Merge tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Don't destroy user event fields when removal fails

   User event fields are destroyed before the event is removed from
   visibility. But that can fail leaving the still visible event with no
   fields. Move the destroying of the fields to after the event is
   successfully removed from visibility.

 - Initialize function graph state is fork before calling
   copy_exec_state()

   For non-CLONE_VM forks, copy_exec_state() allocates a new
   task_exec_state. If that allocation fails, ftrace_graph_exit_task()
   will free the tasks ret_stack pointer. Since that pointer is still
   using the parent's ret_stack, it mistakenly frees the parent's
   pointer too.

   Call ftrace_graph_init() on the task first which will NULL out the
   new tasks's ret_stack and if the copy fails, it will not free
   anything.

 - Remove FGRAPH_MAX_INDEX

   The macro FGRAPH_MAX_INDEX was added but never used. Remove it.

 - Save ent_size in function graph printing of nested functions

   The function graph tracer needs to look at the next event to see if
   the next event is the return of the current function entry. If it is,
   it prints a single line:

	ktime_get();

   Otherwise it prints it like a nested function:

	tick_nohz_irq_exit() {
	    ktime_get();
	    kcpustat_irq_exit();
	}

   In order to look at the next event, it must save the current event so
   that it has the information to print from it. It saves the event in
   the iterator descriptor called "ent". What it doesn't save is the
   ent_size of the event which is now used to know if the function graph
   arguments are to be printed. The peek doesn't save the size so the
   size used happens to be that of the size of the last event that was
   seen.

   Save the entry event size in the iterator descriptor so that the
   correct size is used.

 - Fix several errors with freeing data in the histogram code

   The histogram code had a lot of leaked or or incorrect accounting
   when failures happen. Correct them.

 - Fix histogram regression of .percent and .graph modifiers

   Up until 6.3 histogram values could have "percent" or "graph"
   modifiers that changed how they were printed. But a change that added
   restricting histograms values from being strings, stack traces and
   other modifiers inadvertently prevented them from using the percent
   and graph modifiers, which were legal use cases for values.

   Put back the percent and graph modifiers.

 - Fix various typos in the comments

 - Set the trace_clock before initializing a histogram with clock
   argument

   The histogram API allows the user to specific which trace clock to
   use via a "clock=" string. The histogram is set up first before the
   clock is checked. If the passed in clock is not valid, it exits
   without fully fixing up the histogram leaving it on the list and a
   use-after-free can trigger.

   Update the clock argument first and if it fails then exit gracefully
   before the histogram trigger is placed on any lists.

 - Restore :mod: trailer after parsing in ftrace_set_clr_event

   The function ftrace_set_clr_event() modifies the parse string and
   needs to put it back to what was passed in. It searches for ":mod:"
   via a strsep() but fails to put back the first ':' in the string.

   Add back the ':' in the passed in string.

 - Take trace_array reference when opening a tracer options file

   The options files are dynamically created and some tracers add their
   own options. When a tracer adds their own list of options, the
   trace_array holding them has an array to hold the list of options for
   each tracer. This array increases in size via a krealloc(), and the
   new entry gets a newly allocated array to hold the options of the new
   tracer being added.

   The element in each entry of the tracer's option array holds a
   pointer back to the trace_array, a pointer to the tracer it is
   associated to, a pointer to the flags of the option.

   The issue is that these arrays are freed when the trace_array is
   freed when its instance it represents is removed from the instances
   directory. There's a race that an open of one of these options files
   can happen when the instance is being removed.

   Add a new helper function to be called by the open function of the
   options file to iterate all existing trace_arrays under a lock and
   find the one that has the given option element in one of it's tracer
   arrays. If found, then update the associated trace_array's reference
   counter to keep it from being freed. If not found, have the open call
   return -ENODEV.

 - Disable interrupts when acquiring the lock in rb_wake_up_waiters()

   The function rb_wake_up_waiters() assumes it will be called in
   interrupt context and does not disable irqs when taking
   cpu_buffer->reader_lock, which can be called in hard interrupt
   context. The issue is in PREEMPT_RT, this function is called in
   thread context leaving this lock open to a deadlock.

   Take the lock with interrupts disabled.

 - Use rcu_assign_pointer() for tmp_ops filter hash

   The tmp_ops used in update_ftrace_direct_mod() assigns its
   filter_hash field directly, but that field is annotated as __rcu and
   sparse complains. Assign it with rcu_assign_pointer()

 - Fix use-after-free in enable_trigger_private_data_free()

   The trace_event_call is accessed through the event_trigger_data's
   trace_event_file pointer to put the trace_event_call on freeing. The
   issue is that the trace_event_file data may have been freed already
   causing a use-after-free. Add a field to the event_trigger_data that
   points directly to the trace_event_call so that it can decrement its
   reference directly without needing to go through the
   trace_event_file.

 - Fix accounting of buffer data remote headers

   trace_buffer_desc_size() and trace_remote_alloc_buffer() undercount
   the number of pages is needed for the asked for size as it doesn't
   take into account the meta data on each page. Add a helper function
   to do the calculation properly and use that in these functions.

 - Catch nr_page_va overflow in ring_buffer_desc sizing

   The number of pages per remote ring buffer is capped by
   ring_buffer_desc::nr_page_va (32 bits). A buffer_size large enough to
   overflow that field would silently allocate a descriptor smaller than
   what was asked for.

 - Do not resize the subbuf order if any per_cpu buffer is disabled

   The mmapping of ring buffers disables resizing the subbuffers, but it
   is done per-cpu whereas the subbuf size change is done for all the
   per_cpu buffers under the buffer->mutex. It could change the size of
   some while the mapping is happening on others. Have the resize of the
   subbuf order check all the per_cpu buffers under the lock to see if
   any of them is disabled before starting and causing an inconsistency
   between buffers that are being mapped.

* tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: (25 commits)
  ring-buffer: Check resize_disabled before publishing the new subbuf order
  tracing/remotes: Catch nr_page_va overflow in ring_buffer_desc sizing
  tracing/remotes: Account for ring buffer page header in size calculation
  tracing: Don't dereference trace_event_file in deferred trigger free
  ftrace: Use rcu_assign_pointer() for tmp_ops filter hash
  ring-buffer: Acquire the lock with irqsave in rb_wake_up_waiters()
  tracing: Take trace_array reference when opening a tracer options file
  tracing: Fix ring_buffer_read_page_size() kernel-doc
  tracing: Restore :mod: trailer after parsing in ftrace_set_clr_event()
  tracing: Fix memory corruption from a "STACKTRACE" histogram key
  tracing: Fix memory corruption from the histogram stacktrace modifier
  tracing: Undo the registration when enabling the histogram trigger fails
  tracing: Take the reference before publishing the named histogram trigger
  tracing: Set the trace clock before registering the histogram trigger
  tracing: Fix typo "preceeded" in comment
  tracing: Fix typo "availabe" in comment
  tracing: Let histogram values keep the percent and graph modifiers
  tracing: Keep the entry count when the histogram stats allocation fails
  tracing: Free histogram the field rejected for a bad modifier
  tracing: Free histogram the var ref when its initialization fails
  ...

1 files changed, 1421 insertions, 0 deletions

diff --git a/rust/syn/parse.rs b/rust/syn/parse.rs
new file mode 100644
index 000000000..c6f6c0eec
--- /dev/null
+++ b/rust/syn/parse.rs
@@ -0,0 +1,1421 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+//! 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;
+use crate::punctuated::Punctuated;
+use crate::token::Token;
+use proc_macro2::{Delimiter, Group, Literal, Punct, Span, TokenStream, TokenTree};
+#[cfg(feature = "printing")]
+use quote::ToTokens;
+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::panic::{RefUnwindSafe, UnwindSafe};
+use std::rc::Rc;
+use std::str::FromStr;
+
+pub use crate::error::{Error, Result};
+pub use crate::lookahead::{End, 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, delimiter)) = 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, delimiter));
+            }
+        }
+    }
+}
+
+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)
+    }
+}
+
+impl<'a> UnwindSafe for ParseBuffer<'a> {}
+impl<'a> RefUnwindSafe for ParseBuffer<'a> {}
+
+/// 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, Delimiter),
+    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, delimiter) => Unexpected::Some(*span, *delimiter),
+            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, Delimiter)>) {
+    let mut unexpected = get_unexpected(buffer);
+    loop {
+        match cell_clone(&unexpected) {
+            Unexpected::None => return (unexpected, None),
+            Unexpected::Some(span, delimiter) => return (unexpected, Some((span, delimiter))),
+            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, Delimiter)> {
+    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(), cursor.scope_delimiter()))
+    }
+}
+
+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>(&'a self, function: fn(ParseStream<'a>) -> 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 {
+            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 {
+            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>(
+        &'a self,
+        parser: fn(ParseStream<'a>) -> 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 no more tokens remaining to be parsed from
+    /// this stream.
+    ///
+    /// This method returns true upon reaching the end of the content within a
+    /// set of delimiters, as well as at the end of the tokens provided to the
+    /// outermost parsing entry point.
+    ///
+    /// This is equivalent to
+    /// <code>.<a href="#method.peek">peek</a>(<a href="struct.End.html">syn::parse::End</a>)</code>.
+    /// Use `.peek2(End)` or `.peek3(End)` to look for the end of a parse stream
+    /// further ahead than the current position.
+    ///
+    /// # 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, delimiter)) => Err(err_unexpected_token(span, delimiter)),
+            None => Ok(()),
+        }
+    }
+}
+
+#[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
+impl<T: Parse> Parse for Box<T> {
+    fn parse(input: ParseStream) -> Result<Self> {
+        input.parse().map(Box::new)
+    }
+}
+
+#[cfg_attr(docsrs, 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(docsrs, 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(docsrs, 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(docsrs, 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(docsrs, 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(docsrs, 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 enforces 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 enforces 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(docsrs, 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 enforces 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)]
+    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, delimiter)) =
+            span_of_unexpected_ignoring_nones(state.cursor())
+        {
+            Err(err_unexpected_token(unexpected_span, delimiter))
+        } else {
+            Ok(node)
+        }
+    }
+
+    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, delimiter)) =
+            span_of_unexpected_ignoring_nones(state.cursor())
+        {
+            Err(err_unexpected_token(unexpected_span, delimiter))
+        } else {
+            Ok(node)
+        }
+    }
+}
+
+pub(crate) fn parse_scoped<F: Parser>(f: F, scope: Span, tokens: TokenStream) -> Result<F::Output> {
+    f.__parse_scoped(scope, tokens)
+}
+
+fn err_unexpected_token(span: Span, delimiter: Delimiter) -> Error {
+    let msg = match delimiter {
+        Delimiter::Parenthesis => "unexpected token, expected `)`",
+        Delimiter::Brace => "unexpected token, expected `}`",
+        Delimiter::Bracket => "unexpected token, expected `]`",
+        Delimiter::None => "unexpected token",
+    };
+    Error::new(span, msg)
+}
+
+/// 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 = "printing")]
+#[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
+impl ToTokens for Nothing {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let _ = tokens;
+    }
+}
+
+#[cfg(feature = "clone-impls")]
+#[cfg_attr(docsrs, doc(cfg(feature = "clone-impls")))]
+impl Clone for Nothing {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+#[cfg(feature = "clone-impls")]
+#[cfg_attr(docsrs, doc(cfg(feature = "clone-impls")))]
+impl Copy for Nothing {}
+
+#[cfg(feature = "extra-traits")]
+#[cfg_attr(docsrs, 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(docsrs, doc(cfg(feature = "extra-traits")))]
+impl Eq for Nothing {}
+
+#[cfg(feature = "extra-traits")]
+#[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
+impl PartialEq for Nothing {
+    fn eq(&self, _other: &Self) -> bool {
+        true
+    }
+}
+
+#[cfg(feature = "extra-traits")]
+#[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
+impl Hash for Nothing {
+    fn hash<H: Hasher>(&self, _state: &mut H) {}
+}