Initial vendor packages

Signed-off-by: Valentin Popov <valentin@popov.link>

1 files changed, 1386 insertions, 0 deletions

diff --git a/vendor/syn/src/parse.rs b/vendor/syn/src/parse.rs
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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, Token![,])?,
+//!         })
+//!     }
+//! }
+//! #
+//! # 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);
+//!
+//!     /* ... */
+//! #   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() -> 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(())
+//! }
+//! ```
+
+#[path = "discouraged.rs"]
+pub mod discouraged;
+
+use crate::buffer::{Cursor, TokenBuffer};
+use crate::error;
+use crate::lookahead;
+#[cfg(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(syn::Ident)`&emsp;*(does not accept keywords)*
+    /// - `input.peek(syn::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, Token![,])?,
+    ///             semi_token: input.parse()?,
+    ///         })
+    ///     }
+    /// }
+    /// #
+    /// # let input = quote! {
+    /// #     struct S(A, B);
+    /// # };
+    /// # syn::parse2::<TupleStruct>(input).unwrap();
+    /// ```
+    ///
+    /// # See also
+    ///
+    /// If your separator is anything more complicated than an invocation of the
+    /// `Token!` macro, this method won't be applicable and you can instead
+    /// directly use `Punctuated`'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) -> Result<Self> {
+    ///         Ok(Self(input.parse()?, input.parse()?))
+    ///     }
+    /// }
+    ///
+    /// struct Thing {
+    ///     steps: Punctuated<Expr, Fin>,
+    /// }
+    ///
+    /// impl Parse for Thing {
+    ///     fn parse(input: ParseStream) -> Result<Self> {
+    /// # 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)?,
+    /// #       })
+    /// # }
+    ///     }
+    /// }
+    /// ```
+    pub fn parse_terminated<T, P>(
+        &self,
+        parser: fn(ParseStream) -> Result<T>,
+        separator: P,
+    ) -> Result<Punctuated<T, P::Token>>
+    where
+        P: Peek,
+        P::Token: Parse,
+    {
+        let _ = separator;
+        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, 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<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(LifetimeParam),
+    ///     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.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use proc_macro2::TokenStream;
+    /// use syn::buffer::Cursor;
+    /// use syn::parse::{ParseStream, Result};
+    ///
+    /// // Run a parser that returns T, but get its output as TokenStream instead of T.
+    /// // This works without T needing to implement ToTokens.
+    /// fn recognize_token_stream<T>(
+    ///     recognizer: fn(ParseStream) -> Result<T>,
+    /// ) -> impl Fn(ParseStream) -> Result<TokenStream> {
+    ///     move |input| {
+    ///         let begin = input.cursor();
+    ///         recognizer(input)?;
+    ///         let end = input.cursor();
+    ///         Ok(tokens_between(begin, end))
+    ///     }
+    /// }
+    ///
+    /// // Collect tokens between two cursors as a TokenStream.
+    /// fn tokens_between(begin: Cursor, end: Cursor) -> TokenStream {
+    ///     assert!(begin <= end);
+    ///
+    ///     let mut cursor = begin;
+    ///     let mut tokens = TokenStream::new();
+    ///     while cursor < end {
+    ///         let (token, next) = cursor.token_tree().unwrap();
+    ///         tokens.extend(std::iter::once(token));
+    ///         cursor = next;
+    ///     }
+    ///     tokens
+    /// }
+    ///
+    /// fn main() {
+    ///     use quote::quote;
+    ///     use syn::parse::{Parse, Parser};
+    ///     use syn::Token;
+    ///
+    ///     // Parse syn::Type as a TokenStream, surrounded by angle brackets.
+    ///     fn example(input: ParseStream) -> Result<TokenStream> {
+    ///         let _langle: Token![<] = input.parse()?;
+    ///         let ty = recognize_token_stream(syn::Type::parse)(input)?;
+    ///         let _rangle: Token![>] = input.parse()?;
+    ///         Ok(ty)
+    ///     }
+    ///
+    ///     let tokens = quote! { <fn() -> u8> };
+    ///     println!("{}", example.parse2(tokens).unwrap());
+    /// }
+    /// ```
+    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| {
+            if let Some((group, rest)) = cursor.any_group_token() {
+                if group.delimiter() != Delimiter::None {
+                    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
+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.
+    #[cfg(feature = "proc-macro")]
+    #[cfg_attr(doc_cfg, doc(cfg(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)
+    }
+}
+
+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"))]
+pub(crate) fn parse_scoped<F: Parser>(f: F, scope: Span, tokens: TokenStream) -> Result<F::Output> {
+    f.__parse_scoped(scope, tokens)
+}
+
+/// 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);
+///
+///     /* ... */
+/// #   TokenStream::new()
+/// }
+/// ```
+///
+/// ```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) {}
+}