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    //! 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<T>`] 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<T>`]: 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<Field, Token![,]>,
    //! }
    //! #
    //! # enum ItemEnum {}
    //!
    //! impl Parse for Item {
    //!     fn parse(input: ParseStream) -> Result<Self> {
    //!         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) -> Result<Self> {
    //!         let content;
    //!         Ok(ItemStruct {
    //!             struct_token: input.parse()?,
    //!             ident: input.parse()?,
    //!             brace_token: braced!(content in input),
    //!             fields: content.parse_terminated(Field::parse_named)?,
    //!         })
    //!     }
    //! }
    //! #
    //! # impl Parse for ItemEnum {
    //! #     fn parse(input: ParseStream) -> Result<Self> {
    //! #         unimplemented!()
    //! #     }
    //! # }
    //!
    //! # const IGNORE: &str = stringify! {
    //! #[proc_macro]
    //! # };
    //! pub fn my_macro(tokens: TokenStream) -> TokenStream {
    //!     let input = parse_macro_input!(tokens as Item);
    //!
    //!     /* ... */
    //! #   "".parse().unwrap()
    //! }
    //! ```
    //!
    //! # 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() -> syn::Result<()> {
    //! let t: Type = syn::parse_str("std::collections::HashMap<String, Value>")?;
    //! #     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 "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`]: 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) -> Result<()> {
    //! #
    //! // Can't parse `Punctuated` without knowing whether trailing punctuation
    //! // should be allowed in this context.
    //! let path: Punctuated<PathSegment, Token![::]> = 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) -> Result<()> {
    //!     // Parse a nonempty sequence of path segments separated by `::` punctuation
    //!     // with no trailing punctuation.
    //!     let tokens = input.clone();
    //!     let parser = Punctuated::<PathSegment, Token![::]>::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::<Expr, Token![,]>::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(())
    //! }
    //! ```
    //!
    //! ---
    //!
    //! *This module is available only if Syn is built with the `"parsing"` feature.*

    #[path = "discouraged.rs"]
    pub mod discouraged;

    use crate::buffer::{Cursor, TokenBuffer};
    use crate::error;
    use crate::lookahead;
    #[cfg(all(
        not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
        feature = "proc-macro"
    ))]
    use crate::proc_macro;
    use crate::punctuated::Punctuated;
    use crate::token::Token;
    use proc_macro2::{self, Delimiter, Group, Literal, Punct, Span, TokenStream, TokenTree};
    use std::cell::Cell;
    use std::fmt::{self, Debug, Display};
    #[cfg(feature = "extra-traits")]
    use std::hash::{Hash, Hasher};
    use std::marker::PhantomData;
    use std::mem;
    use std::ops::Deref;
    use std::rc::Rc;
    use std::str::FromStr;

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

    /// 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
    pub trait Parse: Sized {
        fn parse(input: ParseStream) -> Result<Self>;
    }

    /// 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
    pub type ParseStream<'a> = &'a ParseBuffer<'a>;

    /// Cursor position within a buffered token stream.
    ///
    /// This type is more commonly used through the type alias [`ParseStream`] which
    /// is an alias for `&ParseBuffer`.
    ///
    /// `ParseStream` is the input type for all parser functions in Syn. They have
    /// the signature `fn(ParseStream) -> Result<T>`.
    ///
    /// ## 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
    pub struct ParseBuffer<'a> {
        scope: Span,
        // Instead of Cell<Cursor<'a>> so that ParseBuffer<'a> is covariant in '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<Cursor<'a>> cannot be covariant in 'a because then we could take a
        // ParseBuffer<'a>, upcast to ParseBuffer<'short> for some lifetime shorter
        // than 'a, and then assign a Cursor<'short> into the Cell.
        //
        // By extension, it would not be safe to expose an API that accepts a
        // Cursor<'a> and trusts that it lives as long as the cursor currently in
        // the cell.
        cell: Cell<Cursor<'static>>,
        marker: PhantomData<Cursor<'a>>,
        unexpected: Cell<Option<Rc<Cell<Unexpected>>>>,
    }

    impl<'a> Drop for ParseBuffer<'a> {
        fn drop(&mut self) {
            if let Some(unexpected_span) = span_of_unexpected_ignoring_nones(self.cursor()) {
                let (inner, old_span) = inner_unexpected(self);
                if old_span.is_none() {
                    inner.set(Unexpected::Some(unexpected_span));
                }
            }
        }
    }

    impl<'a> Display for ParseBuffer<'a> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            Display::fmt(&self.cursor().token_stream(), f)
        }
    }

    impl<'a> Debug for ParseBuffer<'a> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            Debug::fmt(&self.cursor().token_stream(), f)
        }
    }

    /// 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) -> Result<()> {
    ///     input.step(|cursor| {
    ///         let mut rest = *cursor;
    ///         while let Some((tt, next)) = rest.token_tree() {
    ///             match &tt {
    ///                 TokenTree::Punct(punct) if punct.as_char() == '@' => {
    ///                     return Ok(((), next));
    ///                 }
    ///                 _ => rest = next,
    ///             }
    ///         }
    ///         Err(cursor.error("no `@` was found after this point"))
    ///     })
    /// }
    /// #
    /// # fn remainder_after_skipping_past_next_at(
    /// #     input: ParseStream,
    /// # ) -> Result<proc_macro2::TokenStream> {
    /// #     skip_past_next_at(input)?;
    /// #     input.parse()
    /// # }
    /// #
    /// # use syn::parse::Parser;
    /// # let remainder = remainder_after_skipping_past_next_at
    /// #     .parse_str("a @ b c")
    /// #     .unwrap();
    /// # assert_eq!(remainder.to_string(), "b c");
    /// ```
    pub struct StepCursor<'c, 'a> {
        scope: Span,
        // This field is covariant in 'c.
        cursor: Cursor<'c>,
        // This field is contravariant in 'c. Together these make StepCursor
        // invariant in 'c. Also covariant in 'a. The user cannot cast 'c to a
        // different lifetime but can upcast into a StepCursor with a shorter
        // lifetime 'a.
        //
        // As long as we only ever construct a StepCursor for which 'c outlives 'a,
        // this means if ever a StepCursor<'c, 'a> exists we are guaranteed that 'c
        // outlives 'a.
        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> Copy for StepCursor<'c, 'a> {}

    impl<'c, 'a> Clone for StepCursor<'c, 'a> {
        fn clone(&self) -> Self {
            *self
        }
    }

    impl<'c, 'a> StepCursor<'c, 'a> {
        /// 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.
        pub fn error<T: Display>(self, message: T) -> Error {
            error::new_at(self.scope, self.cursor, message)
        }
    }

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

    pub(crate) fn new_parse_buffer(
        scope: Span,
        cursor: Cursor,
        unexpected: Rc<Cell<Unexpected>>,
    ) -> ParseBuffer {
        ParseBuffer {
            scope,
            // See comment on `cell` in the struct definition.
            cell: Cell::new(unsafe { mem::transmute::<Cursor, Cursor<'static>>(cursor) }),
            marker: PhantomData,
            unexpected: Cell::new(Some(unexpected)),
        }
    }

    pub(crate) enum Unexpected {
        None,
        Some(Span),
        Chain(Rc<Cell<Unexpected>>),
    }

    impl Default for Unexpected {
        fn default() -> Self {
            Unexpected::None
        }
    }

    impl Clone for Unexpected {
        fn clone(&self) -> Self {
            match self {
                Unexpected::None => Unexpected::None,
                Unexpected::Some(span) => Unexpected::Some(*span),
                Unexpected::Chain(next) => Unexpected::Chain(next.clone()),
            }
        }
    }

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

    fn inner_unexpected(buffer: &ParseBuffer) -> (Rc<Cell<Unexpected>>, Option<Span>) {
        let mut unexpected = get_unexpected(buffer);
        loop {
            match cell_clone(&unexpected) {
                Unexpected::None => return (unexpected, None),
                Unexpected::Some(span) => return (unexpected, Some(span)),
                Unexpected::Chain(next) => unexpected = next,
            }
        }
    }

    pub(crate) fn get_unexpected(buffer: &ParseBuffer) -> Rc<Cell<Unexpected>> {
        cell_clone(&buffer.unexpected).unwrap()
    }

    fn span_of_unexpected_ignoring_nones(mut cursor: Cursor) -> Option<Span> {
        if cursor.eof() {
            return None;
        }
        while let Some((inner, _span, rest)) = cursor.group(Delimiter::None) {
            if let Some(unexpected) = span_of_unexpected_ignoring_nones(inner) {
                return Some(unexpected);
            }
            cursor = rest;
        }
        if cursor.eof() {
            None
        } else {
            Some(cursor.span())
        }
    }

    impl<'a> ParseBuffer<'a> {
        /// Parses a syntax tree node of type `T`, advancing the position of our
        /// parse stream past it.
        pub fn parse<T: Parse>(&self) -> Result<T> {
            T::parse(self)
        }

        /// 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 = "s.tmpl"]
        /// //     struct S;
        /// struct UnitStruct {
        ///     attrs: Vec<Attribute>,
        ///     struct_token: Token![struct],
        ///     name: Ident,
        ///     semi_token: Token![;],
        /// }
        ///
        /// impl Parse for UnitStruct {
        ///     fn parse(input: ParseStream) -> Result<Self> {
        ///         Ok(UnitStruct {
        ///             attrs: input.call(Attribute::parse_outer)?,
        ///             struct_token: input.parse()?,
        ///             name: input.parse()?,
        ///             semi_token: input.parse()?,
        ///         })
        ///     }
        /// }
        /// ```
        pub fn call<T>(&self, function: fn(ParseStream) -> Result<T>) -> Result<T> {
            function(self)
        }

        /// 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)`&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<'de, T>: A + B<'de> where Box<T>: Clone {}
        /// struct MarkerTrait {
        ///     trait_token: Token![trait],
        ///     ident: Ident,
        ///     generics: Generics,
        ///     colon_token: Option<Token![:]>,
        ///     supertraits: Punctuated<TypeParamBound, Token![+]>,
        ///     brace_token: token::Brace,
        /// }
        ///
        /// impl Parse for MarkerTrait {
        ///     fn parse(input: ParseStream) -> Result<Self> {
        ///         let trait_token: Token![trait] = input.parse()?;
        ///         let ident: Ident = input.parse()?;
        ///         let mut generics: Generics = input.parse()?;
        ///         let colon_token: Option<Token![:]> = 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,
        ///         })
        ///     }
        /// }
        /// ```
        pub fn peek<T: Peek>(&self, token: T) -> bool {
            let _ = token;
            T::Token::peek(self.cursor())
        }

        /// 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'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<T> { uninit: (), value: T }
        ///     Union(ItemUnion),
        ///     // lazy_static! { ... }
        ///     Macro(Macro),
        /// }
        ///
        /// impl Parse for UnionOrMacro {
        ///     fn parse(input: ParseStream) -> Result<Self> {
        ///         if input.peek(Token![union]) && input.peek2(Ident) {
        ///             input.parse().map(UnionOrMacro::Union)
        ///         } else {
        ///             input.parse().map(UnionOrMacro::Macro)
        ///         }
        ///     }
        /// }
        /// ```
        pub fn peek2<T: Peek>(&self, token: T) -> bool {
            fn peek2(buffer: &ParseBuffer, peek: fn(Cursor) -> bool) -> bool {
                if let Some(group) = buffer.cursor().group(Delimiter::None) {
                    if group.0.skip().map_or(false, peek) {
                        return true;
                    }
                }
                buffer.cursor().skip().map_or(false, peek)
            }

            let _ = token;
            peek2(self, T::Token::peek)
        }

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

            let _ = token;
            peek3(self, T::Token::peek)
        }

        /// 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<Type, Token![,]>,
        ///     semi_token: Token![;],
        /// }
        ///
        /// impl Parse for TupleStruct {
        ///     fn parse(input: ParseStream) -> Result<Self> {
        ///         let content;
        ///         Ok(TupleStruct {
        ///             struct_token: input.parse()?,
        ///             ident: input.parse()?,
        ///             paren_token: parenthesized!(content in input),
        ///             fields: content.parse_terminated(Type::parse)?,
        ///             semi_token: input.parse()?,
        ///         })
        ///     }
        /// }
        /// #
        /// # let input = quote! {
        /// #     struct S(A, B);
        /// # };
        /// # syn::parse2::<TupleStruct>(input).unwrap();
        /// ```
        pub fn parse_terminated<T, P: Parse>(
            &self,
            parser: fn(ParseStream) -> Result<T>,
        ) -> Result<Punctuated<T, P>> {
            Punctuated::parse_terminated_with(self, parser)
        }

        /// 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<Item>,
        /// }
        ///
        /// impl Parse for Mod {
        ///     fn parse(input: ParseStream) -> Result<Self> {
        ///         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
        ///             },
        ///         })
        ///     }
        /// }
        /// ```
        pub fn is_empty(&self) -> bool {
            self.cursor().eof()
        }

        /// 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, LifetimeDef, 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<T: Clone, 'a, 'b: 'a, const N: usize>() { ... }
        /// //
        /// // On invalid input, lookahead gives us a reasonable error message.
        /// //
        /// //     error: expected one of: identifier, lifetime, `const`
        /// //       |
        /// //     5 |     fn f<!Sized>() {}
        /// //       |          ^
        /// enum GenericParam {
        ///     Type(TypeParam),
        ///     Lifetime(LifetimeDef),
        ///     Const(ConstParam),
        /// }
        ///
        /// impl Parse for GenericParam {
        ///     fn parse(input: ParseStream) -> Result<Self> {
        ///         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())
        ///         }
        ///     }
        /// }
        /// ```
        pub fn lookahead1(&self) -> Lookahead1<'a> {
            lookahead::new(self.scope, self.cursor())
        }

        /// 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) -> Result<Expr> {
        /// // Do not do this.
        /// if input.fork().parse::<Expr>().is_ok() {
        ///     return input.parse::<Expr>();
        /// }
        /// # 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'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<Restricted>,
        /// }
        ///
        /// struct Restricted {
        ///     paren_token: token::Paren,
        ///     in_token: Option<Token![in]>,
        ///     path: Path,
        /// }
        ///
        /// impl Parse for PubVisibility {
        ///     fn parse(input: ParseStream) -> Result<Self> {
        ///         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,
        ///         })
        ///     }
        /// }
        /// ```
        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 unless we `advance_to`.
                unexpected: Cell::new(Some(Rc::new(Cell::new(Unexpected::None)))),
            }
        }

        /// 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) -> Result<Self> {
        ///         if input.peek(Token![while])
        ///             || input.peek(Token![for])
        ///             || input.peek(Token![loop])
        ///         {
        ///             Ok(Loop {
        ///                 expr: input.parse()?,
        ///             })
        ///         } else {
        ///             Err(input.error("expected some kind of loop"))
        ///         }
        ///     }
        /// }
        /// ```
        pub fn error<T: Display>(&self, message: T) -> Error {
            error::new_at(self.scope, self.cursor(), message)
        }

        /// 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) -> Result<()> {
        ///     input.step(|cursor| {
        ///         let mut rest = *cursor;
        ///         while let Some((tt, next)) = rest.token_tree() {
        ///             match &tt {
        ///                 TokenTree::Punct(punct) if punct.as_char() == '@' => {
        ///                     return Ok(((), next));
        ///                 }
        ///                 _ => rest = next,
        ///             }
        ///         }
        ///         Err(cursor.error("no `@` was found after this point"))
        ///     })
        /// }
        /// #
        /// # fn remainder_after_skipping_past_next_at(
        /// #     input: ParseStream,
        /// # ) -> Result<proc_macro2::TokenStream> {
        /// #     skip_past_next_at(input)?;
        /// #     input.parse()
        /// # }
        /// #
        /// # use syn::parse::Parser;
        /// # let remainder = remainder_after_skipping_past_next_at
        /// #     .parse_str("a @ b c")
        /// #     .unwrap();
        /// # assert_eq!(remainder.to_string(), "b c");
        /// ```
        pub fn step<F, R>(&self, function: F) -> Result<R>
        where
            F: for<'c> FnOnce(StepCursor<'c, 'a>) -> Result<(R, Cursor<'c>)>,
        {
            // Since the user's function is required to work for any 'c, we know
            // that the Cursor<'c> they return is either derived from the input
            // StepCursor<'c, 'a> or from a Cursor<'static>.
            //
            // It would not be legal to write this function without the invariant
            // lifetime 'c in StepCursor<'c, 'a>. If this function were written only
            // in terms of 'a, the user could take our ParseBuffer<'a>, upcast it to
            // a ParseBuffer<'short> which some shorter lifetime than 'a, invoke
            // `step` on their ParseBuffer<'short> with a closure that returns
            // Cursor<'short>, and we would wrongly write that Cursor<'short> into
            // the Cell intended to hold Cursor<'a>.
            //
            // In some cases it may be necessary for R to contain a Cursor<'a>.
            // Within Syn we solve this using `advance_step_cursor` which uses the
            // existence of a StepCursor<'c, 'a> as proof that it is safe to cast
            // from Cursor<'c> to Cursor<'a>. If needed outside of Syn, it would be
            // safe to expose that API as a method on StepCursor.
            let (node, rest) = function(StepCursor {
                scope: self.scope,
                cursor: self.cell.get(),
                marker: PhantomData,
            })?;
            self.cell.set(rest);
            Ok(node)
        }

        /// 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`.
        pub fn span(&self) -> Span {
            let cursor = self.cursor();
            if cursor.eof() {
                self.scope
            } else {
                crate::buffer::open_span_of_group(cursor)
            }
        }

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

        fn check_unexpected(&self) -> Result<()> {
            match inner_unexpected(self).1 {
                Some(span) => Err(Error::new(span, "unexpected token")),
                None => Ok(()),
            }
        }
    }

    #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
    impl<T: Parse> Parse for Box<T> {
        fn parse(input: ParseStream) -> Result<Self> {
            input.parse().map(Box::new)
        }
    }

    #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
    impl<T: Parse + Token> Parse for Option<T> {
        fn parse(input: ParseStream) -> Result<Self> {
            if T::peek(input.cursor()) {
                Ok(Some(input.parse()?))
            } else {
                Ok(None)
            }
        }
    }

    #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
    impl Parse for TokenStream {
        fn parse(input: ParseStream) -> Result<Self> {
            input.step(|cursor| Ok((cursor.token_stream(), Cursor::empty())))
        }
    }

    #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
    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")),
            })
        }
    }

    #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
    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"))
            })
        }
    }

    #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
    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")),
            })
        }
    }

    #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
    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]: self
    ///
    /// *This trait is available only 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.
        ///
        /// 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.
        fn parse2(self, tokens: TokenStream) -> Result<Self::Output>;

        /// 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.
        ///
        /// *This method is available only if Syn is built with both the `"parsing"` and
        /// `"proc-macro"` features.*
        #[cfg(all(
            not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
            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.
        ///
        /// 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.
        fn parse_str(self, s: &str) -> Result<Self::Output> {
            self.parse2(proc_macro2::TokenStream::from_str(s)?)
        }

        // Not public API.
        #[doc(hidden)]
        #[cfg(any(feature = "full", feature = "derive"))]
        fn __parse_scoped(self, scope: Span, tokens: TokenStream) -> Result<Self::Output> {
            let _ = scope;
            self.parse2(tokens)
        }

        // Not public API.
        #[doc(hidden)]
        #[cfg(any(feature = "full", feature = "derive"))]
        fn __parse_stream(self, input: ParseStream) -> Result<Self::Output> {
            input.parse().and_then(|tokens| self.parse2(tokens))
        }
    }

    fn tokens_to_parse_buffer(tokens: &TokenBuffer) -> ParseBuffer {
        let scope = Span::call_site();
        let cursor = tokens.begin();
        let unexpected = Rc::new(Cell::new(Unexpected::None));
        new_parse_buffer(scope, cursor, unexpected)
    }

    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 state = tokens_to_parse_buffer(&buf);
            let node = self(&state)?;
            state.check_unexpected()?;
            if let Some(unexpected_span) = span_of_unexpected_ignoring_nones(state.cursor()) {
                Err(Error::new(unexpected_span, "unexpected token"))
            } else {
                Ok(node)
            }
        }

        #[cfg(any(feature = "full", feature = "derive"))]
        fn __parse_scoped(self, scope: Span, tokens: TokenStream) -> Result<Self::Output> {
            let buf = TokenBuffer::new2(tokens);
            let cursor = buf.begin();
            let unexpected = Rc::new(Cell::new(Unexpected::None));
            let state = new_parse_buffer(scope, cursor, unexpected);
            let node = self(&state)?;
            state.check_unexpected()?;
            if let Some(unexpected_span) = span_of_unexpected_ignoring_nones(state.cursor()) {
                Err(Error::new(unexpected_span, "unexpected token"))
            } else {
                Ok(node)
            }
        }

        #[cfg(any(feature = "full", feature = "derive"))]
        fn __parse_stream(self, input: ParseStream) -> Result<Self::Output> {
            self(input)
        }
    }

    #[cfg(any(feature = "full", feature = "derive"))]
    pub(crate) fn parse_scoped<F: Parser>(f: F, scope: Span, tokens: TokenStream) -> Result<F::Output> {
        f.__parse_scoped(scope, tokens)
    }

    #[cfg(any(feature = "full", feature = "derive"))]
    pub(crate) fn parse_stream<F: Parser>(f: F, input: ParseStream) -> Result<F::Output> {
        f.__parse_stream(input)
    }

    /// 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: &str = stringify! {
    /// #[proc_macro_attribute]
    /// # };
    /// pub fn my_attr(args: TokenStream, input: TokenStream) -> TokenStream {
    ///     parse_macro_input!(args as Nothing);
    ///
    ///     /* ... */
    /// #   "".parse().unwrap()
    /// }
    /// ```
    ///
    /// ```text
    /// error: unexpected token
    ///  --> src/main.rs:3:19
    ///   |
    /// 3 | #[my_attr(asdf)]
    ///   |           ^^^^
    /// ```
    pub struct Nothing;

    impl Parse for Nothing {
        fn parse(_input: ParseStream) -> Result<Self> {
            Ok(Nothing)
        }
    }

    #[cfg(feature = "extra-traits")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "extra-traits")))]
    impl Debug for Nothing {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.write_str("Nothing")
        }
    }

    #[cfg(feature = "extra-traits")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "extra-traits")))]
    impl Eq for Nothing {}

    #[cfg(feature = "extra-traits")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "extra-traits")))]
    impl PartialEq for Nothing {
        fn eq(&self, _other: &Self) -> bool {
            true
        }
    }

    #[cfg(feature = "extra-traits")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "extra-traits")))]
    impl Hash for Nothing {
        fn hash<H: Hasher>(&self, _state: &mut H) {}
    }