vendor/rustc-ap-syntax/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 (eg stream) of `TokenTree`s,
 //! which are themselves a single `Token` or a `Delimited` subsequence of tokens.
 //!
 //! ## Ownership
 //!
 //! `TokenStreams` 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::ext::base;
 use crate::ext::tt::{macro_parser, quoted};
 use crate::parse::Directory;
 use crate::parse::token::{self, DelimToken, Token, TokenKind};
 use crate::print::pprust;

 use syntax_pos::{BytePos, Mark, Span, DUMMY_SP};
 #[cfg(target_arch = "x86_64")]
 use rustc_data_structures::static_assert_size;
 use rustc_data_structures::sync::Lrc;
 use serialize::{Decoder, Decodable, Encoder, Encodable};
 use smallvec::{SmallVec, smallvec};

 use std::borrow::Cow;
 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, RustcEncodable, RustcDecodable)]
 pub enum TokenTree {
     /// A single token
     Token(Token),
     /// A delimited sequence of token trees
     Delimited(DelimSpan, DelimToken, TokenStream),
 }

 // 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,
 {}

 // These are safe since we ensure that they hold for all fields in the `_dummy` function.
 //
 // These impls are only here because the compiler takes forever to compute the Send and Sync
 // bounds without them.
 // FIXME: Remove these impls when the compiler can compute the bounds quickly again.
 // See https://github.com/rust-lang/rust/issues/60846
 #[cfg(parallel_compiler)]
 unsafe impl Send for TokenTree {}
 #[cfg(parallel_compiler)]
 unsafe impl Sync for TokenTree {}

 impl TokenTree {
     /// Use this token tree as a matcher to parse given tts.
     pub fn parse(cx: &base::ExtCtxt<'_>, mtch: &[quoted::TokenTree], tts: TokenStream)
                  -> macro_parser::NamedParseResult {
         // `None` is because we're not interpolating
         let directory = Directory {
             path: Cow::from(cx.current_expansion.module.directory.as_path()),
             ownership: cx.current_expansion.directory_ownership,
         };
         macro_parser::parse(cx.parse_sess(), tts, mtch, Some(directory), true)
     }

     /// 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,
         }
     }

     // See comments in `Nonterminal::to_tokenstream` for why we care about
     // *probably* equal here rather than actual equality
     //
     // This is otherwise the same as `eq_unspanned`, only recursing with a
     // different method.
     pub fn probably_equal_for_proc_macro(&self, other: &TokenTree) -> bool {
         match (self, other) {
             (TokenTree::Token(token), TokenTree::Token(token2)) => {
                 token.probably_equal_for_proc_macro(token2)
             }
             (TokenTree::Delimited(_, delim, tts), TokenTree::Delimited(_, delim2, tts2)) => {
                 delim == delim2 && tts.probably_equal_for_proc_macro(&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),
         }
     }

     /// Indicates if the stream is a token that is equal to the provided token.
     pub fn eq_token(&self, t: TokenKind) -> bool {
         match self {
             TokenTree::Token(token) => *token == t,
             _ => false,
         }
     }

     pub fn joint(self) -> TokenStream {
         TokenStream::new(vec![(self, Joint)])
     }

     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: Span, delim: DelimToken) -> TokenTree {
         let open_span = if span.is_dummy() {
             span
         } else {
             span.with_hi(span.lo() + BytePos(delim.len() as u32))
         };
         TokenTree::token(token::OpenDelim(delim), open_span)
     }

     /// Returns the closing delimiter as a token tree.
     pub fn close_tt(span: Span, delim: DelimToken) -> TokenTree {
         let close_span = if span.is_dummy() {
             span
         } else {
             span.with_lo(span.hi() - BytePos(delim.len() as u32))
         };
         TokenTree::token(token::CloseDelim(delim), close_span)
     }
 }

 /// # Token Streams
 ///
 /// 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 back-compat.
 ///
 /// The use of `Option` is an optimization that avoids the need for an
 /// allocation when the stream is empty. However, it is not guaranteed that an
 /// empty stream is represented with `None`; it may be represented as a `Some`
 /// around an empty `Vec`.
 #[derive(Clone, Debug)]
 pub struct TokenStream(pub Option<Lrc<Vec<TreeAndJoint>>>);

 pub type TreeAndJoint = (TokenTree, IsJoint);

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

 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum IsJoint {
     Joint,
     NonJoint
 }

 use IsJoint::*;

 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(crate) fn add_comma(&self) -> Option<(TokenStream, Span)> {
         // Used to suggest if a user writes `foo!(a b);`
         if let Some(ref stream) = self.0 {
             let mut suggestion = None;
             let mut iter = stream.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), NonJoint),
                          (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, ..), NonJoint), _) => sp.entire(),
                         _ => continue,
                     };
                     let sp = sp.shrink_to_hi();
                     let comma = (TokenTree::token(token::Comma, sp), NonJoint);
                     suggestion = Some((pos, comma, sp));
                 }
             }
             if let Some((pos, comma, sp)) = suggestion {
                 let mut new_stream = vec![];
                 let parts = stream.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<TokenTree> for TokenStream {
     fn from(tree: TokenTree) -> TokenStream {
         TokenStream::new(vec![(tree, NonJoint)])
     }
 }

 impl From<TokenTree> for TreeAndJoint {
     fn from(tree: TokenTree) -> TreeAndJoint {
         (tree, NonJoint)
     }
 }

 impl<T: Into<TokenStream>> iter::FromIterator<T> for TokenStream {
     fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
         TokenStream::from_streams(iter.into_iter().map(Into::into).collect::<SmallVec<_>>())
     }
 }

 impl Eq for TokenStream {}

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

 impl TokenStream {
     pub fn len(&self) -> usize {
         if let Some(ref slice) = self.0 {
             slice.len()
         } else {
             0
         }
     }

     pub fn empty() -> TokenStream {
         TokenStream(None)
     }

     pub fn is_empty(&self) -> bool {
         match self.0 {
             None => true,
             Some(ref stream) => stream.is_empty(),
         }
     }

     pub(crate) fn from_streams(mut streams: SmallVec<[TokenStream; 2]>) -> TokenStream {
         match streams.len() {
             0 => TokenStream::empty(),
             1 => streams.pop().unwrap(),
             _ => {
                 // rust-lang/rust#57735: pre-allocate vector to avoid
                 // quadratic blow-up due to on-the-fly reallocations.
                 let tree_count = streams.iter()
                     .map(|ts| match &ts.0 { None => 0, Some(s) => s.len() })
                     .sum();
                 let mut vec = Vec::with_capacity(tree_count);

                 for stream in streams {
                     match stream.0 {
                         None => {},
                         Some(stream2) => vec.extend(stream2.iter().cloned()),
                     }
                 }
                 TokenStream::new(vec)
             }
         }
     }

     pub fn new(streams: Vec<TreeAndJoint>) -> TokenStream {
         match streams.len() {
             0 => TokenStream(None),
             _ => TokenStream(Some(Lrc::new(streams))),
         }
     }

     pub fn append_to_tree_and_joint_vec(self, vec: &mut Vec<TreeAndJoint>) {
         if let Some(stream) = self.0 {
             vec.extend(stream.iter().cloned());
         }
     }

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

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

     /// Compares two TokenStreams, 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 t1.by_ref().zip(t2.by_ref()) {
             if !t1.eq_unspanned(&t2) {
                 return false;
             }
         }
         t1.next().is_none() && t2.next().is_none()
     }

     // See comments in `Nonterminal::to_tokenstream` for why we care about
     // *probably* equal here rather than actual equality
     //
     // This is otherwise the same as `eq_unspanned`, only recursing with a
     // different method.
     pub fn probably_equal_for_proc_macro(&self, other: &TokenStream) -> bool {
         // When checking for `probably_eq`, we ignore certain tokens that aren't
         // preserved in the AST. Because they are not preserved, the pretty
         // printer arbitrarily adds or removes them when printing as token
         // streams, making a comparison between a token stream generated from an
         // AST and a token stream which was parsed into an AST more reliable.
         fn semantic_tree(tree: &TokenTree) -> bool {
             if let TokenTree::Token(token) = tree {
                 if let
                     // The pretty printer tends to add trailing commas to
                     // everything, and in particular, after struct fields.
                     | token::Comma
                     // The pretty printer emits `NoDelim` as whitespace.
                     | token::OpenDelim(DelimToken::NoDelim)
                     | token::CloseDelim(DelimToken::NoDelim)
                     // The pretty printer collapses many semicolons into one.
                     | token::Semi
                     // The pretty printer collapses whitespace arbitrarily and can
                     // introduce whitespace from `NoDelim`.
                     | token::Whitespace
                     // The pretty printer can turn `$crate` into `::crate_name`
                     | token::ModSep = token.kind {
                     return false;
                 }
             }
             true
         }

         let mut t1 = self.trees().filter(semantic_tree);
         let mut t2 = other.trees().filter(semantic_tree);
         for (t1, t2) in t1.by_ref().zip(t2.by_ref()) {
             if !t1.probably_equal_for_proc_macro(&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(self.0.map(|stream| {
             Lrc::new(
                 stream
                     .iter()
                     .enumerate()
                     .map(|(i, (tree, is_joint))| (f(i, tree.clone()), *is_joint))
                     .collect())
         }))
     }

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

     fn first_tree_and_joint(&self) -> Option<TreeAndJoint> {
         self.0.as_ref().map(|stream| {
             stream.first().unwrap().clone()
         })
     }

     fn last_tree_if_joint(&self) -> Option<TokenTree> {
         match self.0 {
             None => None,
             Some(ref stream) => {
                 if let (tree, Joint) = stream.last().unwrap() {
                     Some(tree.clone())
                 } else {
                     None
                 }
             }
         }
     }
 }

 // 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 stream = stream.into();
         let last_tree_if_joint = self.0.last().and_then(TokenStream::last_tree_if_joint);
         if let Some(TokenTree::Token(last_token)) = last_tree_if_joint {
             if let Some((TokenTree::Token(token), is_joint)) = stream.first_tree_and_joint() {
                 if let Some(glued_tok) = last_token.kind.glue(token.kind) {
                     let last_stream = self.0.pop().unwrap();
                     self.push_all_but_last_tree(&last_stream);
                     let glued_span = last_token.span.to(token.span);
                     let glued_tt = TokenTree::token(glued_tok, glued_span);
                     let glued_tokenstream = TokenStream::new(vec![(glued_tt, is_joint)]);
                     self.0.push(glued_tokenstream);
                     self.push_all_but_first_tree(&stream);
                     return
                 }
             }
         }
         self.0.push(stream);
     }

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

     fn push_all_but_last_tree(&mut self, stream: &TokenStream) {
         if let Some(ref streams) = stream.0 {
             let len = streams.len();
             match len {
                 1 => {}
                 _ => self.0.push(TokenStream(Some(Lrc::new(streams[0 .. len - 1].to_vec())))),
             }
         }
     }

     fn push_all_but_first_tree(&mut self, stream: &TokenStream) {
         if let Some(ref streams) = stream.0 {
             let len = streams.len();
             match len {
                 1 => {}
                 _ => self.0.push(TokenStream(Some(Lrc::new(streams[1 .. len].to_vec())))),
             }
         }
     }
 }

 #[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_joint().map(|(tree, _)| tree)
     }
 }

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

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

     pub fn append(&mut self, new_stream: TokenStream) {
         if new_stream.is_empty() {
             return;
         }
         let index = self.index;
         let stream = mem::replace(&mut self.stream, TokenStream(None));
         *self = TokenStream::from_streams(smallvec![stream, new_stream]).into_trees();
         self.index = index;
     }

     pub fn look_ahead(&self, n: usize) -> Option<TokenTree> {
         match self.stream.0 {
             None => None,
             Some(ref stream) => stream[self.index ..].get(n).map(|(tree, _)| tree.clone()),
         }
     }
 }

 impl fmt::Display for TokenStream {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_str(&pprust::tokens_to_string(self.clone()))
     }
 }

 impl Encodable for TokenStream {
     fn encode<E: Encoder>(&self, encoder: &mut E) -> Result<(), E::Error> {
         self.trees().collect::<Vec<_>>().encode(encoder)
     }
 }

 impl Decodable for TokenStream {
     fn decode<D: Decoder>(decoder: &mut D) -> Result<TokenStream, D::Error> {
         Vec::<TokenTree>::decode(decoder).map(|vec| vec.into_iter().collect())
     }
 }

 #[derive(Debug, Copy, Clone, PartialEq, RustcEncodable, RustcDecodable)]
 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())
     }

     pub fn apply_mark(self, mark: Mark) -> Self {
         DelimSpan {
             open: self.open.apply_mark(mark),
             close: self.close.apply_mark(mark),
         }
     }
 }

 #[cfg(test)]
 mod tests {
     use super::*;
     use crate::ast::Name;
     use crate::with_default_globals;
     use crate::util::parser_testing::string_to_stream;
     use syntax_pos::{Span, BytePos, NO_EXPANSION};

     fn string_to_ts(string: &str) -> TokenStream {
         string_to_stream(string.to_owned())
     }

     fn sp(a: u32, b: u32) -> Span {
         Span::new(BytePos(a), BytePos(b), NO_EXPANSION)
     }

     #[test]
     fn test_concat() {
         with_default_globals(|| {
             let test_res = string_to_ts("foo::bar::baz");
             let test_fst = string_to_ts("foo::bar");
             let test_snd = string_to_ts("::baz");
             let eq_res = TokenStream::from_streams(smallvec![test_fst, test_snd]);
             assert_eq!(test_res.trees().count(), 5);
             assert_eq!(eq_res.trees().count(), 5);
             assert_eq!(test_res.eq_unspanned(&eq_res), true);
         })
     }

     #[test]
     fn test_to_from_bijection() {
         with_default_globals(|| {
             let test_start = string_to_ts("foo::bar(baz)");
             let test_end = test_start.trees().collect();
             assert_eq!(test_start, test_end)
         })
     }

     #[test]
     fn test_eq_0() {
         with_default_globals(|| {
             let test_res = string_to_ts("foo");
             let test_eqs = string_to_ts("foo");
             assert_eq!(test_res, test_eqs)
         })
     }

     #[test]
     fn test_eq_1() {
         with_default_globals(|| {
             let test_res = string_to_ts("::bar::baz");
             let test_eqs = string_to_ts("::bar::baz");
             assert_eq!(test_res, test_eqs)
         })
     }

     #[test]
     fn test_eq_3() {
         with_default_globals(|| {
             let test_res = string_to_ts("");
             let test_eqs = string_to_ts("");
             assert_eq!(test_res, test_eqs)
         })
     }

     #[test]
     fn test_diseq_0() {
         with_default_globals(|| {
             let test_res = string_to_ts("::bar::baz");
             let test_eqs = string_to_ts("bar::baz");
             assert_eq!(test_res == test_eqs, false)
         })
     }

     #[test]
     fn test_diseq_1() {
         with_default_globals(|| {
             let test_res = string_to_ts("(bar,baz)");
             let test_eqs = string_to_ts("bar,baz");
             assert_eq!(test_res == test_eqs, false)
         })
     }

     #[test]
     fn test_is_empty() {
         with_default_globals(|| {
             let test0: TokenStream = Vec::<TokenTree>::new().into_iter().collect();
             let test1: TokenStream =
                 TokenTree::token(token::Ident(Name::intern("a"), false), sp(0, 1)).into();
             let test2 = string_to_ts("foo(bar::baz)");

             assert_eq!(test0.is_empty(), true);
             assert_eq!(test1.is_empty(), false);
             assert_eq!(test2.is_empty(), false);
         })
     }

     #[test]
     fn test_dotdotdot() {
         with_default_globals(|| {
             let mut builder = TokenStreamBuilder::new();
             builder.push(TokenTree::token(token::Dot, sp(0, 1)).joint());
             builder.push(TokenTree::token(token::Dot, sp(1, 2)).joint());
             builder.push(TokenTree::token(token::Dot, sp(2, 3)));
             let stream = builder.build();
             assert!(stream.eq_unspanned(&string_to_ts("...")));
             assert_eq!(stream.trees().count(), 1);
         })
     }
 }
	//! # Token Streams
	//!
	//! `TokenStream`s represent syntactic objects before they are converted into ASTs.
	//! A `TokenStream` is, roughly speaking, a sequence (eg stream) of `TokenTree`s,
	//! which are themselves a single `Token` or a `Delimited` subsequence of tokens.
	//!
	//! ## Ownership
	//!
	//! `TokenStreams` 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::ext::base;
	use crate::ext::tt::{macro_parser, quoted};
	use crate::parse::Directory;
	use crate::parse::token::{self, DelimToken, Token, TokenKind};
	use crate::print::pprust;

	use syntax_pos::{BytePos, Mark, Span, DUMMY_SP};
	#[cfg(target_arch = "x86_64")]
	use rustc_data_structures::static_assert_size;
	use rustc_data_structures::sync::Lrc;
	use serialize::{Decoder, Decodable, Encoder, Encodable};
	use smallvec::{SmallVec, smallvec};

	use std::borrow::Cow;
	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, RustcEncodable, RustcDecodable)]
	pub enum TokenTree {
	/// A single token
	Token(Token),
	/// A delimited sequence of token trees
	Delimited(DelimSpan, DelimToken, TokenStream),
	}

	// 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,
	{}

	// These are safe since we ensure that they hold for all fields in the `_dummy` function.
	//
	// These impls are only here because the compiler takes forever to compute the Send and Sync
	// bounds without them.
	// FIXME: Remove these impls when the compiler can compute the bounds quickly again.
	// See https://github.com/rust-lang/rust/issues/60846
	#[cfg(parallel_compiler)]
	unsafe impl Send for TokenTree {}
	#[cfg(parallel_compiler)]
	unsafe impl Sync for TokenTree {}

	impl TokenTree {
	/// Use this token tree as a matcher to parse given tts.
	pub fn parse(cx: &base::ExtCtxt<'_>, mtch: &[quoted::TokenTree], tts: TokenStream)
	-> macro_parser::NamedParseResult {
	// `None` is because we're not interpolating
	let directory = Directory {
	path: Cow::from(cx.current_expansion.module.directory.as_path()),
	ownership: cx.current_expansion.directory_ownership,
	};
	macro_parser::parse(cx.parse_sess(), tts, mtch, Some(directory), true)
	}

	/// 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,
	}
	}

	// See comments in `Nonterminal::to_tokenstream` for why we care about
	// probably equal here rather than actual equality
	//
	// This is otherwise the same as `eq_unspanned`, only recursing with a
	// different method.
	pub fn probably_equal_for_proc_macro(&self, other: &TokenTree) -> bool {
	match (self, other) {
	(TokenTree::Token(token), TokenTree::Token(token2)) => {
	token.probably_equal_for_proc_macro(token2)
	}
	(TokenTree::Delimited(_, delim, tts), TokenTree::Delimited(_, delim2, tts2)) => {
	delim == delim2 && tts.probably_equal_for_proc_macro(&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),
	}
	}

	/// Indicates if the stream is a token that is equal to the provided token.
	pub fn eq_token(&self, t: TokenKind) -> bool {
	match self {
	TokenTree::Token(token) => *token == t,
	_ => false,
	}
	}

	pub fn joint(self) -> TokenStream {
	TokenStream::new(vec![(self, Joint)])
	}

	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: Span, delim: DelimToken) -> TokenTree {
	let open_span = if span.is_dummy() {
	span
	} else {
	span.with_hi(span.lo() + BytePos(delim.len() as u32))
	};
	TokenTree::token(token::OpenDelim(delim), open_span)
	}

	/// Returns the closing delimiter as a token tree.
	pub fn close_tt(span: Span, delim: DelimToken) -> TokenTree {
	let close_span = if span.is_dummy() {
	span
	} else {
	span.with_lo(span.hi() - BytePos(delim.len() as u32))
	};
	TokenTree::token(token::CloseDelim(delim), close_span)
	}
	}

	/// # Token Streams
	///
	/// 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 back-compat.
	///
	/// The use of `Option` is an optimization that avoids the need for an
	/// allocation when the stream is empty. However, it is not guaranteed that an
	/// empty stream is represented with `None`; it may be represented as a `Some`
	/// around an empty `Vec`.
	#[derive(Clone, Debug)]
	pub struct TokenStream(pub Option<Lrc<Vec<TreeAndJoint>>>);

	pub type TreeAndJoint = (TokenTree, IsJoint);

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

	#[derive(Clone, Copy, Debug, PartialEq)]
	pub enum IsJoint {
	Joint,
	NonJoint
	}

	use IsJoint::*;

	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(crate) fn add_comma(&self) -> Option<(TokenStream, Span)> {
	// Used to suggest if a user writes `foo!(a b);`
	if let Some(ref stream) = self.0 {
	let mut suggestion = None;
	let mut iter = stream.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), NonJoint),
	(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, ..), NonJoint), _) => sp.entire(),
	_ => continue,
	};
	let sp = sp.shrink_to_hi();
	let comma = (TokenTree::token(token::Comma, sp), NonJoint);
	suggestion = Some((pos, comma, sp));
	}
	}
	if let Some((pos, comma, sp)) = suggestion {
	let mut new_stream = vec![];
	let parts = stream.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<TokenTree> for TokenStream {
	fn from(tree: TokenTree) -> TokenStream {
	TokenStream::new(vec![(tree, NonJoint)])
	}
	}

	impl From<TokenTree> for TreeAndJoint {
	fn from(tree: TokenTree) -> TreeAndJoint {
	(tree, NonJoint)
	}
	}

	impl<T: Into<TokenStream>> iter::FromIterator<T> for TokenStream {
	fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
	TokenStream::from_streams(iter.into_iter().map(Into::into).collect::<SmallVec<_>>())
	}
	}

	impl Eq for TokenStream {}

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

	impl TokenStream {
	pub fn len(&self) -> usize {
	if let Some(ref slice) = self.0 {
	slice.len()
	} else {
	0
	}
	}

	pub fn empty() -> TokenStream {
	TokenStream(None)
	}

	pub fn is_empty(&self) -> bool {
	match self.0 {
	None => true,
	Some(ref stream) => stream.is_empty(),
	}
	}

	pub(crate) fn from_streams(mut streams: SmallVec<[TokenStream; 2]>) -> TokenStream {
	match streams.len() {
	0 => TokenStream::empty(),
	1 => streams.pop().unwrap(),
	_ => {
	// rust-lang/rust#57735: pre-allocate vector to avoid
	// quadratic blow-up due to on-the-fly reallocations.
	let tree_count = streams.iter()
	.map(\|ts\| match &ts.0 { None => 0, Some(s) => s.len() })
	.sum();
	let mut vec = Vec::with_capacity(tree_count);

	for stream in streams {
	match stream.0 {
	None => {},
	Some(stream2) => vec.extend(stream2.iter().cloned()),
	}
	}
	TokenStream::new(vec)
	}
	}
	}

	pub fn new(streams: Vec<TreeAndJoint>) -> TokenStream {
	match streams.len() {
	0 => TokenStream(None),
	_ => TokenStream(Some(Lrc::new(streams))),
	}
	}

	pub fn append_to_tree_and_joint_vec(self, vec: &mut Vec<TreeAndJoint>) {
	if let Some(stream) = self.0 {
	vec.extend(stream.iter().cloned());
	}
	}

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

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

	/// Compares two TokenStreams, 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 t1.by_ref().zip(t2.by_ref()) {
	if !t1.eq_unspanned(&t2) {
	return false;
	}
	}
	t1.next().is_none() && t2.next().is_none()
	}

	// See comments in `Nonterminal::to_tokenstream` for why we care about
	// probably equal here rather than actual equality
	//
	// This is otherwise the same as `eq_unspanned`, only recursing with a
	// different method.
	pub fn probably_equal_for_proc_macro(&self, other: &TokenStream) -> bool {
	// When checking for `probably_eq`, we ignore certain tokens that aren't
	// preserved in the AST. Because they are not preserved, the pretty
	// printer arbitrarily adds or removes them when printing as token
	// streams, making a comparison between a token stream generated from an
	// AST and a token stream which was parsed into an AST more reliable.
	fn semantic_tree(tree: &TokenTree) -> bool {
	if let TokenTree::Token(token) = tree {
	if let
	// The pretty printer tends to add trailing commas to
	// everything, and in particular, after struct fields.
	\| token::Comma
	// The pretty printer emits `NoDelim` as whitespace.
	\| token::OpenDelim(DelimToken::NoDelim)
	\| token::CloseDelim(DelimToken::NoDelim)
	// The pretty printer collapses many semicolons into one.
	\| token::Semi
	// The pretty printer collapses whitespace arbitrarily and can
	// introduce whitespace from `NoDelim`.
	\| token::Whitespace
	// The pretty printer can turn `$crate` into `::crate_name`
	\| token::ModSep = token.kind {
	return false;
	}
	}
	true
	}

	let mut t1 = self.trees().filter(semantic_tree);
	let mut t2 = other.trees().filter(semantic_tree);
	for (t1, t2) in t1.by_ref().zip(t2.by_ref()) {
	if !t1.probably_equal_for_proc_macro(&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(self.0.map(\|stream\| {
	Lrc::new(
	stream
	.iter()
	.enumerate()
	.map(\|(i, (tree, is_joint))\| (f(i, tree.clone()), *is_joint))
	.collect())
	}))
	}

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

	fn first_tree_and_joint(&self) -> Option<TreeAndJoint> {
	self.0.as_ref().map(\|stream\| {
	stream.first().unwrap().clone()
	})
	}

	fn last_tree_if_joint(&self) -> Option<TokenTree> {
	match self.0 {
	None => None,
	Some(ref stream) => {
	if let (tree, Joint) = stream.last().unwrap() {
	Some(tree.clone())
	} else {
	None
	}
	}
	}
	}
	}

	// 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 stream = stream.into();
	let last_tree_if_joint = self.0.last().and_then(TokenStream::last_tree_if_joint);
	if let Some(TokenTree::Token(last_token)) = last_tree_if_joint {
	if let Some((TokenTree::Token(token), is_joint)) = stream.first_tree_and_joint() {
	if let Some(glued_tok) = last_token.kind.glue(token.kind) {
	let last_stream = self.0.pop().unwrap();
	self.push_all_but_last_tree(&last_stream);
	let glued_span = last_token.span.to(token.span);
	let glued_tt = TokenTree::token(glued_tok, glued_span);
	let glued_tokenstream = TokenStream::new(vec![(glued_tt, is_joint)]);
	self.0.push(glued_tokenstream);
	self.push_all_but_first_tree(&stream);
	return
	}
	}
	}
	self.0.push(stream);
	}

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

	fn push_all_but_last_tree(&mut self, stream: &TokenStream) {
	if let Some(ref streams) = stream.0 {
	let len = streams.len();
	match len {
	1 => {}
	_ => self.0.push(TokenStream(Some(Lrc::new(streams[0 .. len - 1].to_vec())))),
	}
	}
	}

	fn push_all_but_first_tree(&mut self, stream: &TokenStream) {
	if let Some(ref streams) = stream.0 {
	let len = streams.len();
	match len {
	1 => {}
	_ => self.0.push(TokenStream(Some(Lrc::new(streams[1 .. len].to_vec())))),
	}
	}
	}
	}

	#[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_joint().map(\|(tree, _)\| tree)
	}
	}

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

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

	pub fn append(&mut self, new_stream: TokenStream) {
	if new_stream.is_empty() {
	return;
	}
	let index = self.index;
	let stream = mem::replace(&mut self.stream, TokenStream(None));
	*self = TokenStream::from_streams(smallvec![stream, new_stream]).into_trees();
	self.index = index;
	}

	pub fn look_ahead(&self, n: usize) -> Option<TokenTree> {
	match self.stream.0 {
	None => None,
	Some(ref stream) => stream[self.index ..].get(n).map(\|(tree, _)\| tree.clone()),
	}
	}
	}

	impl fmt::Display for TokenStream {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	f.write_str(&pprust::tokens_to_string(self.clone()))
	}
	}

	impl Encodable for TokenStream {
	fn encode<E: Encoder>(&self, encoder: &mut E) -> Result<(), E::Error> {
	self.trees().collect::<Vec<_>>().encode(encoder)
	}
	}

	impl Decodable for TokenStream {
	fn decode<D: Decoder>(decoder: &mut D) -> Result<TokenStream, D::Error> {
	Vec::<TokenTree>::decode(decoder).map(\|vec\| vec.into_iter().collect())
	}
	}

	#[derive(Debug, Copy, Clone, PartialEq, RustcEncodable, RustcDecodable)]
	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())
	}

	pub fn apply_mark(self, mark: Mark) -> Self {
	DelimSpan {
	open: self.open.apply_mark(mark),
	close: self.close.apply_mark(mark),
	}
	}
	}

	#[cfg(test)]
	mod tests {
	use super::*;
	use crate::ast::Name;
	use crate::with_default_globals;
	use crate::util::parser_testing::string_to_stream;
	use syntax_pos::{Span, BytePos, NO_EXPANSION};

	fn string_to_ts(string: &str) -> TokenStream {
	string_to_stream(string.to_owned())
	}

	fn sp(a: u32, b: u32) -> Span {
	Span::new(BytePos(a), BytePos(b), NO_EXPANSION)
	}

	#[test]
	fn test_concat() {
	with_default_globals(\|\| {
	let test_res = string_to_ts("foo::bar::baz");
	let test_fst = string_to_ts("foo::bar");
	let test_snd = string_to_ts("::baz");
	let eq_res = TokenStream::from_streams(smallvec![test_fst, test_snd]);
	assert_eq!(test_res.trees().count(), 5);
	assert_eq!(eq_res.trees().count(), 5);
	assert_eq!(test_res.eq_unspanned(&eq_res), true);
	})
	}

	#[test]
	fn test_to_from_bijection() {
	with_default_globals(\|\| {
	let test_start = string_to_ts("foo::bar(baz)");
	let test_end = test_start.trees().collect();
	assert_eq!(test_start, test_end)
	})
	}

	#[test]
	fn test_eq_0() {
	with_default_globals(\|\| {
	let test_res = string_to_ts("foo");
	let test_eqs = string_to_ts("foo");
	assert_eq!(test_res, test_eqs)
	})
	}

	#[test]
	fn test_eq_1() {
	with_default_globals(\|\| {
	let test_res = string_to_ts("::bar::baz");
	let test_eqs = string_to_ts("::bar::baz");
	assert_eq!(test_res, test_eqs)
	})
	}

	#[test]
	fn test_eq_3() {
	with_default_globals(\|\| {
	let test_res = string_to_ts("");
	let test_eqs = string_to_ts("");
	assert_eq!(test_res, test_eqs)
	})
	}

	#[test]
	fn test_diseq_0() {
	with_default_globals(\|\| {
	let test_res = string_to_ts("::bar::baz");
	let test_eqs = string_to_ts("bar::baz");
	assert_eq!(test_res == test_eqs, false)
	})
	}

	#[test]
	fn test_diseq_1() {
	with_default_globals(\|\| {
	let test_res = string_to_ts("(bar,baz)");
	let test_eqs = string_to_ts("bar,baz");
	assert_eq!(test_res == test_eqs, false)
	})
	}

	#[test]
	fn test_is_empty() {
	with_default_globals(\|\| {
	let test0: TokenStream = Vec::<TokenTree>::new().into_iter().collect();
	let test1: TokenStream =
	TokenTree::token(token::Ident(Name::intern("a"), false), sp(0, 1)).into();
	let test2 = string_to_ts("foo(bar::baz)");

	assert_eq!(test0.is_empty(), true);
	assert_eq!(test1.is_empty(), false);
	assert_eq!(test2.is_empty(), false);
	})
	}

	#[test]
	fn test_dotdotdot() {
	with_default_globals(\|\| {
	let mut builder = TokenStreamBuilder::new();
	builder.push(TokenTree::token(token::Dot, sp(0, 1)).joint());
	builder.push(TokenTree::token(token::Dot, sp(1, 2)).joint());
	builder.push(TokenTree::token(token::Dot, sp(2, 3)));
	let stream = builder.build();
	assert!(stream.eq_unspanned(&string_to_ts("...")));
	assert_eq!(stream.trees().count(), 1);
	})
	}
	}