
syn/parse_macro_input.rs

syn/
parse_macro_input.rs
