src/parse.rs - platform/external/rust/crates/syn - Gitiles

 // Copyright 2018 Syn Developers
 //
 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.

 //! Parsing interface for parsing a token stream into a syntax tree node.
 //!
 //! Parsing in Syn is built on parser functions that take in a [`Cursor`] and
 //! produce a [`PResult<T>`] where `T` is some syntax tree node. `Cursor` is a
 //! cheaply copyable cursor over a range of tokens in a token stream, and
 //! `PResult` is a result that packages together a parsed syntax tree node `T`
 //! with a stream of remaining unparsed tokens after `T` represented as another
 //! `Cursor`, or an [`Error`] if parsing failed.
 //!
 //! [`Cursor`]: ../buffer/index.html
 //! [`PResult<T>`]: type.PResult.html
 //! [`Error`]: struct.Error.html
 //!
 //! This `Cursor`- and `PResult`-based interface is convenient for parser
 //! combinators and parser implementations, but not necessarily when you just
 //! have some tokens that you want to parse. For that we expose the following
 //! two entry points.
 //!
 //! ## 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`]: ../fn.parse.html
 //! [`syn::parse2`]: ../fn.parse2.html
 //! [`syn::parse_str`]: ../fn.parse_str.html
 //! [`Parse`]: trait.Parse.html
 //!
 //! ```
 //! use syn::Type;
 //!
 //! # fn run_parser() -> Result<(), syn::parse::Error> {
 //! let t: Type = syn::parse_str("std::collections::HashMap<String, Value>")?;
 //! #     Ok(())
 //! # }
 //! #
 //! # fn main() {
 //! #     run_parser().unwrap();
 //! # }
 //! ```
 //!
 //! The [`parse_quote!`] macro also uses this approach.
 //!
 //! [`parse_quote!`]: ../macro.parse_quote.html
 //!
 //! ## The `Parser` trait
 //!
 //! Some types can be parsed in several ways depending on context. For example
 //! an [`Attribute`] can be either "outer" like `#[...]` or "inner" 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`]: ../struct.Attribute.html
 //! [`Punctuated`]: ../punctuated/index.html
 //!
 //! The `Synom` trait is not implemented in these cases because there is no good
 //! behavior to consider the default.
 //!
 //! ```ignore
 //! // Can't parse `Punctuated` without knowing whether trailing punctuation
 //! // should be allowed in this context.
 //! let path: Punctuated<PathSegment, Token![::]> = syn::parse(tokens)?;
 //! ```
 //!
 //! In these cases the types provide a choice of parser functions rather than a
 //! single `Synom` implementation, and those parser functions can be invoked
 //! through the [`Parser`] trait.
 //!
 //! [`Parser`]: trait.Parser.html
 //!
 //! ```
 //! # #[macro_use]
 //! # extern crate syn;
 //! #
 //! # extern crate proc_macro2;
 //! # use proc_macro2::TokenStream;
 //! #
 //! use syn::synom::Parser;
 //! use syn::punctuated::Punctuated;
 //! use syn::{PathSegment, Expr, Attribute};
 //!
 //! # fn run_parsers() -> Result<(), syn::synom::Error> {
 //! #     let tokens = TokenStream::new().into();
 //! // Parse a nonempty sequence of path segments separated by `::` punctuation
 //! // with no trailing punctuation.
 //! let parser = Punctuated::<PathSegment, Token![::]>::parse_separated_nonempty;
 //! let path = parser.parse(tokens)?;
 //!
 //! #     let tokens = TokenStream::new().into();
 //! // Parse a possibly empty sequence of expressions terminated by commas with
 //! // an optional trailing punctuation.
 //! let parser = Punctuated::<Expr, Token![,]>::parse_terminated;
 //! let args = parser.parse(tokens)?;
 //!
 //! #     let tokens = TokenStream::new().into();
 //! // Parse zero or more outer attributes but not inner attributes.
 //! named!(outer_attrs -> Vec<Attribute>, many0!(Attribute::old_parse_outer));
 //! let attrs = outer_attrs.parse(tokens)?;
 //! #
 //! #     Ok(())
 //! # }
 //! #
 //! # fn main() {}
 //! ```
 //!
 //! # Implementing a parser function
 //!
 //! Parser functions are usually implemented using the [`nom`]-style parser
 //! combinator macros provided by Syn, but may also be implemented without
 //! macros be using the low-level [`Cursor`] API directly.
 //!
 //! [`nom`]: https://github.com/Geal/nom
 //!
 //! The following parser combinator macros are available and a `Synom` parsing
 //! example is provided for each one.
 //!
 //! - [`alt!`](../macro.alt.html)
 //! - [`braces!`](../macro.braces.html)
 //! - [`brackets!`](../macro.brackets.html)
 //! - [`call!`](../macro.call.html)
 //! - [`cond!`](../macro.cond.html)
 //! - [`cond_reduce!`](../macro.cond_reduce.html)
 //! - [`custom_keyword!`](../macro.custom_keyword.html)
 //! - [`do_parse!`](../macro.do_parse.html)
 //! - [`epsilon!`](../macro.epsilon.html)
 //! - [`input_end!`](../macro.input_end.html)
 //! - [`keyword!`](../macro.keyword.html)
 //! - [`many0!`](../macro.many0.html)
 //! - [`map!`](../macro.map.html)
 //! - [`not!`](../macro.not.html)
 //! - [`option!`](../macro.option.html)
 //! - [`parens!`](../macro.parens.html)
 //! - [`punct!`](../macro.punct.html)
 //! - [`reject!`](../macro.reject.html)
 //! - [`switch!`](../macro.switch.html)
 //! - [`syn!`](../macro.syn.html)
 //! - [`tuple!`](../macro.tuple.html)
 //! - [`value!`](../macro.value.html)
 //!
 //! *This module is available if Syn is built with the `"parsing"` feature.*

 use std::cell::Cell;
 use std::fmt::Display;
 use std::marker::PhantomData;
 use std::mem;
 use std::ops::Deref;
 use std::rc::Rc;
 use std::str::FromStr;

 #[cfg(all(
     not(all(target_arch = "wasm32", target_os = "unknown")),
     feature = "proc-macro"
 ))]
 use proc_macro;
 use proc_macro2::{self, Delimiter, Group, Ident, Literal, Punct, Span, TokenStream, TokenTree};

 use buffer::{Cursor, TokenBuffer};
 use error;
 use punctuated::Punctuated;
 use token::Token;

 pub use error::{Error, Result};
 pub use lookahead::{Lookahead1, Peek};

 /// Parsing interface implemented by all types that can be parsed in a default
 /// way from a token stream.
 pub trait Parse: Sized {
     fn parse(input: ParseStream) -> Result<Self>;
 }

 /// Input to a Syn parser function.
 pub type ParseStream<'a> = &'a ParseBuffer<'a>;

 /// Cursor position within a buffered token stream.
 pub struct ParseBuffer<'a> {
     scope: Span,
     cell: Cell<Cursor<'static>>,
     marker: PhantomData<Cursor<'a>>,
     unexpected: Rc<Cell<Option<Span>>>,
 }

 impl<'a> Drop for ParseBuffer<'a> {
     fn drop(&mut self) {
         if !self.is_empty() && self.unexpected.get().is_none() {
             self.unexpected.set(Some(self.cursor().span()));
         }
     }
 }

 // Not public API.
 #[doc(hidden)]
 #[derive(Copy, Clone)]
 pub struct StepCursor<'c, 'a> {
     scope: Span,
     cursor: Cursor<'c>,
     marker: PhantomData<fn(Cursor<'c>) -> Cursor<'a>>,
 }

 impl<'c, 'a> Deref for StepCursor<'c, 'a> {
     type Target = Cursor<'c>;

     fn deref(&self) -> &Self::Target {
         &self.cursor
     }
 }

 impl<'c, 'a> StepCursor<'c, 'a> {
     // Not public API.
     #[doc(hidden)]
     pub fn advance(self, other: Cursor<'c>) -> Cursor<'a> {
         unsafe { mem::transmute::<Cursor<'c>, Cursor<'a>>(other) }
     }

     pub fn error<T: Display>(self, message: T) -> Error {
         error::new_at(self.scope, self.cursor, message)
     }
 }

 fn skip(input: ParseStream) -> bool {
     input.step(|cursor| {
         if let Some((_lifetime, rest)) = cursor.lifetime() {
             Ok((true, rest))
         } else if let Some((_token, rest)) = cursor.token_tree() {
             Ok((true, rest))
         } else {
             Ok((false, *cursor))
         }
     }).unwrap()
 }

 impl<'a> ParseBuffer<'a> {
     // Not public API.
     #[doc(hidden)]
     pub fn new(scope: Span, cursor: Cursor<'a>, unexpected: Rc<Cell<Option<Span>>>) -> Self {
         let extend = unsafe { mem::transmute::<Cursor<'a>, Cursor<'static>>(cursor) };
         ParseBuffer {
             scope: scope,
             cell: Cell::new(extend),
             marker: PhantomData,
             unexpected: unexpected,
         }
     }

     pub fn cursor(&self) -> Cursor<'a> {
         self.cell.get()
     }

     pub fn is_empty(&self) -> bool {
         self.cursor().eof()
     }

     pub fn lookahead1(&self) -> Lookahead1<'a> {
         Lookahead1::new(self.scope, self.cursor())
     }

     pub fn parse<T: Parse>(&self) -> Result<T> {
         self.check_unexpected()?;
         T::parse(self)
     }

     pub fn call<T>(&self, function: fn(ParseStream) -> Result<T>) -> Result<T> {
         function(self)
     }

     pub fn peek<T: Peek>(&self, token: T) -> bool {
         self.lookahead1().peek(token)
     }

     pub fn peek2<T: Peek>(&self, token: T) -> bool {
         let ahead = self.fork();
         skip(&ahead) && ahead.peek(token)
     }

     pub fn peek3<T: Peek>(&self, token: T) -> bool {
         let ahead = self.fork();
         skip(&ahead) && skip(&ahead) && ahead.peek(token)
     }

     pub fn parse_terminated<T, P: Parse>(
         &self,
         parser: fn(ParseStream) -> Result<T>,
     ) -> Result<Punctuated<T, P>> {
         Punctuated::parse_terminated_with(self, parser)
     }

     pub fn fork(&self) -> Self {
         ParseBuffer {
             scope: self.scope,
             cell: self.cell.clone(),
             marker: PhantomData,
             // Not the parent's unexpected. Nothing cares whether the clone
             // parses all the way.
             unexpected: Rc::new(Cell::new(None)),
         }
     }

     pub fn error<T: Display>(&self, message: T) -> Error {
         error::new_at(self.scope, self.cursor(), message)
     }

     pub fn step<F, R>(&self, function: F) -> Result<R>
     where
         F: for<'c> FnOnce(StepCursor<'c, 'a>) -> Result<(R, Cursor<'c>)>,
     {
         self.check_unexpected()?;
         match function(StepCursor {
             scope: self.scope,
             cursor: self.cell.get(),
             marker: PhantomData,
         }) {
             Ok((ret, cursor)) => {
                 self.cell.set(cursor);
                 Ok(ret)
             }
             Err(err) => Err(err),
         }
     }

     // Not public API.
     #[doc(hidden)]
     pub fn get_unexpected(&self) -> Rc<Cell<Option<Span>>> {
         self.unexpected.clone()
     }

     // Not public API.
     #[doc(hidden)]
     pub fn check_unexpected(&self) -> Result<()> {
         match self.unexpected.get() {
             Some(span) => Err(Error::new(span, "unexpected token")),
             None => Ok(()),
         }
     }
 }

 impl Parse for Ident {
     fn parse(input: ParseStream) -> Result<Self> {
         input.step(|cursor| {
             if let Some((ident, rest)) = cursor.ident() {
                 match ident.to_string().as_str() {
                     "_"
                     // Based on https://doc.rust-lang.org/grammar.html#keywords
                     // and https://github.com/rust-lang/rfcs/blob/master/text/2421-unreservations-2018.md
                     | "abstract" | "as" | "become" | "box" | "break" | "const"
                     | "continue" | "crate" | "do" | "else" | "enum" | "extern" | "false" | "final"
                     | "fn" | "for" | "if" | "impl" | "in" | "let" | "loop" | "macro" | "match"
                     | "mod" | "move" | "mut" | "override" | "priv" | "proc" | "pub"
                     | "ref" | "return" | "Self" | "self" | "static" | "struct"
                     | "super" | "trait" | "true" | "type" | "typeof" | "unsafe" | "unsized" | "use"
                     | "virtual" | "where" | "while" | "yield" => {}
                     _ => return Ok((ident, rest)),
                 }
             }
             Err(cursor.error("expected identifier"))
         })
     }
 }

 impl<T: Parse> Parse for Box<T> {
     fn parse(input: ParseStream) -> Result<Self> {
         input.parse().map(Box::new)
     }
 }

 impl<T: Parse + Token> Parse for Option<T> {
     fn parse(input: ParseStream) -> Result<Self> {
         if T::peek(&input.lookahead1()) {
             Ok(Some(input.parse()?))
         } else {
             Ok(None)
         }
     }
 }

 impl Parse for TokenStream {
     fn parse(input: ParseStream) -> Result<Self> {
         input.step(|cursor| Ok((cursor.token_stream(), Cursor::empty())))
     }
 }

 impl Parse for TokenTree {
     fn parse(input: ParseStream) -> Result<Self> {
         input.step(|cursor| match cursor.token_tree() {
             Some((tt, rest)) => Ok((tt, rest)),
             None => Err(cursor.error("expected token tree")),
         })
     }
 }

 impl Parse for Group {
     fn parse(input: ParseStream) -> Result<Self> {
         input.step(|cursor| {
             for delim in &[Delimiter::Parenthesis, Delimiter::Brace, Delimiter::Bracket] {
                 if let Some((inside, span, rest)) = cursor.group(*delim) {
                     let mut group = Group::new(*delim, inside.token_stream());
                     group.set_span(span);
                     return Ok((group, rest));
                 }
             }
             Err(cursor.error("expected group token"))
         })
     }
 }

 impl Parse for Punct {
     fn parse(input: ParseStream) -> Result<Self> {
         input.step(|cursor| match cursor.punct() {
             Some((punct, rest)) => Ok((punct, rest)),
             None => Err(cursor.error("expected punctuation token")),
         })
     }
 }

 impl Parse for Literal {
     fn parse(input: ParseStream) -> Result<Self> {
         input.step(|cursor| match cursor.literal() {
             Some((literal, rest)) => Ok((literal, rest)),
             None => Err(cursor.error("expected literal token")),
         })
     }
 }

 /// 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]: index.html
 ///
 /// *This trait is available if Syn is built with the `"parsing"` feature.*
 pub trait Parser: Sized {
     type Output;

     /// Parse a proc-macro2 token stream into the chosen syntax tree node.
     fn parse2(self, tokens: TokenStream) -> Result<Self::Output>;

     /// Parse tokens of source code into the chosen syntax tree node.
     ///
     /// *This method is available if Syn is built with both the `"parsing"` and
     /// `"proc-macro"` features.*
     #[cfg(all(
         not(all(target_arch = "wasm32", target_os = "unknown")),
         feature = "proc-macro"
     ))]
     fn parse(self, tokens: proc_macro::TokenStream) -> Result<Self::Output> {
         self.parse2(proc_macro2::TokenStream::from(tokens))
     }

     /// Parse a string of Rust code into the chosen syntax tree node.
     ///
     /// # Hygiene
     ///
     /// Every span in the resulting syntax tree will be set to resolve at the
     /// macro call site.
     fn parse_str(self, s: &str) -> Result<Self::Output> {
         self.parse2(proc_macro2::TokenStream::from_str(s)?)
     }
 }

 impl<F, T> Parser for F
 where
     F: FnOnce(ParseStream) -> Result<T>,
 {
     type Output = T;

     fn parse2(self, tokens: TokenStream) -> Result<T> {
         let buf = TokenBuffer::new2(tokens);
         let unexpected = Rc::new(Cell::new(None));
         let state = ParseBuffer::new(Span::call_site(), buf.begin(), unexpected);
         let node = self(&state)?;
         state.check_unexpected()?;
         if state.is_empty() {
             Ok(node)
         } else {
             Err(state.error("unexpected token"))
         }
     }
 }
	// Copyright 2018 Syn Developers
	//
	// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
	// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
	// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
	// option. This file may not be copied, modified, or distributed
	// except according to those terms.

	//! Parsing interface for parsing a token stream into a syntax tree node.
	//!
	//! Parsing in Syn is built on parser functions that take in a [`Cursor`] and
	//! produce a [`PResult<T>`] where `T` is some syntax tree node. `Cursor` is a
	//! cheaply copyable cursor over a range of tokens in a token stream, and
	//! `PResult` is a result that packages together a parsed syntax tree node `T`
	//! with a stream of remaining unparsed tokens after `T` represented as another
	//! `Cursor`, or an [`Error`] if parsing failed.
	//!
	//! [`Cursor`]: ../buffer/index.html
	//! [`PResult<T>`]: type.PResult.html
	//! [`Error`]: struct.Error.html
	//!
	//! This `Cursor`- and `PResult`-based interface is convenient for parser
	//! combinators and parser implementations, but not necessarily when you just
	//! have some tokens that you want to parse. For that we expose the following
	//! two entry points.
	//!
	//! ## 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`]: ../fn.parse.html
	//! [`syn::parse2`]: ../fn.parse2.html
	//! [`syn::parse_str`]: ../fn.parse_str.html
	//! [`Parse`]: trait.Parse.html
	//!
	//! ```
	//! use syn::Type;
	//!
	//! # fn run_parser() -> Result<(), syn::parse::Error> {
	//! let t: Type = syn::parse_str("std::collections::HashMap<String, Value>")?;
	//! # Ok(())
	//! # }
	//! #
	//! # fn main() {
	//! # run_parser().unwrap();
	//! # }
	//! ```
	//!
	//! The [`parse_quote!`] macro also uses this approach.
	//!
	//! [`parse_quote!`]: ../macro.parse_quote.html
	//!
	//! ## The `Parser` trait
	//!
	//! Some types can be parsed in several ways depending on context. For example
	//! an [`Attribute`] can be either "outer" like `#[...]` or "inner" 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`]: ../struct.Attribute.html
	//! [`Punctuated`]: ../punctuated/index.html
	//!
	//! The `Synom` trait is not implemented in these cases because there is no good
	//! behavior to consider the default.
	//!
	//! ```ignore
	//! // Can't parse `Punctuated` without knowing whether trailing punctuation
	//! // should be allowed in this context.
	//! let path: Punctuated<PathSegment, Token![::]> = syn::parse(tokens)?;
	//! ```
	//!
	//! In these cases the types provide a choice of parser functions rather than a
	//! single `Synom` implementation, and those parser functions can be invoked
	//! through the [`Parser`] trait.
	//!
	//! [`Parser`]: trait.Parser.html
	//!
	//! ```
	//! # #[macro_use]
	//! # extern crate syn;
	//! #
	//! # extern crate proc_macro2;
	//! # use proc_macro2::TokenStream;
	//! #
	//! use syn::synom::Parser;
	//! use syn::punctuated::Punctuated;
	//! use syn::{PathSegment, Expr, Attribute};
	//!
	//! # fn run_parsers() -> Result<(), syn::synom::Error> {
	//! # let tokens = TokenStream::new().into();
	//! // Parse a nonempty sequence of path segments separated by `::` punctuation
	//! // with no trailing punctuation.
	//! let parser = Punctuated::<PathSegment, Token![::]>::parse_separated_nonempty;
	//! let path = parser.parse(tokens)?;
	//!
	//! # let tokens = TokenStream::new().into();
	//! // Parse a possibly empty sequence of expressions terminated by commas with
	//! // an optional trailing punctuation.
	//! let parser = Punctuated::<Expr, Token![,]>::parse_terminated;
	//! let args = parser.parse(tokens)?;
	//!
	//! # let tokens = TokenStream::new().into();
	//! // Parse zero or more outer attributes but not inner attributes.
	//! named!(outer_attrs -> Vec<Attribute>, many0!(Attribute::old_parse_outer));
	//! let attrs = outer_attrs.parse(tokens)?;
	//! #
	//! # Ok(())
	//! # }
	//! #
	//! # fn main() {}
	//! ```
	//!
	//! # Implementing a parser function
	//!
	//! Parser functions are usually implemented using the [`nom`]-style parser
	//! combinator macros provided by Syn, but may also be implemented without
	//! macros be using the low-level [`Cursor`] API directly.
	//!
	//! [`nom`]: https://github.com/Geal/nom
	//!
	//! The following parser combinator macros are available and a `Synom` parsing
	//! example is provided for each one.
	//!
	//! - [`alt!`](../macro.alt.html)
	//! - [`braces!`](../macro.braces.html)
	//! - [`brackets!`](../macro.brackets.html)
	//! - [`call!`](../macro.call.html)
	//! - [`cond!`](../macro.cond.html)
	//! - [`cond_reduce!`](../macro.cond_reduce.html)
	//! - [`custom_keyword!`](../macro.custom_keyword.html)
	//! - [`do_parse!`](../macro.do_parse.html)
	//! - [`epsilon!`](../macro.epsilon.html)
	//! - [`input_end!`](../macro.input_end.html)
	//! - [`keyword!`](../macro.keyword.html)
	//! - [`many0!`](../macro.many0.html)
	//! - [`map!`](../macro.map.html)
	//! - [`not!`](../macro.not.html)
	//! - [`option!`](../macro.option.html)
	//! - [`parens!`](../macro.parens.html)
	//! - [`punct!`](../macro.punct.html)
	//! - [`reject!`](../macro.reject.html)
	//! - [`switch!`](../macro.switch.html)
	//! - [`syn!`](../macro.syn.html)
	//! - [`tuple!`](../macro.tuple.html)
	//! - [`value!`](../macro.value.html)
	//!
	//! This module is available if Syn is built with the `"parsing"` feature.

	use std::cell::Cell;
	use std::fmt::Display;
	use std::marker::PhantomData;
	use std::mem;
	use std::ops::Deref;
	use std::rc::Rc;
	use std::str::FromStr;

	#[cfg(all(
	not(all(target_arch = "wasm32", target_os = "unknown")),
	feature = "proc-macro"
	))]
	use proc_macro;
	use proc_macro2::{self, Delimiter, Group, Ident, Literal, Punct, Span, TokenStream, TokenTree};

	use buffer::{Cursor, TokenBuffer};
	use error;
	use punctuated::Punctuated;
	use token::Token;

	pub use error::{Error, Result};
	pub use lookahead::{Lookahead1, Peek};

	/// Parsing interface implemented by all types that can be parsed in a default
	/// way from a token stream.
	pub trait Parse: Sized {
	fn parse(input: ParseStream) -> Result<Self>;
	}

	/// Input to a Syn parser function.
	pub type ParseStream<'a> = &'a ParseBuffer<'a>;

	/// Cursor position within a buffered token stream.
	pub struct ParseBuffer<'a> {
	scope: Span,
	cell: Cell<Cursor<'static>>,
	marker: PhantomData<Cursor<'a>>,
	unexpected: Rc<Cell<Option<Span>>>,
	}

	impl<'a> Drop for ParseBuffer<'a> {
	fn drop(&mut self) {
	if !self.is_empty() && self.unexpected.get().is_none() {
	self.unexpected.set(Some(self.cursor().span()));
	}
	}
	}

	// Not public API.
	#[doc(hidden)]
	#[derive(Copy, Clone)]
	pub struct StepCursor<'c, 'a> {
	scope: Span,
	cursor: Cursor<'c>,
	marker: PhantomData<fn(Cursor<'c>) -> Cursor<'a>>,
	}

	impl<'c, 'a> Deref for StepCursor<'c, 'a> {
	type Target = Cursor<'c>;

	fn deref(&self) -> &Self::Target {
	&self.cursor
	}
	}

	impl<'c, 'a> StepCursor<'c, 'a> {
	// Not public API.
	#[doc(hidden)]
	pub fn advance(self, other: Cursor<'c>) -> Cursor<'a> {
	unsafe { mem::transmute::<Cursor<'c>, Cursor<'a>>(other) }
	}

	pub fn error<T: Display>(self, message: T) -> Error {
	error::new_at(self.scope, self.cursor, message)
	}
	}

	fn skip(input: ParseStream) -> bool {
	input.step(\|cursor\| {
	if let Some((_lifetime, rest)) = cursor.lifetime() {
	Ok((true, rest))
	} else if let Some((_token, rest)) = cursor.token_tree() {
	Ok((true, rest))
	} else {
	Ok((false, *cursor))
	}
	}).unwrap()
	}

	impl<'a> ParseBuffer<'a> {
	// Not public API.
	#[doc(hidden)]
	pub fn new(scope: Span, cursor: Cursor<'a>, unexpected: Rc<Cell<Option<Span>>>) -> Self {
	let extend = unsafe { mem::transmute::<Cursor<'a>, Cursor<'static>>(cursor) };
	ParseBuffer {
	scope: scope,
	cell: Cell::new(extend),
	marker: PhantomData,
	unexpected: unexpected,
	}
	}

	pub fn cursor(&self) -> Cursor<'a> {
	self.cell.get()
	}

	pub fn is_empty(&self) -> bool {
	self.cursor().eof()
	}

	pub fn lookahead1(&self) -> Lookahead1<'a> {
	Lookahead1::new(self.scope, self.cursor())
	}

	pub fn parse<T: Parse>(&self) -> Result<T> {
	self.check_unexpected()?;
	T::parse(self)
	}

	pub fn call<T>(&self, function: fn(ParseStream) -> Result<T>) -> Result<T> {
	function(self)
	}

	pub fn peek<T: Peek>(&self, token: T) -> bool {
	self.lookahead1().peek(token)
	}

	pub fn peek2<T: Peek>(&self, token: T) -> bool {
	let ahead = self.fork();
	skip(&ahead) && ahead.peek(token)
	}

	pub fn peek3<T: Peek>(&self, token: T) -> bool {
	let ahead = self.fork();
	skip(&ahead) && skip(&ahead) && ahead.peek(token)
	}

	pub fn parse_terminated<T, P: Parse>(
	&self,
	parser: fn(ParseStream) -> Result<T>,
	) -> Result<Punctuated<T, P>> {
	Punctuated::parse_terminated_with(self, parser)
	}

	pub fn fork(&self) -> Self {
	ParseBuffer {
	scope: self.scope,
	cell: self.cell.clone(),
	marker: PhantomData,
	// Not the parent's unexpected. Nothing cares whether the clone
	// parses all the way.
	unexpected: Rc::new(Cell::new(None)),
	}
	}

	pub fn error<T: Display>(&self, message: T) -> Error {
	error::new_at(self.scope, self.cursor(), message)
	}

	pub fn step<F, R>(&self, function: F) -> Result<R>
	where
	F: for<'c> FnOnce(StepCursor<'c, 'a>) -> Result<(R, Cursor<'c>)>,
	{
	self.check_unexpected()?;
	match function(StepCursor {
	scope: self.scope,
	cursor: self.cell.get(),
	marker: PhantomData,
	}) {
	Ok((ret, cursor)) => {
	self.cell.set(cursor);
	Ok(ret)
	}
	Err(err) => Err(err),
	}
	}

	// Not public API.
	#[doc(hidden)]
	pub fn get_unexpected(&self) -> Rc<Cell<Option<Span>>> {
	self.unexpected.clone()
	}

	// Not public API.
	#[doc(hidden)]
	pub fn check_unexpected(&self) -> Result<()> {
	match self.unexpected.get() {
	Some(span) => Err(Error::new(span, "unexpected token")),
	None => Ok(()),
	}
	}
	}

	impl Parse for Ident {
	fn parse(input: ParseStream) -> Result<Self> {
	input.step(\|cursor\| {
	if let Some((ident, rest)) = cursor.ident() {
	match ident.to_string().as_str() {
	"_"
	// Based on https://doc.rust-lang.org/grammar.html#keywords
	// and https://github.com/rust-lang/rfcs/blob/master/text/2421-unreservations-2018.md
	\| "abstract" \| "as" \| "become" \| "box" \| "break" \| "const"
	\| "continue" \| "crate" \| "do" \| "else" \| "enum" \| "extern" \| "false" \| "final"
	\| "fn" \| "for" \| "if" \| "impl" \| "in" \| "let" \| "loop" \| "macro" \| "match"
	\| "mod" \| "move" \| "mut" \| "override" \| "priv" \| "proc" \| "pub"
	\| "ref" \| "return" \| "Self" \| "self" \| "static" \| "struct"
	\| "super" \| "trait" \| "true" \| "type" \| "typeof" \| "unsafe" \| "unsized" \| "use"
	\| "virtual" \| "where" \| "while" \| "yield" => {}
	_ => return Ok((ident, rest)),
	}
	}
	Err(cursor.error("expected identifier"))
	})
	}
	}

	impl<T: Parse> Parse for Box<T> {
	fn parse(input: ParseStream) -> Result<Self> {
	input.parse().map(Box::new)
	}
	}

	impl<T: Parse + Token> Parse for Option<T> {
	fn parse(input: ParseStream) -> Result<Self> {
	if T::peek(&input.lookahead1()) {
	Ok(Some(input.parse()?))
	} else {
	Ok(None)
	}
	}
	}

	impl Parse for TokenStream {
	fn parse(input: ParseStream) -> Result<Self> {
	input.step(\|cursor\| Ok((cursor.token_stream(), Cursor::empty())))
	}
	}

	impl Parse for TokenTree {
	fn parse(input: ParseStream) -> Result<Self> {
	input.step(\|cursor\| match cursor.token_tree() {
	Some((tt, rest)) => Ok((tt, rest)),
	None => Err(cursor.error("expected token tree")),
	})
	}
	}

	impl Parse for Group {
	fn parse(input: ParseStream) -> Result<Self> {
	input.step(\|cursor\| {
	for delim in &[Delimiter::Parenthesis, Delimiter::Brace, Delimiter::Bracket] {
	if let Some((inside, span, rest)) = cursor.group(*delim) {
	let mut group = Group::new(*delim, inside.token_stream());
	group.set_span(span);
	return Ok((group, rest));
	}
	}
	Err(cursor.error("expected group token"))
	})
	}
	}

	impl Parse for Punct {
	fn parse(input: ParseStream) -> Result<Self> {
	input.step(\|cursor\| match cursor.punct() {
	Some((punct, rest)) => Ok((punct, rest)),
	None => Err(cursor.error("expected punctuation token")),
	})
	}
	}

	impl Parse for Literal {
	fn parse(input: ParseStream) -> Result<Self> {
	input.step(\|cursor\| match cursor.literal() {
	Some((literal, rest)) => Ok((literal, rest)),
	None => Err(cursor.error("expected literal token")),
	})
	}
	}

	/// 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]: index.html
	///
	/// This trait is available if Syn is built with the `"parsing"` feature.
	pub trait Parser: Sized {
	type Output;

	/// Parse a proc-macro2 token stream into the chosen syntax tree node.
	fn parse2(self, tokens: TokenStream) -> Result<Self::Output>;

	/// Parse tokens of source code into the chosen syntax tree node.
	///
	/// *This method is available if Syn is built with both the `"parsing"` and
	/// `"proc-macro"` features.*
	#[cfg(all(
	not(all(target_arch = "wasm32", target_os = "unknown")),
	feature = "proc-macro"
	))]
	fn parse(self, tokens: proc_macro::TokenStream) -> Result<Self::Output> {
	self.parse2(proc_macro2::TokenStream::from(tokens))
	}

	/// Parse a string of Rust code into the chosen syntax tree node.
	///
	/// # Hygiene
	///
	/// Every span in the resulting syntax tree will be set to resolve at the
	/// macro call site.
	fn parse_str(self, s: &str) -> Result<Self::Output> {
	self.parse2(proc_macro2::TokenStream::from_str(s)?)
	}
	}

	impl<F, T> Parser for F
	where
	F: FnOnce(ParseStream) -> Result<T>,
	{
	type Output = T;

	fn parse2(self, tokens: TokenStream) -> Result<T> {
	let buf = TokenBuffer::new2(tokens);
	let unexpected = Rc::new(Cell::new(None));
	let state = ParseBuffer::new(Span::call_site(), buf.begin(), unexpected);
	let node = self(&state)?;
	state.check_unexpected()?;
	if state.is_empty() {
	Ok(node)
	} else {
	Err(state.error("unexpected token"))
	}
	}
	}