compiler/rustc_ast/src/tokenstream.rs - toolchain/rustc - Git at Google

 //! # Token Streams
 //!
 //! `TokenStream`s represent syntactic objects before they are converted into ASTs.
 //! A `TokenStream` is, roughly speaking, a sequence of [`TokenTree`]s,
 //! which are themselves a single [`Token`] or a `Delimited` subsequence of tokens.
 //!
 //! ## Ownership
 //!
 //! `TokenStream`s are persistent data structures constructed as ropes with reference
 //! counted-children. In general, this means that calling an operation on a `TokenStream`
 //! (such as `slice`) produces an entirely new `TokenStream` from the borrowed reference to
 //! the original. This essentially coerces `TokenStream`s into "views" of their subparts,
 //! and a borrowed `TokenStream` is sufficient to build an owned `TokenStream` without taking
 //! ownership of the original.

 use crate::token::{self, DelimToken, Token, TokenKind};
 use crate::AttrVec;

 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::{self, Lrc};
 use rustc_macros::HashStable_Generic;
 use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use rustc_span::{Span, DUMMY_SP};
 use smallvec::{smallvec, SmallVec};

 use std::{fmt, iter, mem};

 /// When the main Rust parser encounters a syntax-extension invocation, it
 /// parses the arguments to the invocation as a token tree. This is a very
 /// loose structure, such that all sorts of different AST fragments can
 /// be passed to syntax extensions using a uniform type.
 ///
 /// If the syntax extension is an MBE macro, it will attempt to match its
 /// LHS token tree against the provided token tree, and if it finds a
 /// match, will transcribe the RHS token tree, splicing in any captured
 /// `macro_parser::matched_nonterminals` into the `SubstNt`s it finds.
 ///
 /// The RHS of an MBE macro is the only place `SubstNt`s are substituted.
 /// Nothing special happens to misnamed or misplaced `SubstNt`s.
 #[derive(Debug, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
 pub enum TokenTree {
     /// A single token.
     Token(Token),
     /// A delimited sequence of token trees.
     Delimited(DelimSpan, DelimToken, TokenStream),
 }

 #[derive(Copy, Clone)]
 pub enum CanSynthesizeMissingTokens {
     Yes,
     No,
 }

 // Ensure all fields of `TokenTree` is `Send` and `Sync`.
 #[cfg(parallel_compiler)]
 fn _dummy()
 where
     Token: Send + Sync,
     DelimSpan: Send + Sync,
     DelimToken: Send + Sync,
     TokenStream: Send + Sync,
 {
 }

 impl TokenTree {
     /// Checks if this `TokenTree` is equal to the other, regardless of span information.
     pub fn eq_unspanned(&self, other: &TokenTree) -> bool {
         match (self, other) {
             (TokenTree::Token(token), TokenTree::Token(token2)) => token.kind == token2.kind,
             (TokenTree::Delimited(_, delim, tts), TokenTree::Delimited(_, delim2, tts2)) => {
                 delim == delim2 && tts.eq_unspanned(&tts2)
             }
             _ => false,
         }
     }

     /// Retrieves the `TokenTree`'s span.
     pub fn span(&self) -> Span {
         match self {
             TokenTree::Token(token) => token.span,
             TokenTree::Delimited(sp, ..) => sp.entire(),
         }
     }

     /// Modify the `TokenTree`'s span in-place.
     pub fn set_span(&mut self, span: Span) {
         match self {
             TokenTree::Token(token) => token.span = span,
             TokenTree::Delimited(dspan, ..) => *dspan = DelimSpan::from_single(span),
         }
     }

     pub fn token(kind: TokenKind, span: Span) -> TokenTree {
         TokenTree::Token(Token::new(kind, span))
     }

     /// Returns the opening delimiter as a token tree.
     pub fn open_tt(span: DelimSpan, delim: DelimToken) -> TokenTree {
         TokenTree::token(token::OpenDelim(delim), span.open)
     }

     /// Returns the closing delimiter as a token tree.
     pub fn close_tt(span: DelimSpan, delim: DelimToken) -> TokenTree {
         TokenTree::token(token::CloseDelim(delim), span.close)
     }

     pub fn uninterpolate(self) -> TokenTree {
         match self {
             TokenTree::Token(token) => TokenTree::Token(token.uninterpolate().into_owned()),
             tt => tt,
         }
     }
 }

 impl<CTX> HashStable<CTX> for TokenStream
 where
     CTX: crate::HashStableContext,
 {
     fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
         for sub_tt in self.trees() {
             sub_tt.hash_stable(hcx, hasher);
         }
     }
 }

 pub trait CreateTokenStream: sync::Send + sync::Sync {
     fn create_token_stream(&self) -> AttrAnnotatedTokenStream;
 }

 impl CreateTokenStream for AttrAnnotatedTokenStream {
     fn create_token_stream(&self) -> AttrAnnotatedTokenStream {
         self.clone()
     }
 }

 /// A lazy version of [`TokenStream`], which defers creation
 /// of an actual `TokenStream` until it is needed.
 /// `Box` is here only to reduce the structure size.
 #[derive(Clone)]
 pub struct LazyTokenStream(Lrc<Box<dyn CreateTokenStream>>);

 impl LazyTokenStream {
     pub fn new(inner: impl CreateTokenStream + 'static) -> LazyTokenStream {
         LazyTokenStream(Lrc::new(Box::new(inner)))
     }

     pub fn create_token_stream(&self) -> AttrAnnotatedTokenStream {
         self.0.create_token_stream()
     }
 }

 impl fmt::Debug for LazyTokenStream {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "LazyTokenStream({:?})", self.create_token_stream())
     }
 }

 impl<S: Encoder> Encodable<S> for LazyTokenStream {
     fn encode(&self, s: &mut S) -> Result<(), S::Error> {
         // Used by AST json printing.
         Encodable::encode(&self.create_token_stream(), s)
     }
 }

 impl<D: Decoder> Decodable<D> for LazyTokenStream {
     fn decode(_d: &mut D) -> Result<Self, D::Error> {
         panic!("Attempted to decode LazyTokenStream");
     }
 }

 impl<CTX> HashStable<CTX> for LazyTokenStream {
     fn hash_stable(&self, _hcx: &mut CTX, _hasher: &mut StableHasher) {
         panic!("Attempted to compute stable hash for LazyTokenStream");
     }
 }

 /// A `AttrAnnotatedTokenStream` is similar to a `TokenStream`, but with extra
 /// information about the tokens for attribute targets. This is used
 /// during expansion to perform early cfg-expansion, and to process attributes
 /// during proc-macro invocations.
 #[derive(Clone, Debug, Default, Encodable, Decodable)]
 pub struct AttrAnnotatedTokenStream(pub Lrc<Vec<(AttrAnnotatedTokenTree, Spacing)>>);

 /// Like `TokenTree`, but for `AttrAnnotatedTokenStream`
 #[derive(Clone, Debug, Encodable, Decodable)]
 pub enum AttrAnnotatedTokenTree {
     Token(Token),
     Delimited(DelimSpan, DelimToken, AttrAnnotatedTokenStream),
     /// Stores the attributes for an attribute target,
     /// along with the tokens for that attribute target.
     /// See `AttributesData` for more information
     Attributes(AttributesData),
 }

 impl AttrAnnotatedTokenStream {
     pub fn new(tokens: Vec<(AttrAnnotatedTokenTree, Spacing)>) -> AttrAnnotatedTokenStream {
         AttrAnnotatedTokenStream(Lrc::new(tokens))
     }

     /// Converts this `AttrAnnotatedTokenStream` to a plain `TokenStream
     /// During conversion, `AttrAnnotatedTokenTree::Attributes` get 'flattened'
     /// back to a `TokenStream` of the form `outer_attr attr_target`.
     /// If there are inner attributes, they are inserted into the proper
     /// place in the attribute target tokens.
     pub fn to_tokenstream(&self) -> TokenStream {
         let trees: Vec<_> = self
             .0
             .iter()
             .flat_map(|tree| match &tree.0 {
                 AttrAnnotatedTokenTree::Token(inner) => {
                     smallvec![(TokenTree::Token(inner.clone()), tree.1)].into_iter()
                 }
                 AttrAnnotatedTokenTree::Delimited(span, delim, stream) => smallvec![(
                     TokenTree::Delimited(*span, *delim, stream.to_tokenstream()),
                     tree.1,
                 )]
                 .into_iter(),
                 AttrAnnotatedTokenTree::Attributes(data) => {
                     let mut outer_attrs = Vec::new();
                     let mut inner_attrs = Vec::new();
                     let attrs: Vec<_> = data.attrs.clone().into();
                     for attr in attrs {
                         match attr.style {
                             crate::AttrStyle::Outer => {
                                 assert!(
                                     inner_attrs.len() == 0,
                                     "Found outer attribute {:?} after inner attrs {:?}",
                                     attr,
                                     inner_attrs
                                 );
                                 outer_attrs.push(attr);
                             }
                             crate::AttrStyle::Inner => {
                                 inner_attrs.push(attr);
                             }
                         }
                     }

                     let mut target_tokens: Vec<_> = data
                         .tokens
                         .create_token_stream()
                         .to_tokenstream()
                         .0
                         .iter()
                         .cloned()
                         .collect();
                     if !inner_attrs.is_empty() {
                         let mut found = false;
                         // Check the last two trees (to account for a trailing semi)
                         for (tree, _) in target_tokens.iter_mut().rev().take(2) {
                             if let TokenTree::Delimited(span, delim, delim_tokens) = tree {
                                 // Inner attributes are only supported on extern blocks, functions, impls,
                                 // and modules. All of these have their inner attributes placed at
                                 // the beginning of the rightmost outermost braced group:
                                 // e.g. fn foo() { #![my_attr} }
                                 //
                                 // Therefore, we can insert them back into the right location
                                 // without needing to do any extra position tracking.
                                 //
                                 // Note: Outline modules are an exception - they can
                                 // have attributes like `#![my_attr]` at the start of a file.
                                 // Support for custom attributes in this position is not
                                 // properly implemented - we always synthesize fake tokens,
                                 // so we never reach this code.

                                 let mut builder = TokenStreamBuilder::new();
                                 for inner_attr in &inner_attrs {
                                     builder.push(inner_attr.tokens().to_tokenstream());
                                 }
                                 builder.push(delim_tokens.clone());
                                 *tree = TokenTree::Delimited(*span, *delim, builder.build());
                                 found = true;
                                 break;
                             }
                         }

                         assert!(
                             found,
                             "Failed to find trailing delimited group in: {:?}",
                             target_tokens
                         );
                     }
                     let mut flat: SmallVec<[_; 1]> = SmallVec::new();
                     for attr in outer_attrs {
                         // FIXME: Make this more efficient
                         flat.extend(attr.tokens().to_tokenstream().0.clone().iter().cloned());
                     }
                     flat.extend(target_tokens);
                     flat.into_iter()
                 }
             })
             .collect();
         TokenStream::new(trees)
     }
 }

 /// Stores the tokens for an attribute target, along
 /// with its attributes.
 ///
 /// This is constructed during parsing when we need to capture
 /// tokens.
 ///
 /// For example, `#[cfg(FALSE)] struct Foo {}` would
 /// have an `attrs` field containing the `#[cfg(FALSE)]` attr,
 /// and a `tokens` field storing the (unparesd) tokens `struct Foo {}`
 #[derive(Clone, Debug, Encodable, Decodable)]
 pub struct AttributesData {
     /// Attributes, both outer and inner.
     /// These are stored in the original order that they were parsed in.
     pub attrs: AttrVec,
     /// The underlying tokens for the attribute target that `attrs`
     /// are applied to
     pub tokens: LazyTokenStream,
 }

 /// A `TokenStream` is an abstract sequence of tokens, organized into [`TokenTree`]s.
 ///
 /// The goal is for procedural macros to work with `TokenStream`s and `TokenTree`s
 /// instead of a representation of the abstract syntax tree.
 /// Today's `TokenTree`s can still contain AST via `token::Interpolated` for
 /// backwards compatibility.
 #[derive(Clone, Debug, Default, Encodable, Decodable)]
 pub struct TokenStream(pub(crate) Lrc<Vec<TreeAndSpacing>>);

 pub type TreeAndSpacing = (TokenTree, Spacing);

 // `TokenStream` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 rustc_data_structures::static_assert_size!(TokenStream, 8);

 #[derive(Clone, Copy, Debug, PartialEq, Encodable, Decodable)]
 pub enum Spacing {
     Alone,
     Joint,
 }

 impl TokenStream {
     /// Given a `TokenStream` with a `Stream` of only two arguments, return a new `TokenStream`
     /// separating the two arguments with a comma for diagnostic suggestions.
     pub fn add_comma(&self) -> Option<(TokenStream, Span)> {
         // Used to suggest if a user writes `foo!(a b);`
         let mut suggestion = None;
         let mut iter = self.0.iter().enumerate().peekable();
         while let Some((pos, ts)) = iter.next() {
             if let Some((_, next)) = iter.peek() {
                 let sp = match (&ts, &next) {
                     (_, (TokenTree::Token(Token { kind: token::Comma, .. }), _)) => continue,
                     (
                         (TokenTree::Token(token_left), Spacing::Alone),
                         (TokenTree::Token(token_right), _),
                     ) if ((token_left.is_ident() && !token_left.is_reserved_ident())
                         || token_left.is_lit())
                         && ((token_right.is_ident() && !token_right.is_reserved_ident())
                             || token_right.is_lit()) =>
                     {
                         token_left.span
                     }
                     ((TokenTree::Delimited(sp, ..), Spacing::Alone), _) => sp.entire(),
                     _ => continue,
                 };
                 let sp = sp.shrink_to_hi();
                 let comma = (TokenTree::token(token::Comma, sp), Spacing::Alone);
                 suggestion = Some((pos, comma, sp));
             }
         }
         if let Some((pos, comma, sp)) = suggestion {
             let mut new_stream = Vec::with_capacity(self.0.len() + 1);
             let parts = self.0.split_at(pos + 1);
             new_stream.extend_from_slice(parts.0);
             new_stream.push(comma);
             new_stream.extend_from_slice(parts.1);
             return Some((TokenStream::new(new_stream), sp));
         }
         None
     }
 }

 impl From<(AttrAnnotatedTokenTree, Spacing)> for AttrAnnotatedTokenStream {
     fn from((tree, spacing): (AttrAnnotatedTokenTree, Spacing)) -> AttrAnnotatedTokenStream {
         AttrAnnotatedTokenStream::new(vec![(tree, spacing)])
     }
 }

 impl From<TokenTree> for TokenStream {
     fn from(tree: TokenTree) -> TokenStream {
         TokenStream::new(vec![(tree, Spacing::Alone)])
     }
 }

 impl From<TokenTree> for TreeAndSpacing {
     fn from(tree: TokenTree) -> TreeAndSpacing {
         (tree, Spacing::Alone)
     }
 }

 impl iter::FromIterator<TokenTree> for TokenStream {
     fn from_iter<I: IntoIterator<Item = TokenTree>>(iter: I) -> Self {
         TokenStream::new(iter.into_iter().map(Into::into).collect::<Vec<TreeAndSpacing>>())
     }
 }

 impl Eq for TokenStream {}

 impl PartialEq<TokenStream> for TokenStream {
     fn eq(&self, other: &TokenStream) -> bool {
         self.trees().eq(other.trees())
     }
 }

 impl TokenStream {
     pub fn new(streams: Vec<TreeAndSpacing>) -> TokenStream {
         TokenStream(Lrc::new(streams))
     }

     pub fn is_empty(&self) -> bool {
         self.0.is_empty()
     }

     pub fn len(&self) -> usize {
         self.0.len()
     }

     pub fn from_streams(mut streams: SmallVec<[TokenStream; 2]>) -> TokenStream {
         match streams.len() {
             0 => TokenStream::default(),
             1 => streams.pop().unwrap(),
             _ => {
                 // We are going to extend the first stream in `streams` with
                 // the elements from the subsequent streams. This requires
                 // using `make_mut()` on the first stream, and in practice this
                 // doesn't cause cloning 99.9% of the time.
                 //
                 // One very common use case is when `streams` has two elements,
                 // where the first stream has any number of elements within
                 // (often 1, but sometimes many more) and the second stream has
                 // a single element within.

                 // Determine how much the first stream will be extended.
                 // Needed to avoid quadratic blow up from on-the-fly
                 // reallocations (#57735).
                 let num_appends = streams.iter().skip(1).map(|ts| ts.len()).sum();

                 // Get the first stream. If it's `None`, create an empty
                 // stream.
                 let mut iter = streams.drain(..);
                 let mut first_stream_lrc = iter.next().unwrap().0;

                 // Append the elements to the first stream, after reserving
                 // space for them.
                 let first_vec_mut = Lrc::make_mut(&mut first_stream_lrc);
                 first_vec_mut.reserve(num_appends);
                 for stream in iter {
                     first_vec_mut.extend(stream.0.iter().cloned());
                 }

                 // Create the final `TokenStream`.
                 TokenStream(first_stream_lrc)
             }
         }
     }

     pub fn trees(&self) -> Cursor {
         self.clone().into_trees()
     }

     pub fn into_trees(self) -> Cursor {
         Cursor::new(self)
     }

     /// Compares two `TokenStream`s, checking equality without regarding span information.
     pub fn eq_unspanned(&self, other: &TokenStream) -> bool {
         let mut t1 = self.trees();
         let mut t2 = other.trees();
         for (t1, t2) in iter::zip(&mut t1, &mut t2) {
             if !t1.eq_unspanned(&t2) {
                 return false;
             }
         }
         t1.next().is_none() && t2.next().is_none()
     }

     pub fn map_enumerated<F: FnMut(usize, &TokenTree) -> TokenTree>(self, mut f: F) -> TokenStream {
         TokenStream(Lrc::new(
             self.0
                 .iter()
                 .enumerate()
                 .map(|(i, (tree, is_joint))| (f(i, tree), *is_joint))
                 .collect(),
         ))
     }
 }

 // 99.5%+ of the time we have 1 or 2 elements in this vector.
 #[derive(Clone)]
 pub struct TokenStreamBuilder(SmallVec<[TokenStream; 2]>);

 impl TokenStreamBuilder {
     pub fn new() -> TokenStreamBuilder {
         TokenStreamBuilder(SmallVec::new())
     }

     pub fn push<T: Into<TokenStream>>(&mut self, stream: T) {
         let mut stream = stream.into();

         // If `self` is not empty and the last tree within the last stream is a
         // token tree marked with `Joint`...
         if let Some(TokenStream(ref mut last_stream_lrc)) = self.0.last_mut() {
             if let Some((TokenTree::Token(last_token), Spacing::Joint)) = last_stream_lrc.last() {
                 // ...and `stream` is not empty and the first tree within it is
                 // a token tree...
                 let TokenStream(ref mut stream_lrc) = stream;
                 if let Some((TokenTree::Token(token), spacing)) = stream_lrc.first() {
                     // ...and the two tokens can be glued together...
                     if let Some(glued_tok) = last_token.glue(&token) {
                         // ...then do so, by overwriting the last token
                         // tree in `self` and removing the first token tree
                         // from `stream`. This requires using `make_mut()`
                         // on the last stream in `self` and on `stream`,
                         // and in practice this doesn't cause cloning 99.9%
                         // of the time.

                         // Overwrite the last token tree with the merged
                         // token.
                         let last_vec_mut = Lrc::make_mut(last_stream_lrc);
                         *last_vec_mut.last_mut().unwrap() = (TokenTree::Token(glued_tok), *spacing);

                         // Remove the first token tree from `stream`. (This
                         // is almost always the only tree in `stream`.)
                         let stream_vec_mut = Lrc::make_mut(stream_lrc);
                         stream_vec_mut.remove(0);

                         // Don't push `stream` if it's empty -- that could
                         // block subsequent token gluing, by getting
                         // between two token trees that should be glued
                         // together.
                         if !stream.is_empty() {
                             self.0.push(stream);
                         }
                         return;
                     }
                 }
             }
         }
         self.0.push(stream);
     }

     pub fn build(self) -> TokenStream {
         TokenStream::from_streams(self.0)
     }
 }

 /// By-reference iterator over a [`TokenStream`].
 #[derive(Clone)]
 pub struct CursorRef<'t> {
     stream: &'t TokenStream,
     index: usize,
 }

 impl<'t> CursorRef<'t> {
     fn next_with_spacing(&mut self) -> Option<&'t TreeAndSpacing> {
         self.stream.0.get(self.index).map(|tree| {
             self.index += 1;
             tree
         })
     }
 }

 impl<'t> Iterator for CursorRef<'t> {
     type Item = &'t TokenTree;

     fn next(&mut self) -> Option<&'t TokenTree> {
         self.next_with_spacing().map(|(tree, _)| tree)
     }
 }

 /// Owning by-value iterator over a [`TokenStream`].
 // FIXME: Many uses of this can be replaced with by-reference iterator to avoid clones.
 #[derive(Clone)]
 pub struct Cursor {
     pub stream: TokenStream,
     index: usize,
 }

 impl Iterator for Cursor {
     type Item = TokenTree;

     fn next(&mut self) -> Option<TokenTree> {
         self.next_with_spacing().map(|(tree, _)| tree)
     }
 }

 impl Cursor {
     fn new(stream: TokenStream) -> Self {
         Cursor { stream, index: 0 }
     }

     pub fn next_with_spacing(&mut self) -> Option<TreeAndSpacing> {
         if self.index < self.stream.len() {
             self.index += 1;
             Some(self.stream.0[self.index - 1].clone())
         } else {
             None
         }
     }

     pub fn index(&self) -> usize {
         self.index
     }

     pub fn append(&mut self, new_stream: TokenStream) {
         if new_stream.is_empty() {
             return;
         }
         let index = self.index;
         let stream = mem::take(&mut self.stream);
         *self = TokenStream::from_streams(smallvec![stream, new_stream]).into_trees();
         self.index = index;
     }

     pub fn look_ahead(&self, n: usize) -> Option<&TokenTree> {
         self.stream.0[self.index..].get(n).map(|(tree, _)| tree)
     }
 }

 #[derive(Debug, Copy, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
 pub struct DelimSpan {
     pub open: Span,
     pub close: Span,
 }

 impl DelimSpan {
     pub fn from_single(sp: Span) -> Self {
         DelimSpan { open: sp, close: sp }
     }

     pub fn from_pair(open: Span, close: Span) -> Self {
         DelimSpan { open, close }
     }

     pub fn dummy() -> Self {
         Self::from_single(DUMMY_SP)
     }

     pub fn entire(self) -> Span {
         self.open.with_hi(self.close.hi())
     }
 }
	//! # Token Streams
	//!
	//! `TokenStream`s represent syntactic objects before they are converted into ASTs.
	//! A `TokenStream` is, roughly speaking, a sequence of [`TokenTree`]s,
	//! which are themselves a single [`Token`] or a `Delimited` subsequence of tokens.
	//!
	//! ## Ownership
	//!
	//! `TokenStream`s are persistent data structures constructed as ropes with reference
	//! counted-children. In general, this means that calling an operation on a `TokenStream`
	//! (such as `slice`) produces an entirely new `TokenStream` from the borrowed reference to
	//! the original. This essentially coerces `TokenStream`s into "views" of their subparts,
	//! and a borrowed `TokenStream` is sufficient to build an owned `TokenStream` without taking
	//! ownership of the original.

	use crate::token::{self, DelimToken, Token, TokenKind};
	use crate::AttrVec;

	use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
	use rustc_data_structures::sync::{self, Lrc};
	use rustc_macros::HashStable_Generic;
	use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
	use rustc_span::{Span, DUMMY_SP};
	use smallvec::{smallvec, SmallVec};

	use std::{fmt, iter, mem};

	/// When the main Rust parser encounters a syntax-extension invocation, it
	/// parses the arguments to the invocation as a token tree. This is a very
	/// loose structure, such that all sorts of different AST fragments can
	/// be passed to syntax extensions using a uniform type.
	///
	/// If the syntax extension is an MBE macro, it will attempt to match its
	/// LHS token tree against the provided token tree, and if it finds a
	/// match, will transcribe the RHS token tree, splicing in any captured
	/// `macro_parser::matched_nonterminals` into the `SubstNt`s it finds.
	///
	/// The RHS of an MBE macro is the only place `SubstNt`s are substituted.
	/// Nothing special happens to misnamed or misplaced `SubstNt`s.
	#[derive(Debug, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
	pub enum TokenTree {
	/// A single token.
	Token(Token),
	/// A delimited sequence of token trees.
	Delimited(DelimSpan, DelimToken, TokenStream),
	}

	#[derive(Copy, Clone)]
	pub enum CanSynthesizeMissingTokens {
	Yes,
	No,
	}

	// Ensure all fields of `TokenTree` is `Send` and `Sync`.
	#[cfg(parallel_compiler)]
	fn _dummy()
	where
	Token: Send + Sync,
	DelimSpan: Send + Sync,
	DelimToken: Send + Sync,
	TokenStream: Send + Sync,
	{
	}

	impl TokenTree {
	/// Checks if this `TokenTree` is equal to the other, regardless of span information.
	pub fn eq_unspanned(&self, other: &TokenTree) -> bool {
	match (self, other) {
	(TokenTree::Token(token), TokenTree::Token(token2)) => token.kind == token2.kind,
	(TokenTree::Delimited(_, delim, tts), TokenTree::Delimited(_, delim2, tts2)) => {
	delim == delim2 && tts.eq_unspanned(&tts2)
	}
	_ => false,
	}
	}

	/// Retrieves the `TokenTree`'s span.
	pub fn span(&self) -> Span {
	match self {
	TokenTree::Token(token) => token.span,
	TokenTree::Delimited(sp, ..) => sp.entire(),
	}
	}

	/// Modify the `TokenTree`'s span in-place.
	pub fn set_span(&mut self, span: Span) {
	match self {
	TokenTree::Token(token) => token.span = span,
	TokenTree::Delimited(dspan, ..) => *dspan = DelimSpan::from_single(span),
	}
	}

	pub fn token(kind: TokenKind, span: Span) -> TokenTree {
	TokenTree::Token(Token::new(kind, span))
	}

	/// Returns the opening delimiter as a token tree.
	pub fn open_tt(span: DelimSpan, delim: DelimToken) -> TokenTree {
	TokenTree::token(token::OpenDelim(delim), span.open)
	}

	/// Returns the closing delimiter as a token tree.
	pub fn close_tt(span: DelimSpan, delim: DelimToken) -> TokenTree {
	TokenTree::token(token::CloseDelim(delim), span.close)
	}

	pub fn uninterpolate(self) -> TokenTree {
	match self {
	TokenTree::Token(token) => TokenTree::Token(token.uninterpolate().into_owned()),
	tt => tt,
	}
	}
	}

	impl<CTX> HashStable<CTX> for TokenStream
	where
	CTX: crate::HashStableContext,
	{
	fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
	for sub_tt in self.trees() {
	sub_tt.hash_stable(hcx, hasher);
	}
	}
	}

	pub trait CreateTokenStream: sync::Send + sync::Sync {
	fn create_token_stream(&self) -> AttrAnnotatedTokenStream;
	}

	impl CreateTokenStream for AttrAnnotatedTokenStream {
	fn create_token_stream(&self) -> AttrAnnotatedTokenStream {
	self.clone()
	}
	}

	/// A lazy version of [`TokenStream`], which defers creation
	/// of an actual `TokenStream` until it is needed.
	/// `Box` is here only to reduce the structure size.
	#[derive(Clone)]
	pub struct LazyTokenStream(Lrc<Box<dyn CreateTokenStream>>);

	impl LazyTokenStream {
	pub fn new(inner: impl CreateTokenStream + 'static) -> LazyTokenStream {
	LazyTokenStream(Lrc::new(Box::new(inner)))
	}

	pub fn create_token_stream(&self) -> AttrAnnotatedTokenStream {
	self.0.create_token_stream()
	}
	}

	impl fmt::Debug for LazyTokenStream {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	write!(f, "LazyTokenStream({:?})", self.create_token_stream())
	}
	}

	impl<S: Encoder> Encodable<S> for LazyTokenStream {
	fn encode(&self, s: &mut S) -> Result<(), S::Error> {
	// Used by AST json printing.
	Encodable::encode(&self.create_token_stream(), s)
	}
	}

	impl<D: Decoder> Decodable<D> for LazyTokenStream {
	fn decode(_d: &mut D) -> Result<Self, D::Error> {
	panic!("Attempted to decode LazyTokenStream");
	}
	}

	impl<CTX> HashStable<CTX> for LazyTokenStream {
	fn hash_stable(&self, _hcx: &mut CTX, _hasher: &mut StableHasher) {
	panic!("Attempted to compute stable hash for LazyTokenStream");
	}
	}

	/// A `AttrAnnotatedTokenStream` is similar to a `TokenStream`, but with extra
	/// information about the tokens for attribute targets. This is used
	/// during expansion to perform early cfg-expansion, and to process attributes
	/// during proc-macro invocations.
	#[derive(Clone, Debug, Default, Encodable, Decodable)]
	pub struct AttrAnnotatedTokenStream(pub Lrc<Vec<(AttrAnnotatedTokenTree, Spacing)>>);

	/// Like `TokenTree`, but for `AttrAnnotatedTokenStream`
	#[derive(Clone, Debug, Encodable, Decodable)]
	pub enum AttrAnnotatedTokenTree {
	Token(Token),
	Delimited(DelimSpan, DelimToken, AttrAnnotatedTokenStream),
	/// Stores the attributes for an attribute target,
	/// along with the tokens for that attribute target.
	/// See `AttributesData` for more information
	Attributes(AttributesData),
	}

	impl AttrAnnotatedTokenStream {
	pub fn new(tokens: Vec<(AttrAnnotatedTokenTree, Spacing)>) -> AttrAnnotatedTokenStream {
	AttrAnnotatedTokenStream(Lrc::new(tokens))
	}

	/// Converts this `AttrAnnotatedTokenStream` to a plain `TokenStream
	/// During conversion, `AttrAnnotatedTokenTree::Attributes` get 'flattened'
	/// back to a `TokenStream` of the form `outer_attr attr_target`.
	/// If there are inner attributes, they are inserted into the proper
	/// place in the attribute target tokens.
	pub fn to_tokenstream(&self) -> TokenStream {
	let trees: Vec<_> = self
	.0
	.iter()
	.flat_map(\|tree\| match &tree.0 {
	AttrAnnotatedTokenTree::Token(inner) => {
	smallvec![(TokenTree::Token(inner.clone()), tree.1)].into_iter()
	}
	AttrAnnotatedTokenTree::Delimited(span, delim, stream) => smallvec![(
	TokenTree::Delimited(span, delim, stream.to_tokenstream()),
	tree.1,
	)]
	.into_iter(),
	AttrAnnotatedTokenTree::Attributes(data) => {
	let mut outer_attrs = Vec::new();
	let mut inner_attrs = Vec::new();
	let attrs: Vec<_> = data.attrs.clone().into();
	for attr in attrs {
	match attr.style {
	crate::AttrStyle::Outer => {
	assert!(
	inner_attrs.len() == 0,
	"Found outer attribute {:?} after inner attrs {:?}",
	attr,
	inner_attrs
	);
	outer_attrs.push(attr);
	}
	crate::AttrStyle::Inner => {
	inner_attrs.push(attr);
	}
	}
	}

	let mut target_tokens: Vec<_> = data
	.tokens
	.create_token_stream()
	.to_tokenstream()
	.0
	.iter()
	.cloned()
	.collect();
	if !inner_attrs.is_empty() {
	let mut found = false;
	// Check the last two trees (to account for a trailing semi)
	for (tree, _) in target_tokens.iter_mut().rev().take(2) {
	if let TokenTree::Delimited(span, delim, delim_tokens) = tree {
	// Inner attributes are only supported on extern blocks, functions, impls,
	// and modules. All of these have their inner attributes placed at
	// the beginning of the rightmost outermost braced group:
	// e.g. fn foo() { #![my_attr} }
	//
	// Therefore, we can insert them back into the right location
	// without needing to do any extra position tracking.
	//
	// Note: Outline modules are an exception - they can
	// have attributes like `#![my_attr]` at the start of a file.
	// Support for custom attributes in this position is not
	// properly implemented - we always synthesize fake tokens,
	// so we never reach this code.

	let mut builder = TokenStreamBuilder::new();
	for inner_attr in &inner_attrs {
	builder.push(inner_attr.tokens().to_tokenstream());
	}
	builder.push(delim_tokens.clone());
	tree = TokenTree::Delimited(span, *delim, builder.build());
	found = true;
	break;
	}
	}

	assert!(
	found,
	"Failed to find trailing delimited group in: {:?}",
	target_tokens
	);
	}
	let mut flat: SmallVec<[_; 1]> = SmallVec::new();
	for attr in outer_attrs {
	// FIXME: Make this more efficient
	flat.extend(attr.tokens().to_tokenstream().0.clone().iter().cloned());
	}
	flat.extend(target_tokens);
	flat.into_iter()
	}
	})
	.collect();
	TokenStream::new(trees)
	}
	}

	/// Stores the tokens for an attribute target, along
	/// with its attributes.
	///
	/// This is constructed during parsing when we need to capture
	/// tokens.
	///
	/// For example, `#[cfg(FALSE)] struct Foo {}` would
	/// have an `attrs` field containing the `#[cfg(FALSE)]` attr,
	/// and a `tokens` field storing the (unparesd) tokens `struct Foo {}`
	#[derive(Clone, Debug, Encodable, Decodable)]
	pub struct AttributesData {
	/// Attributes, both outer and inner.
	/// These are stored in the original order that they were parsed in.
	pub attrs: AttrVec,
	/// The underlying tokens for the attribute target that `attrs`
	/// are applied to
	pub tokens: LazyTokenStream,
	}

	/// A `TokenStream` is an abstract sequence of tokens, organized into [`TokenTree`]s.
	///
	/// The goal is for procedural macros to work with `TokenStream`s and `TokenTree`s
	/// instead of a representation of the abstract syntax tree.
	/// Today's `TokenTree`s can still contain AST via `token::Interpolated` for
	/// backwards compatibility.
	#[derive(Clone, Debug, Default, Encodable, Decodable)]
	pub struct TokenStream(pub(crate) Lrc<Vec<TreeAndSpacing>>);

	pub type TreeAndSpacing = (TokenTree, Spacing);

	// `TokenStream` is used a lot. Make sure it doesn't unintentionally get bigger.
	#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
	rustc_data_structures::static_assert_size!(TokenStream, 8);

	#[derive(Clone, Copy, Debug, PartialEq, Encodable, Decodable)]
	pub enum Spacing {
	Alone,
	Joint,
	}

	impl TokenStream {
	/// Given a `TokenStream` with a `Stream` of only two arguments, return a new `TokenStream`
	/// separating the two arguments with a comma for diagnostic suggestions.
	pub fn add_comma(&self) -> Option<(TokenStream, Span)> {
	// Used to suggest if a user writes `foo!(a b);`
	let mut suggestion = None;
	let mut iter = self.0.iter().enumerate().peekable();
	while let Some((pos, ts)) = iter.next() {
	if let Some((_, next)) = iter.peek() {
	let sp = match (&ts, &next) {
	(_, (TokenTree::Token(Token { kind: token::Comma, .. }), _)) => continue,
	(
	(TokenTree::Token(token_left), Spacing::Alone),
	(TokenTree::Token(token_right), _),
	) if ((token_left.is_ident() && !token_left.is_reserved_ident())
	\|\| token_left.is_lit())
	&& ((token_right.is_ident() && !token_right.is_reserved_ident())
	\|\| token_right.is_lit()) =>
	{
	token_left.span
	}
	((TokenTree::Delimited(sp, ..), Spacing::Alone), _) => sp.entire(),
	_ => continue,
	};
	let sp = sp.shrink_to_hi();
	let comma = (TokenTree::token(token::Comma, sp), Spacing::Alone);
	suggestion = Some((pos, comma, sp));
	}
	}
	if let Some((pos, comma, sp)) = suggestion {
	let mut new_stream = Vec::with_capacity(self.0.len() + 1);
	let parts = self.0.split_at(pos + 1);
	new_stream.extend_from_slice(parts.0);
	new_stream.push(comma);
	new_stream.extend_from_slice(parts.1);
	return Some((TokenStream::new(new_stream), sp));
	}
	None
	}
	}

	impl From<(AttrAnnotatedTokenTree, Spacing)> for AttrAnnotatedTokenStream {
	fn from((tree, spacing): (AttrAnnotatedTokenTree, Spacing)) -> AttrAnnotatedTokenStream {
	AttrAnnotatedTokenStream::new(vec![(tree, spacing)])
	}
	}

	impl From<TokenTree> for TokenStream {
	fn from(tree: TokenTree) -> TokenStream {
	TokenStream::new(vec![(tree, Spacing::Alone)])
	}
	}

	impl From<TokenTree> for TreeAndSpacing {
	fn from(tree: TokenTree) -> TreeAndSpacing {
	(tree, Spacing::Alone)
	}
	}

	impl iter::FromIterator<TokenTree> for TokenStream {
	fn from_iter<I: IntoIterator<Item = TokenTree>>(iter: I) -> Self {
	TokenStream::new(iter.into_iter().map(Into::into).collect::<Vec<TreeAndSpacing>>())
	}
	}

	impl Eq for TokenStream {}

	impl PartialEq<TokenStream> for TokenStream {
	fn eq(&self, other: &TokenStream) -> bool {
	self.trees().eq(other.trees())
	}
	}

	impl TokenStream {
	pub fn new(streams: Vec<TreeAndSpacing>) -> TokenStream {
	TokenStream(Lrc::new(streams))
	}

	pub fn is_empty(&self) -> bool {
	self.0.is_empty()
	}

	pub fn len(&self) -> usize {
	self.0.len()
	}

	pub fn from_streams(mut streams: SmallVec<[TokenStream; 2]>) -> TokenStream {
	match streams.len() {
	0 => TokenStream::default(),
	1 => streams.pop().unwrap(),
	_ => {
	// We are going to extend the first stream in `streams` with
	// the elements from the subsequent streams. This requires
	// using `make_mut()` on the first stream, and in practice this
	// doesn't cause cloning 99.9% of the time.
	//
	// One very common use case is when `streams` has two elements,
	// where the first stream has any number of elements within
	// (often 1, but sometimes many more) and the second stream has
	// a single element within.

	// Determine how much the first stream will be extended.
	// Needed to avoid quadratic blow up from on-the-fly
	// reallocations (#57735).
	let num_appends = streams.iter().skip(1).map(\|ts\| ts.len()).sum();

	// Get the first stream. If it's `None`, create an empty
	// stream.
	let mut iter = streams.drain(..);
	let mut first_stream_lrc = iter.next().unwrap().0;

	// Append the elements to the first stream, after reserving
	// space for them.
	let first_vec_mut = Lrc::make_mut(&mut first_stream_lrc);
	first_vec_mut.reserve(num_appends);
	for stream in iter {
	first_vec_mut.extend(stream.0.iter().cloned());
	}

	// Create the final `TokenStream`.
	TokenStream(first_stream_lrc)
	}
	}
	}

	pub fn trees(&self) -> Cursor {
	self.clone().into_trees()
	}

	pub fn into_trees(self) -> Cursor {
	Cursor::new(self)
	}

	/// Compares two `TokenStream`s, checking equality without regarding span information.
	pub fn eq_unspanned(&self, other: &TokenStream) -> bool {
	let mut t1 = self.trees();
	let mut t2 = other.trees();
	for (t1, t2) in iter::zip(&mut t1, &mut t2) {
	if !t1.eq_unspanned(&t2) {
	return false;
	}
	}
	t1.next().is_none() && t2.next().is_none()
	}

	pub fn map_enumerated<F: FnMut(usize, &TokenTree) -> TokenTree>(self, mut f: F) -> TokenStream {
	TokenStream(Lrc::new(
	self.0
	.iter()
	.enumerate()
	.map(\|(i, (tree, is_joint))\| (f(i, tree), *is_joint))
	.collect(),
	))
	}
	}

	// 99.5%+ of the time we have 1 or 2 elements in this vector.
	#[derive(Clone)]
	pub struct TokenStreamBuilder(SmallVec<[TokenStream; 2]>);

	impl TokenStreamBuilder {
	pub fn new() -> TokenStreamBuilder {
	TokenStreamBuilder(SmallVec::new())
	}

	pub fn push<T: Into<TokenStream>>(&mut self, stream: T) {
	let mut stream = stream.into();

	// If `self` is not empty and the last tree within the last stream is a
	// token tree marked with `Joint`...
	if let Some(TokenStream(ref mut last_stream_lrc)) = self.0.last_mut() {
	if let Some((TokenTree::Token(last_token), Spacing::Joint)) = last_stream_lrc.last() {
	// ...and `stream` is not empty and the first tree within it is
	// a token tree...
	let TokenStream(ref mut stream_lrc) = stream;
	if let Some((TokenTree::Token(token), spacing)) = stream_lrc.first() {
	// ...and the two tokens can be glued together...
	if let Some(glued_tok) = last_token.glue(&token) {
	// ...then do so, by overwriting the last token
	// tree in `self` and removing the first token tree
	// from `stream`. This requires using `make_mut()`
	// on the last stream in `self` and on `stream`,
	// and in practice this doesn't cause cloning 99.9%
	// of the time.

	// Overwrite the last token tree with the merged
	// token.
	let last_vec_mut = Lrc::make_mut(last_stream_lrc);
	last_vec_mut.last_mut().unwrap() = (TokenTree::Token(glued_tok), spacing);

	// Remove the first token tree from `stream`. (This
	// is almost always the only tree in `stream`.)
	let stream_vec_mut = Lrc::make_mut(stream_lrc);
	stream_vec_mut.remove(0);

	// Don't push `stream` if it's empty -- that could
	// block subsequent token gluing, by getting
	// between two token trees that should be glued
	// together.
	if !stream.is_empty() {
	self.0.push(stream);
	}
	return;
	}
	}
	}
	}
	self.0.push(stream);
	}

	pub fn build(self) -> TokenStream {
	TokenStream::from_streams(self.0)
	}
	}

	/// By-reference iterator over a [`TokenStream`].
	#[derive(Clone)]
	pub struct CursorRef<'t> {
	stream: &'t TokenStream,
	index: usize,
	}

	impl<'t> CursorRef<'t> {
	fn next_with_spacing(&mut self) -> Option<&'t TreeAndSpacing> {
	self.stream.0.get(self.index).map(\|tree\| {
	self.index += 1;
	tree
	})
	}
	}

	impl<'t> Iterator for CursorRef<'t> {
	type Item = &'t TokenTree;

	fn next(&mut self) -> Option<&'t TokenTree> {
	self.next_with_spacing().map(\|(tree, _)\| tree)
	}
	}

	/// Owning by-value iterator over a [`TokenStream`].
	// FIXME: Many uses of this can be replaced with by-reference iterator to avoid clones.
	#[derive(Clone)]
	pub struct Cursor {
	pub stream: TokenStream,
	index: usize,
	}

	impl Iterator for Cursor {
	type Item = TokenTree;

	fn next(&mut self) -> Option<TokenTree> {
	self.next_with_spacing().map(\|(tree, _)\| tree)
	}
	}

	impl Cursor {
	fn new(stream: TokenStream) -> Self {
	Cursor { stream, index: 0 }
	}

	pub fn next_with_spacing(&mut self) -> Option<TreeAndSpacing> {
	if self.index < self.stream.len() {
	self.index += 1;
	Some(self.stream.0[self.index - 1].clone())
	} else {
	None
	}
	}

	pub fn index(&self) -> usize {
	self.index
	}

	pub fn append(&mut self, new_stream: TokenStream) {
	if new_stream.is_empty() {
	return;
	}
	let index = self.index;
	let stream = mem::take(&mut self.stream);
	*self = TokenStream::from_streams(smallvec![stream, new_stream]).into_trees();
	self.index = index;
	}

	pub fn look_ahead(&self, n: usize) -> Option<&TokenTree> {
	self.stream.0[self.index..].get(n).map(\|(tree, _)\| tree)
	}
	}

	#[derive(Debug, Copy, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
	pub struct DelimSpan {
	pub open: Span,
	pub close: Span,
	}

	impl DelimSpan {
	pub fn from_single(sp: Span) -> Self {
	DelimSpan { open: sp, close: sp }
	}

	pub fn from_pair(open: Span, close: Span) -> Self {
	DelimSpan { open, close }
	}

	pub fn dummy() -> Self {
	Self::from_single(DUMMY_SP)
	}

	pub fn entire(self) -> Span {
	self.open.with_hi(self.close.hi())
	}
	}