Add case to parse_macro_input! to use a Parser function

src/parse_macro_input.rs

@@ -46,6 +46,42 @@
 ///
 /// <br>
 ///
+/// # Usage with Parser
+///
+/// This macro can also be used with the [`Parser` trait] for types that have
+/// multiple ways that they can be parsed.
+///
+/// [`Parser` trait]: crate::rustdoc_workaround::parse_module::Parser
+///
+/// ```
+/// # extern crate proc_macro;
+/// #
+/// # use proc_macro::TokenStream;
+/// # use syn::{parse_macro_input, Result};
+/// # use syn::parse::ParseStream;
+/// #
+/// # struct MyMacroInput {}
+/// #
+/// impl MyMacroInput {
+///     fn parse_alternate(input: ParseStream) -> Result<Self> {
+///         /* ... */
+/// #       Ok(MyMacroInput {})
+///     }
+/// }
+///
+/// # const IGNORE: &str = stringify! {
+/// #[proc_macro]
+/// # };
+/// pub fn my_macro(tokens: TokenStream) -> TokenStream {
+///     let input = parse_macro_input!(tokens with MyMacroInput::parse_alternate);
+///
+///     /* ... */
+/// #   "".parse().unwrap()
+/// }
+/// ```
+///
+/// <br>
+///
 /// # Expansion
 ///
 /// `parse_macro_input!($variable as $Type)` expands to something like:
@@ -80,6 +116,15 @@ macro_rules! parse_macro_input {
     ($tokenstream:ident) => {
         $crate::parse_macro_input!($tokenstream as _)
     };
+    ($tokenstream:ident with $parser:path) => {{
+        use $crate::parse::Parser;
+        match $parser.parse($tokenstream) {
+            $crate::export::Ok(data) => data,
+            $crate::export::Err(err) => {
+                return $crate::export::TokenStream::from(err.to_compile_error());
+            }
+        }
+    }};
 }
 
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