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    /// Parse the input TokenStream of a macro, triggering a compile error if the
    /// tokens fail to parse.
    ///
    /// Refer to the [`parse` module] documentation for more details about parsing
    /// in Syn.
    ///
    /// [`parse` module]: mod@crate::parse
    ///
    /// <br>
    ///
    /// # Intended usage
    ///
    /// This macro must be called from a function that returns
    /// `proc_macro::TokenStream`. Usually this will be your proc macro entry point,
    /// the function that has the #\[proc_macro\] / #\[proc_macro_derive\] /
    /// #\[proc_macro_attribute\] attribute.
    ///
    /// ```
    /// # extern crate proc_macro;
    /// #
    /// use proc_macro::TokenStream;
    /// use syn::{parse_macro_input, Result};
    /// use syn::parse::{Parse, ParseStream};
    ///
    /// struct MyMacroInput {
    ///     /* ... */
    /// }
    ///
    /// impl Parse for MyMacroInput {
    ///     fn parse(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 as MyMacroInput);
    ///
    ///     /* ... */
    /// #   "".parse().unwrap()
    /// }
    /// ```
    ///
    /// <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::parse::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 $Type)` expands to something like:
    ///
    /// ```no_run
    /// # extern crate proc_macro;
    /// #
    /// # macro_rules! doc_test {
    /// #     ($variable:ident as $Type:ty) => {
    /// match syn::parse::<$Type>($variable) {
    ///     Ok(syntax_tree) => syntax_tree,
    ///     Err(err) => return proc_macro::TokenStream::from(err.to_compile_error()),
    /// }
    /// #     };
    /// # }
    /// #
    /// # fn test(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    /// #     let _ = doc_test!(input as syn::Ident);
    /// #     proc_macro::TokenStream::new()
    /// # }
    /// ```
    #[macro_export]
    #[cfg_attr(doc_cfg, doc(cfg(all(feature = "parsing", feature = "proc-macro"))))]
    macro_rules! parse_macro_input {
        ($tokenstream:ident as $ty:ty) => {
            match $crate::parse_macro_input::parse::<$ty>($tokenstream) {
                $crate::__private::Ok(data) => data,
                $crate::__private::Err(err) => {
                    return $crate::__private::TokenStream::from(err.to_compile_error());
                }
            }
        };
        ($tokenstream:ident with $parser:path) => {
            match $crate::parse::Parser::parse($parser, $tokenstream) {
                $crate::__private::Ok(data) => data,
                $crate::__private::Err(err) => {
                    return $crate::__private::TokenStream::from(err.to_compile_error());
                }
            }
        };
        ($tokenstream:ident) => {
            $crate::parse_macro_input!($tokenstream as _)
        };
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Can parse any type that implements Parse.

    use crate::parse::{Parse, ParseStream, Parser, Result};
    use proc_macro::TokenStream;

    // Not public API.
    #[doc(hidden)]
    pub fn parse<T: ParseMacroInput>(token_stream: TokenStream) -> Result<T> {
        T::parse.parse(token_stream)
    }

    // Not public API.
    #[doc(hidden)]
    pub trait ParseMacroInput: Sized {
        fn parse(input: ParseStream) -> Result<Self>;
    }

    impl<T: Parse> ParseMacroInput for T {
        fn parse(input: ParseStream) -> Result<Self> {
            <T as Parse>::parse(input)
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Any other types that we want `parse_macro_input!` to be able to parse.

    #[cfg(any(feature = "full", feature = "derive"))]
    use crate::AttributeArgs;

    #[cfg(any(feature = "full", feature = "derive"))]
    impl ParseMacroInput for AttributeArgs {
        fn parse(input: ParseStream) -> Result<Self> {
            let mut metas = Vec::new();

            loop {
                if input.is_empty() {
                    break;
                }
                let value = input.parse()?;
                metas.push(value);
                if input.is_empty() {
                    break;
                }
                input.parse::<Token![,]>()?;
            }

            Ok(metas)
        }
    }