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 <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-2.0.18/src/parse.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>parse.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../normalize.css"><link rel="stylesheet" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../ayu.css" disabled><link rel="stylesheet" href="../../dark.css" disabled><link rel="stylesheet" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script defer src="../../source-script.js"></script><script defer src="../../source-files.js"></script><script defer src="../../main.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../syn/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../syn/index.html"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span>
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 </pre><pre class="rust"><code><span class="doccomment">//! Parsing interface for parsing a token stream into a syntax tree node.
 //!
 //! Parsing in Syn is built on parser functions that take in a [`ParseStream`]
 //! and produce a [`Result&lt;T&gt;`] where `T` is some syntax tree node. Underlying
 //! these parser functions is a lower level mechanism built around the
 //! [`Cursor`] type. `Cursor` is a cheaply copyable cursor over a range of
 //! tokens in a token stream.
 //!
 //! [`Result&lt;T&gt;`]: Result
 //! [`Cursor`]: crate::buffer::Cursor
 //!
 //! # Example
 //!
 //! Here is a snippet of parsing code to get a feel for the style of the
 //! library. We define data structures for a subset of Rust syntax including
 //! enums (not shown) and structs, then provide implementations of the [`Parse`]
 //! trait to parse these syntax tree data structures from a token stream.
 //!
 //! Once `Parse` impls have been defined, they can be called conveniently from a
 //! procedural macro through [`parse_macro_input!`] as shown at the bottom of
 //! the snippet. If the caller provides syntactically invalid input to the
 //! procedural macro, they will receive a helpful compiler error message
 //! pointing out the exact token that triggered the failure to parse.
 //!
 //! [`parse_macro_input!`]: crate::parse_macro_input!
 //!
 //! ```
 //! # extern crate proc_macro;
 //! #
 //! use proc_macro::TokenStream;
 //! use syn::{braced, parse_macro_input, token, Field, Ident, Result, Token};
 //! use syn::parse::{Parse, ParseStream};
 //! use syn::punctuated::Punctuated;
 //!
 //! enum Item {
 //!     Struct(ItemStruct),
 //!     Enum(ItemEnum),
 //! }
 //!
 //! struct ItemStruct {
 //!     struct_token: Token![struct],
 //!     ident: Ident,
 //!     brace_token: token::Brace,
 //!     fields: Punctuated&lt;Field, Token![,]&gt;,
 //! }
 //! #
 //! # enum ItemEnum {}
 //!
 //! impl Parse for Item {
 //!     fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
 //!         let lookahead = input.lookahead1();
 //!         if lookahead.peek(Token![struct]) {
 //!             input.parse().map(Item::Struct)
 //!         } else if lookahead.peek(Token![enum]) {
 //!             input.parse().map(Item::Enum)
 //!         } else {
 //!             Err(lookahead.error())
 //!         }
 //!     }
 //! }
 //!
 //! impl Parse for ItemStruct {
 //!     fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
 //!         let content;
 //!         Ok(ItemStruct {
 //!             struct_token: input.parse()?,
 //!             ident: input.parse()?,
 //!             brace_token: braced!(content in input),
 //!             fields: content.parse_terminated(Field::parse_named, Token![,])?,
 //!         })
 //!     }
 //! }
 //! #
 //! # impl Parse for ItemEnum {
 //! #     fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
 //! #         unimplemented!()
 //! #     }
 //! # }
 //!
 //! # const IGNORE: &amp;str = stringify! {
 //! #[proc_macro]
 //! # };
 //! pub fn my_macro(tokens: TokenStream) -&gt; TokenStream {
 //!     let input = parse_macro_input!(tokens as Item);
 //!
 //!     /* ... */
 //! #   TokenStream::new()
 //! }
 //! ```
 //!
 //! # The `syn::parse*` functions
 //!
 //! The [`syn::parse`], [`syn::parse2`], and [`syn::parse_str`] functions serve
 //! as an entry point for parsing syntax tree nodes that can be parsed in an
 //! obvious default way. These functions can return any syntax tree node that
 //! implements the [`Parse`] trait, which includes most types in Syn.
 //!
 //! [`syn::parse`]: crate::parse()
 //! [`syn::parse2`]: crate::parse2()
 //! [`syn::parse_str`]: crate::parse_str()
 //!
 //! ```
 //! use syn::Type;
 //!
 //! # fn run_parser() -&gt; syn::Result&lt;()&gt; {
 //! let t: Type = syn::parse_str(&quot;std::collections::HashMap&lt;String, Value&gt;&quot;)?;
 //! #     Ok(())
 //! # }
 //! #
 //! # run_parser().unwrap();
 //! ```
 //!
 //! The [`parse_quote!`] macro also uses this approach.
 //!
 //! [`parse_quote!`]: crate::parse_quote!
 //!
 //! # The `Parser` trait
 //!
 //! Some types can be parsed in several ways depending on context. For example
 //! an [`Attribute`] can be either &quot;outer&quot; like `#[...]` or &quot;inner&quot; like
 //! `#![...]` and parsing the wrong one would be a bug. Similarly [`Punctuated`]
 //! may or may not allow trailing punctuation, and parsing it the wrong way
 //! would either reject valid input or accept invalid input.
 //!
 //! [`Attribute`]: crate::Attribute
 //! [`Punctuated`]: crate::punctuated
 //!
 //! The `Parse` trait is not implemented in these cases because there is no good
 //! behavior to consider the default.
 //!
 //! ```compile_fail
 //! # extern crate proc_macro;
 //! #
 //! # use syn::punctuated::Punctuated;
 //! # use syn::{PathSegment, Result, Token};
 //! #
 //! # fn f(tokens: proc_macro::TokenStream) -&gt; Result&lt;()&gt; {
 //! #
 //! // Can&#39;t parse `Punctuated` without knowing whether trailing punctuation
 //! // should be allowed in this context.
 //! let path: Punctuated&lt;PathSegment, Token![::]&gt; = syn::parse(tokens)?;
 //! #
 //! #     Ok(())
 //! # }
 //! ```
 //!
 //! In these cases the types provide a choice of parser functions rather than a
 //! single `Parse` implementation, and those parser functions can be invoked
 //! through the [`Parser`] trait.
 //!
 //!
 //! ```
 //! # extern crate proc_macro;
 //! #
 //! use proc_macro::TokenStream;
 //! use syn::parse::Parser;
 //! use syn::punctuated::Punctuated;
 //! use syn::{Attribute, Expr, PathSegment, Result, Token};
 //!
 //! fn call_some_parser_methods(input: TokenStream) -&gt; Result&lt;()&gt; {
 //!     // Parse a nonempty sequence of path segments separated by `::` punctuation
 //!     // with no trailing punctuation.
 //!     let tokens = input.clone();
 //!     let parser = Punctuated::&lt;PathSegment, Token![::]&gt;::parse_separated_nonempty;
 //!     let _path = parser.parse(tokens)?;
 //!
 //!     // Parse a possibly empty sequence of expressions terminated by commas with
 //!     // an optional trailing punctuation.
 //!     let tokens = input.clone();
 //!     let parser = Punctuated::&lt;Expr, Token![,]&gt;::parse_terminated;
 //!     let _args = parser.parse(tokens)?;
 //!
 //!     // Parse zero or more outer attributes but not inner attributes.
 //!     let tokens = input.clone();
 //!     let parser = Attribute::parse_outer;
 //!     let _attrs = parser.parse(tokens)?;
 //!
 //!     Ok(())
 //! }
 //! ```

 </span><span class="attribute">#[path = <span class="string">&quot;discouraged.rs&quot;</span>]
 </span><span class="kw">pub mod </span>discouraged;

 <span class="kw">use </span><span class="kw">crate</span>::buffer::{Cursor, TokenBuffer};
 <span class="kw">use </span><span class="kw">crate</span>::error;
 <span class="kw">use </span><span class="kw">crate</span>::lookahead;
 <span class="attribute">#[cfg(feature = <span class="string">&quot;proc-macro&quot;</span>)]
 </span><span class="kw">use </span><span class="kw">crate</span>::proc_macro;
 <span class="kw">use </span><span class="kw">crate</span>::punctuated::Punctuated;
 <span class="kw">use </span><span class="kw">crate</span>::token::Token;
 <span class="kw">use </span>proc_macro2::{<span class="self">self</span>, Delimiter, Group, Literal, Punct, Span, TokenStream, TokenTree};
 <span class="kw">use </span>std::cell::Cell;
 <span class="kw">use </span>std::fmt::{<span class="self">self</span>, Debug, Display};
 <span class="attribute">#[cfg(feature = <span class="string">&quot;extra-traits&quot;</span>)]
 </span><span class="kw">use </span>std::hash::{Hash, Hasher};
 <span class="kw">use </span>std::marker::PhantomData;
 <span class="kw">use </span>std::mem;
 <span class="kw">use </span>std::ops::Deref;
 <span class="kw">use </span>std::rc::Rc;
 <span class="kw">use </span>std::str::FromStr;

 <span class="kw">pub use </span><span class="kw">crate</span>::error::{Error, <span class="prelude-ty">Result</span>};
 <span class="kw">pub use </span><span class="kw">crate</span>::lookahead::{Lookahead1, Peek};

 <span class="doccomment">/// Parsing interface implemented by all types that can be parsed in a default
 /// way from a token stream.
 ///
 /// Refer to the [module documentation] for details about implementing and using
 /// the `Parse` trait.
 ///
 /// [module documentation]: self
 </span><span class="kw">pub trait </span>Parse: Sized {
     <span class="kw">fn </span>parse(input: ParseStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>&gt;;
 }

 <span class="doccomment">/// Input to a Syn parser function.
 ///
 /// See the methods of this type under the documentation of [`ParseBuffer`]. For
 /// an overview of parsing in Syn, refer to the [module documentation].
 ///
 /// [module documentation]: self
 </span><span class="kw">pub type </span>ParseStream&lt;<span class="lifetime">&#39;a</span>&gt; = <span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>ParseBuffer&lt;<span class="lifetime">&#39;a</span>&gt;;

 <span class="doccomment">/// Cursor position within a buffered token stream.
 ///
 /// This type is more commonly used through the type alias [`ParseStream`] which
 /// is an alias for `&amp;ParseBuffer`.
 ///
 /// `ParseStream` is the input type for all parser functions in Syn. They have
 /// the signature `fn(ParseStream) -&gt; Result&lt;T&gt;`.
 ///
 /// ## Calling a parser function
 ///
 /// There is no public way to construct a `ParseBuffer`. Instead, if you are
 /// looking to invoke a parser function that requires `ParseStream` as input,
 /// you will need to go through one of the public parsing entry points.
 ///
 /// - The [`parse_macro_input!`] macro if parsing input of a procedural macro;
 /// - One of [the `syn::parse*` functions][syn-parse]; or
 /// - A method of the [`Parser`] trait.
 ///
 /// [`parse_macro_input!`]: crate::parse_macro_input!
 /// [syn-parse]: self#the-synparse-functions
 </span><span class="kw">pub struct </span>ParseBuffer&lt;<span class="lifetime">&#39;a</span>&gt; {
     scope: Span,
     <span class="comment">// Instead of Cell&lt;Cursor&lt;&#39;a&gt;&gt; so that ParseBuffer&lt;&#39;a&gt; is covariant in &#39;a.
     // The rest of the code in this module needs to be careful that only a
     // cursor derived from this `cell` is ever assigned to this `cell`.
     //
     // Cell&lt;Cursor&lt;&#39;a&gt;&gt; cannot be covariant in &#39;a because then we could take a
     // ParseBuffer&lt;&#39;a&gt;, upcast to ParseBuffer&lt;&#39;short&gt; for some lifetime shorter
     // than &#39;a, and then assign a Cursor&lt;&#39;short&gt; into the Cell.
     //
     // By extension, it would not be safe to expose an API that accepts a
     // Cursor&lt;&#39;a&gt; and trusts that it lives as long as the cursor currently in
     // the cell.
     </span>cell: Cell&lt;Cursor&lt;<span class="lifetime">&#39;static</span>&gt;&gt;,
     marker: PhantomData&lt;Cursor&lt;<span class="lifetime">&#39;a</span>&gt;&gt;,
     unexpected: Cell&lt;<span class="prelude-ty">Option</span>&lt;Rc&lt;Cell&lt;Unexpected&gt;&gt;&gt;&gt;,
 }

 <span class="kw">impl</span>&lt;<span class="lifetime">&#39;a</span>&gt; Drop <span class="kw">for </span>ParseBuffer&lt;<span class="lifetime">&#39;a</span>&gt; {
     <span class="kw">fn </span>drop(<span class="kw-2">&amp;mut </span><span class="self">self</span>) {
         <span class="kw">if let </span><span class="prelude-val">Some</span>(unexpected_span) = span_of_unexpected_ignoring_nones(<span class="self">self</span>.cursor()) {
             <span class="kw">let </span>(inner, old_span) = inner_unexpected(<span class="self">self</span>);
             <span class="kw">if </span>old_span.is_none() {
                 inner.set(Unexpected::Some(unexpected_span));
             }
         }
     }
 }

 <span class="kw">impl</span>&lt;<span class="lifetime">&#39;a</span>&gt; Display <span class="kw">for </span>ParseBuffer&lt;<span class="lifetime">&#39;a</span>&gt; {
     <span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, f: <span class="kw-2">&amp;mut </span>fmt::Formatter) -&gt; fmt::Result {
         Display::fmt(<span class="kw-2">&amp;</span><span class="self">self</span>.cursor().token_stream(), f)
     }
 }

 <span class="kw">impl</span>&lt;<span class="lifetime">&#39;a</span>&gt; Debug <span class="kw">for </span>ParseBuffer&lt;<span class="lifetime">&#39;a</span>&gt; {
     <span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, f: <span class="kw-2">&amp;mut </span>fmt::Formatter) -&gt; fmt::Result {
         Debug::fmt(<span class="kw-2">&amp;</span><span class="self">self</span>.cursor().token_stream(), f)
     }
 }

 <span class="doccomment">/// Cursor state associated with speculative parsing.
 ///
 /// This type is the input of the closure provided to [`ParseStream::step`].
 ///
 /// [`ParseStream::step`]: ParseBuffer::step
 ///
 /// # Example
 ///
 /// ```
 /// use proc_macro2::TokenTree;
 /// use syn::Result;
 /// use syn::parse::ParseStream;
 ///
 /// // This function advances the stream past the next occurrence of `@`. If
 /// // no `@` is present in the stream, the stream position is unchanged and
 /// // an error is returned.
 /// fn skip_past_next_at(input: ParseStream) -&gt; Result&lt;()&gt; {
 ///     input.step(|cursor| {
 ///         let mut rest = *cursor;
 ///         while let Some((tt, next)) = rest.token_tree() {
 ///             match &amp;tt {
 ///                 TokenTree::Punct(punct) if punct.as_char() == &#39;@&#39; =&gt; {
 ///                     return Ok(((), next));
 ///                 }
 ///                 _ =&gt; rest = next,
 ///             }
 ///         }
 ///         Err(cursor.error(&quot;no `@` was found after this point&quot;))
 ///     })
 /// }
 /// #
 /// # fn remainder_after_skipping_past_next_at(
 /// #     input: ParseStream,
 /// # ) -&gt; Result&lt;proc_macro2::TokenStream&gt; {
 /// #     skip_past_next_at(input)?;
 /// #     input.parse()
 /// # }
 /// #
 /// # use syn::parse::Parser;
 /// # let remainder = remainder_after_skipping_past_next_at
 /// #     .parse_str(&quot;a @ b c&quot;)
 /// #     .unwrap();
 /// # assert_eq!(remainder.to_string(), &quot;b c&quot;);
 /// ```
 </span><span class="kw">pub struct </span>StepCursor&lt;<span class="lifetime">&#39;c</span>, <span class="lifetime">&#39;a</span>&gt; {
     scope: Span,
     <span class="comment">// This field is covariant in &#39;c.
     </span>cursor: Cursor&lt;<span class="lifetime">&#39;c</span>&gt;,
     <span class="comment">// This field is contravariant in &#39;c. Together these make StepCursor
     // invariant in &#39;c. Also covariant in &#39;a. The user cannot cast &#39;c to a
     // different lifetime but can upcast into a StepCursor with a shorter
     // lifetime &#39;a.
     //
     // As long as we only ever construct a StepCursor for which &#39;c outlives &#39;a,
     // this means if ever a StepCursor&lt;&#39;c, &#39;a&gt; exists we are guaranteed that &#39;c
     // outlives &#39;a.
     </span>marker: PhantomData&lt;<span class="kw">fn</span>(Cursor&lt;<span class="lifetime">&#39;c</span>&gt;) -&gt; Cursor&lt;<span class="lifetime">&#39;a</span>&gt;&gt;,
 }

 <span class="kw">impl</span>&lt;<span class="lifetime">&#39;c</span>, <span class="lifetime">&#39;a</span>&gt; Deref <span class="kw">for </span>StepCursor&lt;<span class="lifetime">&#39;c</span>, <span class="lifetime">&#39;a</span>&gt; {
     <span class="kw">type </span>Target = Cursor&lt;<span class="lifetime">&#39;c</span>&gt;;

     <span class="kw">fn </span>deref(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span><span class="self">Self</span>::Target {
         <span class="kw-2">&amp;</span><span class="self">self</span>.cursor
     }
 }

 <span class="kw">impl</span>&lt;<span class="lifetime">&#39;c</span>, <span class="lifetime">&#39;a</span>&gt; Copy <span class="kw">for </span>StepCursor&lt;<span class="lifetime">&#39;c</span>, <span class="lifetime">&#39;a</span>&gt; {}

 <span class="kw">impl</span>&lt;<span class="lifetime">&#39;c</span>, <span class="lifetime">&#39;a</span>&gt; Clone <span class="kw">for </span>StepCursor&lt;<span class="lifetime">&#39;c</span>, <span class="lifetime">&#39;a</span>&gt; {
     <span class="kw">fn </span>clone(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="self">Self </span>{
         <span class="kw-2">*</span><span class="self">self
     </span>}
 }

 <span class="kw">impl</span>&lt;<span class="lifetime">&#39;c</span>, <span class="lifetime">&#39;a</span>&gt; StepCursor&lt;<span class="lifetime">&#39;c</span>, <span class="lifetime">&#39;a</span>&gt; {
     <span class="doccomment">/// Triggers an error at the current position of the parse stream.
     ///
     /// The `ParseStream::step` invocation will return this same error without
     /// advancing the stream state.
     </span><span class="kw">pub fn </span>error&lt;T: Display&gt;(<span class="self">self</span>, message: T) -&gt; Error {
         error::new_at(<span class="self">self</span>.scope, <span class="self">self</span>.cursor, message)
     }
 }

 <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>advance_step_cursor&lt;<span class="lifetime">&#39;c</span>, <span class="lifetime">&#39;a</span>&gt;(proof: StepCursor&lt;<span class="lifetime">&#39;c</span>, <span class="lifetime">&#39;a</span>&gt;, to: Cursor&lt;<span class="lifetime">&#39;c</span>&gt;) -&gt; Cursor&lt;<span class="lifetime">&#39;a</span>&gt; {
     <span class="comment">// Refer to the comments within the StepCursor definition. We use the
     // fact that a StepCursor&lt;&#39;c, &#39;a&gt; exists as proof that &#39;c outlives &#39;a.
     // Cursor is covariant in its lifetime parameter so we can cast a
     // Cursor&lt;&#39;c&gt; to one with the shorter lifetime Cursor&lt;&#39;a&gt;.
     </span><span class="kw">let _ </span>= proof;
     <span class="kw">unsafe </span>{ mem::transmute::&lt;Cursor&lt;<span class="lifetime">&#39;c</span>&gt;, Cursor&lt;<span class="lifetime">&#39;a</span>&gt;&gt;(to) }
 }

 <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>new_parse_buffer(
     scope: Span,
     cursor: Cursor,
     unexpected: Rc&lt;Cell&lt;Unexpected&gt;&gt;,
 ) -&gt; ParseBuffer {
     ParseBuffer {
         scope,
         <span class="comment">// See comment on `cell` in the struct definition.
         </span>cell: Cell::new(<span class="kw">unsafe </span>{ mem::transmute::&lt;Cursor, Cursor&lt;<span class="lifetime">&#39;static</span>&gt;&gt;(cursor) }),
         marker: PhantomData,
         unexpected: Cell::new(<span class="prelude-val">Some</span>(unexpected)),
     }
 }

 <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">enum </span>Unexpected {
     <span class="prelude-val">None</span>,
     <span class="prelude-val">Some</span>(Span),
     Chain(Rc&lt;Cell&lt;Unexpected&gt;&gt;),
 }

 <span class="kw">impl </span>Default <span class="kw">for </span>Unexpected {
     <span class="kw">fn </span>default() -&gt; <span class="self">Self </span>{
         Unexpected::None
     }
 }

 <span class="kw">impl </span>Clone <span class="kw">for </span>Unexpected {
     <span class="kw">fn </span>clone(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="self">Self </span>{
         <span class="kw">match </span><span class="self">self </span>{
             Unexpected::None =&gt; Unexpected::None,
             Unexpected::Some(span) =&gt; Unexpected::Some(<span class="kw-2">*</span>span),
             Unexpected::Chain(next) =&gt; Unexpected::Chain(next.clone()),
         }
     }
 }

 <span class="comment">// We call this on Cell&lt;Unexpected&gt; and Cell&lt;Option&lt;T&gt;&gt; where temporarily
 // swapping in a None is cheap.
 </span><span class="kw">fn </span>cell_clone&lt;T: Default + Clone&gt;(cell: <span class="kw-2">&amp;</span>Cell&lt;T&gt;) -&gt; T {
     <span class="kw">let </span>prev = cell.take();
     <span class="kw">let </span>ret = prev.clone();
     cell.set(prev);
     ret
 }

 <span class="kw">fn </span>inner_unexpected(buffer: <span class="kw-2">&amp;</span>ParseBuffer) -&gt; (Rc&lt;Cell&lt;Unexpected&gt;&gt;, <span class="prelude-ty">Option</span>&lt;Span&gt;) {
     <span class="kw">let </span><span class="kw-2">mut </span>unexpected = get_unexpected(buffer);
     <span class="kw">loop </span>{
         <span class="kw">match </span>cell_clone(<span class="kw-2">&amp;</span>unexpected) {
             Unexpected::None =&gt; <span class="kw">return </span>(unexpected, <span class="prelude-val">None</span>),
             Unexpected::Some(span) =&gt; <span class="kw">return </span>(unexpected, <span class="prelude-val">Some</span>(span)),
             Unexpected::Chain(next) =&gt; unexpected = next,
         }
     }
 }

 <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>get_unexpected(buffer: <span class="kw-2">&amp;</span>ParseBuffer) -&gt; Rc&lt;Cell&lt;Unexpected&gt;&gt; {
     cell_clone(<span class="kw-2">&amp;</span>buffer.unexpected).unwrap()
 }

 <span class="kw">fn </span>span_of_unexpected_ignoring_nones(<span class="kw-2">mut </span>cursor: Cursor) -&gt; <span class="prelude-ty">Option</span>&lt;Span&gt; {
     <span class="kw">if </span>cursor.eof() {
         <span class="kw">return </span><span class="prelude-val">None</span>;
     }
     <span class="kw">while let </span><span class="prelude-val">Some</span>((inner, _span, rest)) = cursor.group(Delimiter::None) {
         <span class="kw">if let </span><span class="prelude-val">Some</span>(unexpected) = span_of_unexpected_ignoring_nones(inner) {
             <span class="kw">return </span><span class="prelude-val">Some</span>(unexpected);
         }
         cursor = rest;
     }
     <span class="kw">if </span>cursor.eof() {
         <span class="prelude-val">None
     </span>} <span class="kw">else </span>{
         <span class="prelude-val">Some</span>(cursor.span())
     }
 }

 <span class="kw">impl</span>&lt;<span class="lifetime">&#39;a</span>&gt; ParseBuffer&lt;<span class="lifetime">&#39;a</span>&gt; {
     <span class="doccomment">/// Parses a syntax tree node of type `T`, advancing the position of our
     /// parse stream past it.
     </span><span class="kw">pub fn </span>parse&lt;T: Parse&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="prelude-ty">Result</span>&lt;T&gt; {
         T::parse(<span class="self">self</span>)
     }

     <span class="doccomment">/// Calls the given parser function to parse a syntax tree node of type `T`
     /// from this stream.
     ///
     /// # Example
     ///
     /// The parser below invokes [`Attribute::parse_outer`] to parse a vector of
     /// zero or more outer attributes.
     ///
     /// [`Attribute::parse_outer`]: crate::Attribute::parse_outer
     ///
     /// ```
     /// use syn::{Attribute, Ident, Result, Token};
     /// use syn::parse::{Parse, ParseStream};
     ///
     /// // Parses a unit struct with attributes.
     /// //
     /// //     #[path = &quot;s.tmpl&quot;]
     /// //     struct S;
     /// struct UnitStruct {
     ///     attrs: Vec&lt;Attribute&gt;,
     ///     struct_token: Token![struct],
     ///     name: Ident,
     ///     semi_token: Token![;],
     /// }
     ///
     /// impl Parse for UnitStruct {
     ///     fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
     ///         Ok(UnitStruct {
     ///             attrs: input.call(Attribute::parse_outer)?,
     ///             struct_token: input.parse()?,
     ///             name: input.parse()?,
     ///             semi_token: input.parse()?,
     ///         })
     ///     }
     /// }
     /// ```
     </span><span class="kw">pub fn </span>call&lt;T&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, function: <span class="kw">fn</span>(ParseStream) -&gt; <span class="prelude-ty">Result</span>&lt;T&gt;) -&gt; <span class="prelude-ty">Result</span>&lt;T&gt; {
         function(<span class="self">self</span>)
     }

     <span class="doccomment">/// Looks at the next token in the parse stream to determine whether it
     /// matches the requested type of token.
     ///
     /// Does not advance the position of the parse stream.
     ///
     /// # Syntax
     ///
     /// Note that this method does not use turbofish syntax. Pass the peek type
     /// inside of parentheses.
     ///
     /// - `input.peek(Token![struct])`
     /// - `input.peek(Token![==])`
     /// - `input.peek(Ident)`&amp;emsp;*(does not accept keywords)*
     /// - `input.peek(Ident::peek_any)`
     /// - `input.peek(Lifetime)`
     /// - `input.peek(token::Brace)`
     ///
     /// # Example
     ///
     /// In this example we finish parsing the list of supertraits when the next
     /// token in the input is either `where` or an opening curly brace.
     ///
     /// ```
     /// use syn::{braced, token, Generics, Ident, Result, Token, TypeParamBound};
     /// use syn::parse::{Parse, ParseStream};
     /// use syn::punctuated::Punctuated;
     ///
     /// // Parses a trait definition containing no associated items.
     /// //
     /// //     trait Marker&lt;&#39;de, T&gt;: A + B&lt;&#39;de&gt; where Box&lt;T&gt;: Clone {}
     /// struct MarkerTrait {
     ///     trait_token: Token![trait],
     ///     ident: Ident,
     ///     generics: Generics,
     ///     colon_token: Option&lt;Token![:]&gt;,
     ///     supertraits: Punctuated&lt;TypeParamBound, Token![+]&gt;,
     ///     brace_token: token::Brace,
     /// }
     ///
     /// impl Parse for MarkerTrait {
     ///     fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
     ///         let trait_token: Token![trait] = input.parse()?;
     ///         let ident: Ident = input.parse()?;
     ///         let mut generics: Generics = input.parse()?;
     ///         let colon_token: Option&lt;Token![:]&gt; = input.parse()?;
     ///
     ///         let mut supertraits = Punctuated::new();
     ///         if colon_token.is_some() {
     ///             loop {
     ///                 supertraits.push_value(input.parse()?);
     ///                 if input.peek(Token![where]) || input.peek(token::Brace) {
     ///                     break;
     ///                 }
     ///                 supertraits.push_punct(input.parse()?);
     ///             }
     ///         }
     ///
     ///         generics.where_clause = input.parse()?;
     ///         let content;
     ///         let empty_brace_token = braced!(content in input);
     ///
     ///         Ok(MarkerTrait {
     ///             trait_token,
     ///             ident,
     ///             generics,
     ///             colon_token,
     ///             supertraits,
     ///             brace_token: empty_brace_token,
     ///         })
     ///     }
     /// }
     /// ```
     </span><span class="kw">pub fn </span>peek&lt;T: Peek&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, token: T) -&gt; bool {
         <span class="kw">let _ </span>= token;
         T::Token::peek(<span class="self">self</span>.cursor())
     }

     <span class="doccomment">/// Looks at the second-next token in the parse stream.
     ///
     /// This is commonly useful as a way to implement contextual keywords.
     ///
     /// # Example
     ///
     /// This example needs to use `peek2` because the symbol `union` is not a
     /// keyword in Rust. We can&#39;t use just `peek` and decide to parse a union if
     /// the very next token is `union`, because someone is free to write a `mod
     /// union` and a macro invocation that looks like `union::some_macro! { ...
     /// }`. In other words `union` is a contextual keyword.
     ///
     /// ```
     /// use syn::{Ident, ItemUnion, Macro, Result, Token};
     /// use syn::parse::{Parse, ParseStream};
     ///
     /// // Parses either a union or a macro invocation.
     /// enum UnionOrMacro {
     ///     // union MaybeUninit&lt;T&gt; { uninit: (), value: T }
     ///     Union(ItemUnion),
     ///     // lazy_static! { ... }
     ///     Macro(Macro),
     /// }
     ///
     /// impl Parse for UnionOrMacro {
     ///     fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
     ///         if input.peek(Token![union]) &amp;&amp; input.peek2(Ident) {
     ///             input.parse().map(UnionOrMacro::Union)
     ///         } else {
     ///             input.parse().map(UnionOrMacro::Macro)
     ///         }
     ///     }
     /// }
     /// ```
     </span><span class="kw">pub fn </span>peek2&lt;T: Peek&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, token: T) -&gt; bool {
         <span class="kw">fn </span>peek2(buffer: <span class="kw-2">&amp;</span>ParseBuffer, peek: <span class="kw">fn</span>(Cursor) -&gt; bool) -&gt; bool {
             <span class="kw">if let </span><span class="prelude-val">Some</span>(group) = buffer.cursor().group(Delimiter::None) {
                 <span class="kw">if </span>group.<span class="number">0</span>.skip().map_or(<span class="bool-val">false</span>, peek) {
                     <span class="kw">return </span><span class="bool-val">true</span>;
                 }
             }
             buffer.cursor().skip().map_or(<span class="bool-val">false</span>, peek)
         }

         <span class="kw">let _ </span>= token;
         peek2(<span class="self">self</span>, T::Token::peek)
     }

     <span class="doccomment">/// Looks at the third-next token in the parse stream.
     </span><span class="kw">pub fn </span>peek3&lt;T: Peek&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, token: T) -&gt; bool {
         <span class="kw">fn </span>peek3(buffer: <span class="kw-2">&amp;</span>ParseBuffer, peek: <span class="kw">fn</span>(Cursor) -&gt; bool) -&gt; bool {
             <span class="kw">if let </span><span class="prelude-val">Some</span>(group) = buffer.cursor().group(Delimiter::None) {
                 <span class="kw">if </span>group.<span class="number">0</span>.skip().and_then(Cursor::skip).map_or(<span class="bool-val">false</span>, peek) {
                     <span class="kw">return </span><span class="bool-val">true</span>;
                 }
             }
             buffer
                 .cursor()
                 .skip()
                 .and_then(Cursor::skip)
                 .map_or(<span class="bool-val">false</span>, peek)
         }

         <span class="kw">let _ </span>= token;
         peek3(<span class="self">self</span>, T::Token::peek)
     }

     <span class="doccomment">/// Parses zero or more occurrences of `T` separated by punctuation of type
     /// `P`, with optional trailing punctuation.
     ///
     /// Parsing continues until the end of this parse stream. The entire content
     /// of this parse stream must consist of `T` and `P`.
     ///
     /// # Example
     ///
     /// ```
     /// # use quote::quote;
     /// #
     /// use syn::{parenthesized, token, Ident, Result, Token, Type};
     /// use syn::parse::{Parse, ParseStream};
     /// use syn::punctuated::Punctuated;
     ///
     /// // Parse a simplified tuple struct syntax like:
     /// //
     /// //     struct S(A, B);
     /// struct TupleStruct {
     ///     struct_token: Token![struct],
     ///     ident: Ident,
     ///     paren_token: token::Paren,
     ///     fields: Punctuated&lt;Type, Token![,]&gt;,
     ///     semi_token: Token![;],
     /// }
     ///
     /// impl Parse for TupleStruct {
     ///     fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
     ///         let content;
     ///         Ok(TupleStruct {
     ///             struct_token: input.parse()?,
     ///             ident: input.parse()?,
     ///             paren_token: parenthesized!(content in input),
     ///             fields: content.parse_terminated(Type::parse, Token![,])?,
     ///             semi_token: input.parse()?,
     ///         })
     ///     }
     /// }
     /// #
     /// # let input = quote! {
     /// #     struct S(A, B);
     /// # };
     /// # syn::parse2::&lt;TupleStruct&gt;(input).unwrap();
     /// ```
     ///
     /// # See also
     ///
     /// If your separator is anything more complicated than an invocation of the
     /// `Token!` macro, this method won&#39;t be applicable and you can instead
     /// directly use `Punctuated`&#39;s parser functions: [`parse_terminated`],
     /// [`parse_separated_nonempty`] etc.
     ///
     /// [`parse_terminated`]: Punctuated::parse_terminated
     /// [`parse_separated_nonempty`]: Punctuated::parse_separated_nonempty
     ///
     /// ```
     /// use syn::{custom_keyword, Expr, Result, Token};
     /// use syn::parse::{Parse, ParseStream};
     /// use syn::punctuated::Punctuated;
     ///
     /// mod kw {
     ///     syn::custom_keyword!(fin);
     /// }
     ///
     /// struct Fin(kw::fin, Token![;]);
     ///
     /// impl Parse for Fin {
     ///     fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
     ///         Ok(Self(input.parse()?, input.parse()?))
     ///     }
     /// }
     ///
     /// struct Thing {
     ///     steps: Punctuated&lt;Expr, Fin&gt;,
     /// }
     ///
     /// impl Parse for Thing {
     ///     fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
     /// # if true {
     ///         Ok(Thing {
     ///             steps: Punctuated::parse_terminated(input)?,
     ///         })
     /// # } else {
     ///         // or equivalently, this means the same thing:
     /// #       Ok(Thing {
     ///             steps: input.call(Punctuated::parse_terminated)?,
     /// #       })
     /// # }
     ///     }
     /// }
     /// ```
     </span><span class="kw">pub fn </span>parse_terminated&lt;T, P&gt;(
         <span class="kw-2">&amp;</span><span class="self">self</span>,
         parser: <span class="kw">fn</span>(ParseStream) -&gt; <span class="prelude-ty">Result</span>&lt;T&gt;,
         separator: P,
     ) -&gt; <span class="prelude-ty">Result</span>&lt;Punctuated&lt;T, P::Token&gt;&gt;
     <span class="kw">where
         </span>P: Peek,
         P::Token: Parse,
     {
         <span class="kw">let _ </span>= separator;
         Punctuated::parse_terminated_with(<span class="self">self</span>, parser)
     }

     <span class="doccomment">/// Returns whether there are tokens remaining in this stream.
     ///
     /// This method returns true at the end of the content of a set of
     /// delimiters, as well as at the very end of the complete macro input.
     ///
     /// # Example
     ///
     /// ```
     /// use syn::{braced, token, Ident, Item, Result, Token};
     /// use syn::parse::{Parse, ParseStream};
     ///
     /// // Parses a Rust `mod m { ... }` containing zero or more items.
     /// struct Mod {
     ///     mod_token: Token![mod],
     ///     name: Ident,
     ///     brace_token: token::Brace,
     ///     items: Vec&lt;Item&gt;,
     /// }
     ///
     /// impl Parse for Mod {
     ///     fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
     ///         let content;
     ///         Ok(Mod {
     ///             mod_token: input.parse()?,
     ///             name: input.parse()?,
     ///             brace_token: braced!(content in input),
     ///             items: {
     ///                 let mut items = Vec::new();
     ///                 while !content.is_empty() {
     ///                     items.push(content.parse()?);
     ///                 }
     ///                 items
     ///             },
     ///         })
     ///     }
     /// }
     /// ```
     </span><span class="kw">pub fn </span>is_empty(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="self">self</span>.cursor().eof()
     }

     <span class="doccomment">/// Constructs a helper for peeking at the next token in this stream and
     /// building an error message if it is not one of a set of expected tokens.
     ///
     /// # Example
     ///
     /// ```
     /// use syn::{ConstParam, Ident, Lifetime, LifetimeParam, Result, Token, TypeParam};
     /// use syn::parse::{Parse, ParseStream};
     ///
     /// // A generic parameter, a single one of the comma-separated elements inside
     /// // angle brackets in:
     /// //
     /// //     fn f&lt;T: Clone, &#39;a, &#39;b: &#39;a, const N: usize&gt;() { ... }
     /// //
     /// // On invalid input, lookahead gives us a reasonable error message.
     /// //
     /// //     error: expected one of: identifier, lifetime, `const`
     /// //       |
     /// //     5 |     fn f&lt;!Sized&gt;() {}
     /// //       |          ^
     /// enum GenericParam {
     ///     Type(TypeParam),
     ///     Lifetime(LifetimeParam),
     ///     Const(ConstParam),
     /// }
     ///
     /// impl Parse for GenericParam {
     ///     fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
     ///         let lookahead = input.lookahead1();
     ///         if lookahead.peek(Ident) {
     ///             input.parse().map(GenericParam::Type)
     ///         } else if lookahead.peek(Lifetime) {
     ///             input.parse().map(GenericParam::Lifetime)
     ///         } else if lookahead.peek(Token![const]) {
     ///             input.parse().map(GenericParam::Const)
     ///         } else {
     ///             Err(lookahead.error())
     ///         }
     ///     }
     /// }
     /// ```
     </span><span class="kw">pub fn </span>lookahead1(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; Lookahead1&lt;<span class="lifetime">&#39;a</span>&gt; {
         lookahead::new(<span class="self">self</span>.scope, <span class="self">self</span>.cursor())
     }

     <span class="doccomment">/// Forks a parse stream so that parsing tokens out of either the original
     /// or the fork does not advance the position of the other.
     ///
     /// # Performance
     ///
     /// Forking a parse stream is a cheap fixed amount of work and does not
     /// involve copying token buffers. Where you might hit performance problems
     /// is if your macro ends up parsing a large amount of content more than
     /// once.
     ///
     /// ```
     /// # use syn::{Expr, Result};
     /// # use syn::parse::ParseStream;
     /// #
     /// # fn bad(input: ParseStream) -&gt; Result&lt;Expr&gt; {
     /// // Do not do this.
     /// if input.fork().parse::&lt;Expr&gt;().is_ok() {
     ///     return input.parse::&lt;Expr&gt;();
     /// }
     /// # unimplemented!()
     /// # }
     /// ```
     ///
     /// As a rule, avoid parsing an unbounded amount of tokens out of a forked
     /// parse stream. Only use a fork when the amount of work performed against
     /// the fork is small and bounded.
     ///
     /// When complex speculative parsing against the forked stream is
     /// unavoidable, use [`parse::discouraged::Speculative`] to advance the
     /// original stream once the fork&#39;s parse is determined to have been
     /// successful.
     ///
     /// For a lower level way to perform speculative parsing at the token level,
     /// consider using [`ParseStream::step`] instead.
     ///
     /// [`parse::discouraged::Speculative`]: discouraged::Speculative
     /// [`ParseStream::step`]: ParseBuffer::step
     ///
     /// # Example
     ///
     /// The parse implementation shown here parses possibly restricted `pub`
     /// visibilities.
     ///
     /// - `pub`
     /// - `pub(crate)`
     /// - `pub(self)`
     /// - `pub(super)`
     /// - `pub(in some::path)`
     ///
     /// To handle the case of visibilities inside of tuple structs, the parser
     /// needs to distinguish parentheses that specify visibility restrictions
     /// from parentheses that form part of a tuple type.
     ///
     /// ```
     /// # struct A;
     /// # struct B;
     /// # struct C;
     /// #
     /// struct S(pub(crate) A, pub (B, C));
     /// ```
     ///
     /// In this example input the first tuple struct element of `S` has
     /// `pub(crate)` visibility while the second tuple struct element has `pub`
     /// visibility; the parentheses around `(B, C)` are part of the type rather
     /// than part of a visibility restriction.
     ///
     /// The parser uses a forked parse stream to check the first token inside of
     /// parentheses after the `pub` keyword. This is a small bounded amount of
     /// work performed against the forked parse stream.
     ///
     /// ```
     /// use syn::{parenthesized, token, Ident, Path, Result, Token};
     /// use syn::ext::IdentExt;
     /// use syn::parse::{Parse, ParseStream};
     ///
     /// struct PubVisibility {
     ///     pub_token: Token![pub],
     ///     restricted: Option&lt;Restricted&gt;,
     /// }
     ///
     /// struct Restricted {
     ///     paren_token: token::Paren,
     ///     in_token: Option&lt;Token![in]&gt;,
     ///     path: Path,
     /// }
     ///
     /// impl Parse for PubVisibility {
     ///     fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
     ///         let pub_token: Token![pub] = input.parse()?;
     ///
     ///         if input.peek(token::Paren) {
     ///             let ahead = input.fork();
     ///             let mut content;
     ///             parenthesized!(content in ahead);
     ///
     ///             if content.peek(Token![crate])
     ///                 || content.peek(Token![self])
     ///                 || content.peek(Token![super])
     ///             {
     ///                 return Ok(PubVisibility {
     ///                     pub_token,
     ///                     restricted: Some(Restricted {
     ///                         paren_token: parenthesized!(content in input),
     ///                         in_token: None,
     ///                         path: Path::from(content.call(Ident::parse_any)?),
     ///                     }),
     ///                 });
     ///             } else if content.peek(Token![in]) {
     ///                 return Ok(PubVisibility {
     ///                     pub_token,
     ///                     restricted: Some(Restricted {
     ///                         paren_token: parenthesized!(content in input),
     ///                         in_token: Some(content.parse()?),
     ///                         path: content.call(Path::parse_mod_style)?,
     ///                     }),
     ///                 });
     ///             }
     ///         }
     ///
     ///         Ok(PubVisibility {
     ///             pub_token,
     ///             restricted: None,
     ///         })
     ///     }
     /// }
     /// ```
     </span><span class="kw">pub fn </span>fork(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="self">Self </span>{
         ParseBuffer {
             scope: <span class="self">self</span>.scope,
             cell: <span class="self">self</span>.cell.clone(),
             marker: PhantomData,
             <span class="comment">// Not the parent&#39;s unexpected. Nothing cares whether the clone
             // parses all the way unless we `advance_to`.
             </span>unexpected: Cell::new(<span class="prelude-val">Some</span>(Rc::new(Cell::new(Unexpected::None)))),
         }
     }

     <span class="doccomment">/// Triggers an error at the current position of the parse stream.
     ///
     /// # Example
     ///
     /// ```
     /// use syn::{Expr, Result, Token};
     /// use syn::parse::{Parse, ParseStream};
     ///
     /// // Some kind of loop: `while` or `for` or `loop`.
     /// struct Loop {
     ///     expr: Expr,
     /// }
     ///
     /// impl Parse for Loop {
     ///     fn parse(input: ParseStream) -&gt; Result&lt;Self&gt; {
     ///         if input.peek(Token![while])
     ///             || input.peek(Token![for])
     ///             || input.peek(Token![loop])
     ///         {
     ///             Ok(Loop {
     ///                 expr: input.parse()?,
     ///             })
     ///         } else {
     ///             Err(input.error(&quot;expected some kind of loop&quot;))
     ///         }
     ///     }
     /// }
     /// ```
     </span><span class="kw">pub fn </span>error&lt;T: Display&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, message: T) -&gt; Error {
         error::new_at(<span class="self">self</span>.scope, <span class="self">self</span>.cursor(), message)
     }

     <span class="doccomment">/// Speculatively parses tokens from this parse stream, advancing the
     /// position of this stream only if parsing succeeds.
     ///
     /// This is a powerful low-level API used for defining the `Parse` impls of
     /// the basic built-in token types. It is not something that will be used
     /// widely outside of the Syn codebase.
     ///
     /// # Example
     ///
     /// ```
     /// use proc_macro2::TokenTree;
     /// use syn::Result;
     /// use syn::parse::ParseStream;
     ///
     /// // This function advances the stream past the next occurrence of `@`. If
     /// // no `@` is present in the stream, the stream position is unchanged and
     /// // an error is returned.
     /// fn skip_past_next_at(input: ParseStream) -&gt; Result&lt;()&gt; {
     ///     input.step(|cursor| {
     ///         let mut rest = *cursor;
     ///         while let Some((tt, next)) = rest.token_tree() {
     ///             match &amp;tt {
     ///                 TokenTree::Punct(punct) if punct.as_char() == &#39;@&#39; =&gt; {
     ///                     return Ok(((), next));
     ///                 }
     ///                 _ =&gt; rest = next,
     ///             }
     ///         }
     ///         Err(cursor.error(&quot;no `@` was found after this point&quot;))
     ///     })
     /// }
     /// #
     /// # fn remainder_after_skipping_past_next_at(
     /// #     input: ParseStream,
     /// # ) -&gt; Result&lt;proc_macro2::TokenStream&gt; {
     /// #     skip_past_next_at(input)?;
     /// #     input.parse()
     /// # }
     /// #
     /// # use syn::parse::Parser;
     /// # let remainder = remainder_after_skipping_past_next_at
     /// #     .parse_str(&quot;a @ b c&quot;)
     /// #     .unwrap();
     /// # assert_eq!(remainder.to_string(), &quot;b c&quot;);
     /// ```
     </span><span class="kw">pub fn </span>step&lt;F, R&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, function: F) -&gt; <span class="prelude-ty">Result</span>&lt;R&gt;
     <span class="kw">where
         </span>F: <span class="kw">for</span>&lt;<span class="lifetime">&#39;c</span>&gt; FnOnce(StepCursor&lt;<span class="lifetime">&#39;c</span>, <span class="lifetime">&#39;a</span>&gt;) -&gt; <span class="prelude-ty">Result</span>&lt;(R, Cursor&lt;<span class="lifetime">&#39;c</span>&gt;)&gt;,
     {
         <span class="comment">// Since the user&#39;s function is required to work for any &#39;c, we know
         // that the Cursor&lt;&#39;c&gt; they return is either derived from the input
         // StepCursor&lt;&#39;c, &#39;a&gt; or from a Cursor&lt;&#39;static&gt;.
         //
         // It would not be legal to write this function without the invariant
         // lifetime &#39;c in StepCursor&lt;&#39;c, &#39;a&gt;. If this function were written only
         // in terms of &#39;a, the user could take our ParseBuffer&lt;&#39;a&gt;, upcast it to
         // a ParseBuffer&lt;&#39;short&gt; which some shorter lifetime than &#39;a, invoke
         // `step` on their ParseBuffer&lt;&#39;short&gt; with a closure that returns
         // Cursor&lt;&#39;short&gt;, and we would wrongly write that Cursor&lt;&#39;short&gt; into
         // the Cell intended to hold Cursor&lt;&#39;a&gt;.
         //
         // In some cases it may be necessary for R to contain a Cursor&lt;&#39;a&gt;.
         // Within Syn we solve this using `advance_step_cursor` which uses the
         // existence of a StepCursor&lt;&#39;c, &#39;a&gt; as proof that it is safe to cast
         // from Cursor&lt;&#39;c&gt; to Cursor&lt;&#39;a&gt;. If needed outside of Syn, it would be
         // safe to expose that API as a method on StepCursor.
         </span><span class="kw">let </span>(node, rest) = function(StepCursor {
             scope: <span class="self">self</span>.scope,
             cursor: <span class="self">self</span>.cell.get(),
             marker: PhantomData,
         })<span class="question-mark">?</span>;
         <span class="self">self</span>.cell.set(rest);
         <span class="prelude-val">Ok</span>(node)
     }

     <span class="doccomment">/// Returns the `Span` of the next token in the parse stream, or
     /// `Span::call_site()` if this parse stream has completely exhausted its
     /// input `TokenStream`.
     </span><span class="kw">pub fn </span>span(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; Span {
         <span class="kw">let </span>cursor = <span class="self">self</span>.cursor();
         <span class="kw">if </span>cursor.eof() {
             <span class="self">self</span>.scope
         } <span class="kw">else </span>{
             <span class="kw">crate</span>::buffer::open_span_of_group(cursor)
         }
     }

     <span class="doccomment">/// Provides low-level access to the token representation underlying this
     /// parse stream.
     ///
     /// Cursors are immutable so no operations you perform against the cursor
     /// will affect the state of this parse stream.
     </span><span class="kw">pub fn </span>cursor(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; Cursor&lt;<span class="lifetime">&#39;a</span>&gt; {
         <span class="self">self</span>.cell.get()
     }

     <span class="kw">fn </span>check_unexpected(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="prelude-ty">Result</span>&lt;()&gt; {
         <span class="kw">match </span>inner_unexpected(<span class="self">self</span>).<span class="number">1 </span>{
             <span class="prelude-val">Some</span>(span) =&gt; <span class="prelude-val">Err</span>(Error::new(span, <span class="string">&quot;unexpected token&quot;</span>)),
             <span class="prelude-val">None </span>=&gt; <span class="prelude-val">Ok</span>(()),
         }
     }
 }

 <span class="attribute">#[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;parsing&quot;</span>)))]
 </span><span class="kw">impl</span>&lt;T: Parse&gt; Parse <span class="kw">for </span>Box&lt;T&gt; {
     <span class="kw">fn </span>parse(input: ParseStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>&gt; {
         input.parse().map(Box::new)
     }
 }

 <span class="attribute">#[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;parsing&quot;</span>)))]
 </span><span class="kw">impl</span>&lt;T: Parse + Token&gt; Parse <span class="kw">for </span><span class="prelude-ty">Option</span>&lt;T&gt; {
     <span class="kw">fn </span>parse(input: ParseStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>&gt; {
         <span class="kw">if </span>T::peek(input.cursor()) {
             <span class="prelude-val">Ok</span>(<span class="prelude-val">Some</span>(input.parse()<span class="question-mark">?</span>))
         } <span class="kw">else </span>{
             <span class="prelude-val">Ok</span>(<span class="prelude-val">None</span>)
         }
     }
 }

 <span class="attribute">#[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;parsing&quot;</span>)))]
 </span><span class="kw">impl </span>Parse <span class="kw">for </span>TokenStream {
     <span class="kw">fn </span>parse(input: ParseStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>&gt; {
         input.step(|cursor| <span class="prelude-val">Ok</span>((cursor.token_stream(), Cursor::empty())))
     }
 }

 <span class="attribute">#[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;parsing&quot;</span>)))]
 </span><span class="kw">impl </span>Parse <span class="kw">for </span>TokenTree {
     <span class="kw">fn </span>parse(input: ParseStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>&gt; {
         input.step(|cursor| <span class="kw">match </span>cursor.token_tree() {
             <span class="prelude-val">Some</span>((tt, rest)) =&gt; <span class="prelude-val">Ok</span>((tt, rest)),
             <span class="prelude-val">None </span>=&gt; <span class="prelude-val">Err</span>(cursor.error(<span class="string">&quot;expected token tree&quot;</span>)),
         })
     }
 }

 <span class="attribute">#[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;parsing&quot;</span>)))]
 </span><span class="kw">impl </span>Parse <span class="kw">for </span>Group {
     <span class="kw">fn </span>parse(input: ParseStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>&gt; {
         input.step(|cursor| {
             <span class="kw">if let </span><span class="prelude-val">Some</span>((group, rest)) = cursor.any_group_token() {
                 <span class="kw">if </span>group.delimiter() != Delimiter::None {
                     <span class="kw">return </span><span class="prelude-val">Ok</span>((group, rest));
                 }
             }
             <span class="prelude-val">Err</span>(cursor.error(<span class="string">&quot;expected group token&quot;</span>))
         })
     }
 }

 <span class="attribute">#[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;parsing&quot;</span>)))]
 </span><span class="kw">impl </span>Parse <span class="kw">for </span>Punct {
     <span class="kw">fn </span>parse(input: ParseStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>&gt; {
         input.step(|cursor| <span class="kw">match </span>cursor.punct() {
             <span class="prelude-val">Some</span>((punct, rest)) =&gt; <span class="prelude-val">Ok</span>((punct, rest)),
             <span class="prelude-val">None </span>=&gt; <span class="prelude-val">Err</span>(cursor.error(<span class="string">&quot;expected punctuation token&quot;</span>)),
         })
     }
 }

 <span class="attribute">#[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;parsing&quot;</span>)))]
 </span><span class="kw">impl </span>Parse <span class="kw">for </span>Literal {
     <span class="kw">fn </span>parse(input: ParseStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>&gt; {
         input.step(|cursor| <span class="kw">match </span>cursor.literal() {
             <span class="prelude-val">Some</span>((literal, rest)) =&gt; <span class="prelude-val">Ok</span>((literal, rest)),
             <span class="prelude-val">None </span>=&gt; <span class="prelude-val">Err</span>(cursor.error(<span class="string">&quot;expected literal token&quot;</span>)),
         })
     }
 }

 <span class="doccomment">/// Parser that can parse Rust tokens into a particular syntax tree node.
 ///
 /// Refer to the [module documentation] for details about parsing in Syn.
 ///
 /// [module documentation]: self
 </span><span class="kw">pub trait </span>Parser: Sized {
     <span class="kw">type </span>Output;

     <span class="doccomment">/// Parse a proc-macro2 token stream into the chosen syntax tree node.
     ///
     /// This function will check that the input is fully parsed. If there are
     /// any unparsed tokens at the end of the stream, an error is returned.
     </span><span class="kw">fn </span>parse2(<span class="self">self</span>, tokens: TokenStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>::Output&gt;;

     <span class="doccomment">/// Parse tokens of source code into the chosen syntax tree node.
     ///
     /// This function will check that the input is fully parsed. If there are
     /// any unparsed tokens at the end of the stream, an error is returned.
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;proc-macro&quot;</span>)]
     #[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;proc-macro&quot;</span>)))]
     </span><span class="kw">fn </span>parse(<span class="self">self</span>, tokens: proc_macro::TokenStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>::Output&gt; {
         <span class="self">self</span>.parse2(proc_macro2::TokenStream::from(tokens))
     }

     <span class="doccomment">/// Parse a string of Rust code into the chosen syntax tree node.
     ///
     /// This function will check that the input is fully parsed. If there are
     /// any unparsed tokens at the end of the string, an error is returned.
     ///
     /// # Hygiene
     ///
     /// Every span in the resulting syntax tree will be set to resolve at the
     /// macro call site.
     </span><span class="kw">fn </span>parse_str(<span class="self">self</span>, s: <span class="kw-2">&amp;</span>str) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>::Output&gt; {
         <span class="self">self</span>.parse2(proc_macro2::TokenStream::from_str(s)<span class="question-mark">?</span>)
     }

     <span class="comment">// Not public API.
     </span><span class="attribute">#[doc(hidden)]
     #[cfg(any(feature = <span class="string">&quot;full&quot;</span>, feature = <span class="string">&quot;derive&quot;</span>))]
     </span><span class="kw">fn </span>__parse_scoped(<span class="self">self</span>, scope: Span, tokens: TokenStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>::Output&gt; {
         <span class="kw">let _ </span>= scope;
         <span class="self">self</span>.parse2(tokens)
     }
 }

 <span class="kw">fn </span>tokens_to_parse_buffer(tokens: <span class="kw-2">&amp;</span>TokenBuffer) -&gt; ParseBuffer {
     <span class="kw">let </span>scope = Span::call_site();
     <span class="kw">let </span>cursor = tokens.begin();
     <span class="kw">let </span>unexpected = Rc::new(Cell::new(Unexpected::None));
     new_parse_buffer(scope, cursor, unexpected)
 }

 <span class="kw">impl</span>&lt;F, T&gt; Parser <span class="kw">for </span>F
 <span class="kw">where
     </span>F: FnOnce(ParseStream) -&gt; <span class="prelude-ty">Result</span>&lt;T&gt;,
 {
     <span class="kw">type </span>Output = T;

     <span class="kw">fn </span>parse2(<span class="self">self</span>, tokens: TokenStream) -&gt; <span class="prelude-ty">Result</span>&lt;T&gt; {
         <span class="kw">let </span>buf = TokenBuffer::new2(tokens);
         <span class="kw">let </span>state = tokens_to_parse_buffer(<span class="kw-2">&amp;</span>buf);
         <span class="kw">let </span>node = <span class="self">self</span>(<span class="kw-2">&amp;</span>state)<span class="question-mark">?</span>;
         state.check_unexpected()<span class="question-mark">?</span>;
         <span class="kw">if let </span><span class="prelude-val">Some</span>(unexpected_span) = span_of_unexpected_ignoring_nones(state.cursor()) {
             <span class="prelude-val">Err</span>(Error::new(unexpected_span, <span class="string">&quot;unexpected token&quot;</span>))
         } <span class="kw">else </span>{
             <span class="prelude-val">Ok</span>(node)
         }
     }

     <span class="attribute">#[cfg(any(feature = <span class="string">&quot;full&quot;</span>, feature = <span class="string">&quot;derive&quot;</span>))]
     </span><span class="kw">fn </span>__parse_scoped(<span class="self">self</span>, scope: Span, tokens: TokenStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>::Output&gt; {
         <span class="kw">let </span>buf = TokenBuffer::new2(tokens);
         <span class="kw">let </span>cursor = buf.begin();
         <span class="kw">let </span>unexpected = Rc::new(Cell::new(Unexpected::None));
         <span class="kw">let </span>state = new_parse_buffer(scope, cursor, unexpected);
         <span class="kw">let </span>node = <span class="self">self</span>(<span class="kw-2">&amp;</span>state)<span class="question-mark">?</span>;
         state.check_unexpected()<span class="question-mark">?</span>;
         <span class="kw">if let </span><span class="prelude-val">Some</span>(unexpected_span) = span_of_unexpected_ignoring_nones(state.cursor()) {
             <span class="prelude-val">Err</span>(Error::new(unexpected_span, <span class="string">&quot;unexpected token&quot;</span>))
         } <span class="kw">else </span>{
             <span class="prelude-val">Ok</span>(node)
         }
     }
 }

 <span class="attribute">#[cfg(any(feature = <span class="string">&quot;full&quot;</span>, feature = <span class="string">&quot;derive&quot;</span>))]
 </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>parse_scoped&lt;F: Parser&gt;(f: F, scope: Span, tokens: TokenStream) -&gt; <span class="prelude-ty">Result</span>&lt;F::Output&gt; {
     f.__parse_scoped(scope, tokens)
 }

 <span class="doccomment">/// An empty syntax tree node that consumes no tokens when parsed.
 ///
 /// This is useful for attribute macros that want to ensure they are not
 /// provided any attribute args.
 ///
 /// ```
 /// # extern crate proc_macro;
 /// #
 /// use proc_macro::TokenStream;
 /// use syn::parse_macro_input;
 /// use syn::parse::Nothing;
 ///
 /// # const IGNORE: &amp;str = stringify! {
 /// #[proc_macro_attribute]
 /// # };
 /// pub fn my_attr(args: TokenStream, input: TokenStream) -&gt; TokenStream {
 ///     parse_macro_input!(args as Nothing);
 ///
 ///     /* ... */
 /// #   TokenStream::new()
 /// }
 /// ```
 ///
 /// ```text
 /// error: unexpected token
 ///  --&gt; src/main.rs:3:19
 ///   |
 /// 3 | #[my_attr(asdf)]
 ///   |           ^^^^
 /// ```
 </span><span class="kw">pub struct </span>Nothing;

 <span class="kw">impl </span>Parse <span class="kw">for </span>Nothing {
     <span class="kw">fn </span>parse(_input: ParseStream) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="self">Self</span>&gt; {
         <span class="prelude-val">Ok</span>(Nothing)
     }
 }

 <span class="attribute">#[cfg(feature = <span class="string">&quot;extra-traits&quot;</span>)]
 #[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;extra-traits&quot;</span>)))]
 </span><span class="kw">impl </span>Debug <span class="kw">for </span>Nothing {
     <span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, f: <span class="kw-2">&amp;mut </span>fmt::Formatter) -&gt; fmt::Result {
         f.write_str(<span class="string">&quot;Nothing&quot;</span>)
     }
 }

 <span class="attribute">#[cfg(feature = <span class="string">&quot;extra-traits&quot;</span>)]
 #[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;extra-traits&quot;</span>)))]
 </span><span class="kw">impl </span>Eq <span class="kw">for </span>Nothing {}

 <span class="attribute">#[cfg(feature = <span class="string">&quot;extra-traits&quot;</span>)]
 #[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;extra-traits&quot;</span>)))]
 </span><span class="kw">impl </span>PartialEq <span class="kw">for </span>Nothing {
     <span class="kw">fn </span>eq(<span class="kw-2">&amp;</span><span class="self">self</span>, _other: <span class="kw-2">&amp;</span><span class="self">Self</span>) -&gt; bool {
         <span class="bool-val">true
     </span>}
 }

 <span class="attribute">#[cfg(feature = <span class="string">&quot;extra-traits&quot;</span>)]
 #[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;extra-traits&quot;</span>)))]
 </span><span class="kw">impl </span>Hash <span class="kw">for </span>Nothing {
     <span class="kw">fn </span>hash&lt;H: Hasher&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, _state: <span class="kw-2">&amp;mut </span>H) {}
 }
 </code></pre></div>
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