src/libsyntax/ext/tt/quoted.rs - toolchain/rustc - Git at Google

 use crate::ast;
 use crate::ast::NodeId;
 use crate::ext::tt::macro_parser;
 use crate::feature_gate::Features;
 use crate::parse::token::{self, Token, TokenKind};
 use crate::parse::ParseSess;
 use crate::print::pprust;
 use crate::symbol::kw;
 use crate::tokenstream::{self, DelimSpan};

 use syntax_pos::{edition::Edition, BytePos, Span};

 use rustc_data_structures::sync::Lrc;
 use std::iter::Peekable;

 /// Contains the sub-token-trees of a "delimited" token tree, such as the contents of `(`. Note
 /// that the delimiter itself might be `NoDelim`.
 #[derive(Clone, PartialEq, RustcEncodable, RustcDecodable, Debug)]
 pub struct Delimited {
     pub delim: token::DelimToken,
     pub tts: Vec<TokenTree>,
 }

 impl Delimited {
     /// Returns a `self::TokenTree` with a `Span` corresponding to the opening delimiter.
     pub fn open_tt(&self, span: Span) -> TokenTree {
         let open_span = if span.is_dummy() {
             span
         } else {
             span.with_lo(span.lo() + BytePos(self.delim.len() as u32))
         };
         TokenTree::token(token::OpenDelim(self.delim), open_span)
     }

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

 #[derive(Clone, PartialEq, RustcEncodable, RustcDecodable, Debug)]
 pub struct SequenceRepetition {
     /// The sequence of token trees
     pub tts: Vec<TokenTree>,
     /// The optional separator
     pub separator: Option<Token>,
     /// Whether the sequence can be repeated zero (*), or one or more times (+)
     pub op: KleeneOp,
     /// The number of `Match`s that appear in the sequence (and subsequences)
     pub num_captures: usize,
 }

 /// A Kleene-style [repetition operator](http://en.wikipedia.org/wiki/Kleene_star)
 /// for token sequences.
 #[derive(Clone, PartialEq, RustcEncodable, RustcDecodable, Hash, Debug, Copy)]
 pub enum KleeneOp {
     /// Kleene star (`*`) for zero or more repetitions
     ZeroOrMore,
     /// Kleene plus (`+`) for one or more repetitions
     OneOrMore,
     /// Kleene optional (`?`) for zero or one reptitions
     ZeroOrOne,
 }

 /// Similar to `tokenstream::TokenTree`, except that `$i`, `$i:ident`, and `$(...)`
 /// are "first-class" token trees. Useful for parsing macros.
 #[derive(Debug, Clone, PartialEq, RustcEncodable, RustcDecodable)]
 pub enum TokenTree {
     Token(Token),
     Delimited(DelimSpan, Lrc<Delimited>),
     /// A kleene-style repetition sequence
     Sequence(DelimSpan, Lrc<SequenceRepetition>),
     /// e.g., `$var`
     MetaVar(Span, ast::Ident),
     /// e.g., `$var:expr`. This is only used in the left hand side of MBE macros.
     MetaVarDecl(
         Span,
         ast::Ident, /* name to bind */
         ast::Ident, /* kind of nonterminal */
     ),
 }

 impl TokenTree {
     /// Return the number of tokens in the tree.
     pub fn len(&self) -> usize {
         match *self {
             TokenTree::Delimited(_, ref delimed) => match delimed.delim {
                 token::NoDelim => delimed.tts.len(),
                 _ => delimed.tts.len() + 2,
             },
             TokenTree::Sequence(_, ref seq) => seq.tts.len(),
             _ => 0,
         }
     }

     /// Returns `true` if the given token tree contains no other tokens. This is vacuously true for
     /// single tokens or metavar/decls, but may be false for delimited trees or sequences.
     pub fn is_empty(&self) -> bool {
         match *self {
             TokenTree::Delimited(_, ref delimed) => match delimed.delim {
                 token::NoDelim => delimed.tts.is_empty(),
                 _ => false,
             },
             TokenTree::Sequence(_, ref seq) => seq.tts.is_empty(),
             _ => true,
         }
     }

     /// Gets the `index`-th sub-token-tree. This only makes sense for delimited trees and sequences.
     pub fn get_tt(&self, index: usize) -> TokenTree {
         match (self, index) {
             (&TokenTree::Delimited(_, ref delimed), _) if delimed.delim == token::NoDelim => {
                 delimed.tts[index].clone()
             }
             (&TokenTree::Delimited(span, ref delimed), _) => {
                 if index == 0 {
                     return delimed.open_tt(span.open);
                 }
                 if index == delimed.tts.len() + 1 {
                     return delimed.close_tt(span.close);
                 }
                 delimed.tts[index - 1].clone()
             }
             (&TokenTree::Sequence(_, ref seq), _) => seq.tts[index].clone(),
             _ => panic!("Cannot expand a token tree"),
         }
     }

     /// Retrieves the `TokenTree`'s span.
     pub fn span(&self) -> Span {
         match *self {
             TokenTree::Token(Token { span, .. })
             | TokenTree::MetaVar(span, _)
             | TokenTree::MetaVarDecl(span, _, _) => span,
             TokenTree::Delimited(span, _) | TokenTree::Sequence(span, _) => span.entire(),
         }
     }

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

 /// Takes a `tokenstream::TokenStream` and returns a `Vec<self::TokenTree>`. Specifically, this
 /// takes a generic `TokenStream`, such as is used in the rest of the compiler, and returns a
 /// collection of `TokenTree` for use in parsing a macro.
 ///
 /// # Parameters
 ///
 /// - `input`: a token stream to read from, the contents of which we are parsing.
 /// - `expect_matchers`: `parse` can be used to parse either the "patterns" or the "body" of a
 ///   macro. Both take roughly the same form _except_ that in a pattern, metavars are declared with
 ///   their "matcher" type. For example `$var:expr` or `$id:ident`. In this example, `expr` and
 ///   `ident` are "matchers". They are not present in the body of a macro rule -- just in the
 ///   pattern, so we pass a parameter to indicate whether to expect them or not.
 /// - `sess`: the parsing session. Any errors will be emitted to this session.
 /// - `features`, `attrs`: language feature flags and attributes so that we know whether to use
 ///   unstable features or not.
 /// - `edition`: which edition are we in.
 /// - `macro_node_id`: the NodeId of the macro we are parsing.
 ///
 /// # Returns
 ///
 /// A collection of `self::TokenTree`. There may also be some errors emitted to `sess`.
 pub fn parse(
     input: tokenstream::TokenStream,
     expect_matchers: bool,
     sess: &ParseSess,
     features: &Features,
     attrs: &[ast::Attribute],
     edition: Edition,
     macro_node_id: NodeId,
 ) -> Vec<TokenTree> {
     // Will contain the final collection of `self::TokenTree`
     let mut result = Vec::new();

     // For each token tree in `input`, parse the token into a `self::TokenTree`, consuming
     // additional trees if need be.
     let mut trees = input.trees().peekable();
     while let Some(tree) = trees.next() {
         // Given the parsed tree, if there is a metavar and we are expecting matchers, actually
         // parse out the matcher (i.e., in `$id:ident` this would parse the `:` and `ident`).
         let tree = parse_tree(
             tree,
             &mut trees,
             expect_matchers,
             sess,
             features,
             attrs,
             edition,
             macro_node_id,
         );
         match tree {
             TokenTree::MetaVar(start_sp, ident) if expect_matchers => {
                 let span = match trees.next() {
                     Some(tokenstream::TokenTree::Token(Token { kind: token::Colon, span })) => {
                         match trees.next() {
                             Some(tokenstream::TokenTree::Token(token)) => match token.ident() {
                                 Some((kind, _)) => {
                                     let span = token.span.with_lo(start_sp.lo());
                                     result.push(TokenTree::MetaVarDecl(span, ident, kind));
                                     continue;
                                 }
                                 _ => token.span,
                             },
                             tree => tree.as_ref().map(tokenstream::TokenTree::span).unwrap_or(span),
                         }
                     }
                     tree => tree.as_ref().map(tokenstream::TokenTree::span).unwrap_or(start_sp),
                 };
                 sess.missing_fragment_specifiers.borrow_mut().insert(span);
                 result.push(TokenTree::MetaVarDecl(span, ident, ast::Ident::invalid()));
             }

             // Not a metavar or no matchers allowed, so just return the tree
             _ => result.push(tree),
         }
     }
     result
 }

 /// Takes a `tokenstream::TokenTree` and returns a `self::TokenTree`. Specifically, this takes a
 /// generic `TokenTree`, such as is used in the rest of the compiler, and returns a `TokenTree`
 /// for use in parsing a macro.
 ///
 /// Converting the given tree may involve reading more tokens.
 ///
 /// # Parameters
 ///
 /// - `tree`: the tree we wish to convert.
 /// - `trees`: an iterator over trees. We may need to read more tokens from it in order to finish
 ///   converting `tree`
 /// - `expect_matchers`: same as for `parse` (see above).
 /// - `sess`: the parsing session. Any errors will be emitted to this session.
 /// - `features`, `attrs`: language feature flags and attributes so that we know whether to use
 ///   unstable features or not.
 fn parse_tree(
     tree: tokenstream::TokenTree,
     trees: &mut Peekable<impl Iterator<Item = tokenstream::TokenTree>>,
     expect_matchers: bool,
     sess: &ParseSess,
     features: &Features,
     attrs: &[ast::Attribute],
     edition: Edition,
     macro_node_id: NodeId,
 ) -> TokenTree {
     // Depending on what `tree` is, we could be parsing different parts of a macro
     match tree {
         // `tree` is a `$` token. Look at the next token in `trees`
         tokenstream::TokenTree::Token(Token { kind: token::Dollar, span }) => match trees.next() {
             // `tree` is followed by a delimited set of token trees. This indicates the beginning
             // of a repetition sequence in the macro (e.g. `$(pat)*`).
             Some(tokenstream::TokenTree::Delimited(span, delim, tts)) => {
                 // Must have `(` not `{` or `[`
                 if delim != token::Paren {
                     let tok = pprust::token_kind_to_string(&token::OpenDelim(delim));
                     let msg = format!("expected `(`, found `{}`", tok);
                     sess.span_diagnostic.span_err(span.entire(), &msg);
                 }
                 // Parse the contents of the sequence itself
                 let sequence = parse(
                     tts.into(),
                     expect_matchers,
                     sess,
                     features,
                     attrs,
                     edition,
                     macro_node_id,
                 );
                 // Get the Kleene operator and optional separator
                 let (separator, op) = parse_sep_and_kleene_op(trees, span.entire(), sess);
                 // Count the number of captured "names" (i.e., named metavars)
                 let name_captures = macro_parser::count_names(&sequence);
                 TokenTree::Sequence(
                     span,
                     Lrc::new(SequenceRepetition {
                         tts: sequence,
                         separator,
                         op,
                         num_captures: name_captures,
                     }),
                 )
             }

             // `tree` is followed by an `ident`. This could be `$meta_var` or the `$crate` special
             // metavariable that names the crate of the invocation.
             Some(tokenstream::TokenTree::Token(token)) if token.is_ident() => {
                 let (ident, is_raw) = token.ident().unwrap();
                 let span = ident.span.with_lo(span.lo());
                 if ident.name == kw::Crate && !is_raw {
                     TokenTree::token(token::Ident(kw::DollarCrate, is_raw), span)
                 } else {
                     TokenTree::MetaVar(span, ident)
                 }
             }

             // `tree` is followed by a random token. This is an error.
             Some(tokenstream::TokenTree::Token(token)) => {
                 let msg =
                     format!("expected identifier, found `{}`", pprust::token_to_string(&token),);
                 sess.span_diagnostic.span_err(token.span, &msg);
                 TokenTree::MetaVar(token.span, ast::Ident::invalid())
             }

             // There are no more tokens. Just return the `$` we already have.
             None => TokenTree::token(token::Dollar, span),
         },

         // `tree` is an arbitrary token. Keep it.
         tokenstream::TokenTree::Token(token) => TokenTree::Token(token),

         // `tree` is the beginning of a delimited set of tokens (e.g., `(` or `{`). We need to
         // descend into the delimited set and further parse it.
         tokenstream::TokenTree::Delimited(span, delim, tts) => TokenTree::Delimited(
             span,
             Lrc::new(Delimited {
                 delim,
                 tts: parse(
                     tts.into(),
                     expect_matchers,
                     sess,
                     features,
                     attrs,
                     edition,
                     macro_node_id,
                 ),
             }),
         ),
     }
 }

 /// Takes a token and returns `Some(KleeneOp)` if the token is `+` `*` or `?`. Otherwise, return
 /// `None`.
 fn kleene_op(token: &Token) -> Option<KleeneOp> {
     match token.kind {
         token::BinOp(token::Star) => Some(KleeneOp::ZeroOrMore),
         token::BinOp(token::Plus) => Some(KleeneOp::OneOrMore),
         token::Question => Some(KleeneOp::ZeroOrOne),
         _ => None,
     }
 }

 /// Parse the next token tree of the input looking for a KleeneOp. Returns
 ///
 /// - Ok(Ok((op, span))) if the next token tree is a KleeneOp
 /// - Ok(Err(tok, span)) if the next token tree is a token but not a KleeneOp
 /// - Err(span) if the next token tree is not a token
 fn parse_kleene_op(
     input: &mut impl Iterator<Item = tokenstream::TokenTree>,
     span: Span,
 ) -> Result<Result<(KleeneOp, Span), Token>, Span> {
     match input.next() {
         Some(tokenstream::TokenTree::Token(token)) => match kleene_op(&token) {
             Some(op) => Ok(Ok((op, token.span))),
             None => Ok(Err(token)),
         },
         tree => Err(tree.as_ref().map(tokenstream::TokenTree::span).unwrap_or(span)),
     }
 }

 /// Attempt to parse a single Kleene star, possibly with a separator.
 ///
 /// For example, in a pattern such as `$(a),*`, `a` is the pattern to be repeated, `,` is the
 /// separator, and `*` is the Kleene operator. This function is specifically concerned with parsing
 /// the last two tokens of such a pattern: namely, the optional separator and the Kleene operator
 /// itself. Note that here we are parsing the _macro_ itself, rather than trying to match some
 /// stream of tokens in an invocation of a macro.
 ///
 /// This function will take some input iterator `input` corresponding to `span` and a parsing
 /// session `sess`. If the next one (or possibly two) tokens in `input` correspond to a Kleene
 /// operator and separator, then a tuple with `(separator, KleeneOp)` is returned. Otherwise, an
 /// error with the appropriate span is emitted to `sess` and a dummy value is returned.
 fn parse_sep_and_kleene_op(
     input: &mut Peekable<impl Iterator<Item = tokenstream::TokenTree>>,
     span: Span,
     sess: &ParseSess,
 ) -> (Option<Token>, KleeneOp) {
     // We basically look at two token trees here, denoted as #1 and #2 below
     let span = match parse_kleene_op(input, span) {
         // #1 is a `?`, `+`, or `*` KleeneOp
         Ok(Ok((op, _))) => return (None, op),

         // #1 is a separator followed by #2, a KleeneOp
         Ok(Err(token)) => match parse_kleene_op(input, token.span) {
             // #2 is the `?` Kleene op, which does not take a separator (error)
             Ok(Ok((KleeneOp::ZeroOrOne, _))) => {
                 // Error!
                 sess.span_diagnostic.span_err(
                     token.span,
                     "the `?` macro repetition operator does not take a separator",
                 );

                 // Return a dummy
                 return (None, KleeneOp::ZeroOrMore);
             }

             // #2 is a KleeneOp :D
             Ok(Ok((op, _))) => return (Some(token), op),

             // #2 is a random token or not a token at all :(
             Ok(Err(Token { span, .. })) | Err(span) => span,
         },

         // #1 is not a token
         Err(span) => span,
     };

     // If we ever get to this point, we have experienced an "unexpected token" error
     sess.span_diagnostic.span_err(span, "expected one of: `*`, `+`, or `?`");

     // Return a dummy
     (None, KleeneOp::ZeroOrMore)
 }
	use crate::ast;
	use crate::ast::NodeId;
	use crate::ext::tt::macro_parser;
	use crate::feature_gate::Features;
	use crate::parse::token::{self, Token, TokenKind};
	use crate::parse::ParseSess;
	use crate::print::pprust;
	use crate::symbol::kw;
	use crate::tokenstream::{self, DelimSpan};

	use syntax_pos::{edition::Edition, BytePos, Span};

	use rustc_data_structures::sync::Lrc;
	use std::iter::Peekable;

	/// Contains the sub-token-trees of a "delimited" token tree, such as the contents of `(`. Note
	/// that the delimiter itself might be `NoDelim`.
	#[derive(Clone, PartialEq, RustcEncodable, RustcDecodable, Debug)]
	pub struct Delimited {
	pub delim: token::DelimToken,
	pub tts: Vec<TokenTree>,
	}

	impl Delimited {
	/// Returns a `self::TokenTree` with a `Span` corresponding to the opening delimiter.
	pub fn open_tt(&self, span: Span) -> TokenTree {
	let open_span = if span.is_dummy() {
	span
	} else {
	span.with_lo(span.lo() + BytePos(self.delim.len() as u32))
	};
	TokenTree::token(token::OpenDelim(self.delim), open_span)
	}

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

	#[derive(Clone, PartialEq, RustcEncodable, RustcDecodable, Debug)]
	pub struct SequenceRepetition {
	/// The sequence of token trees
	pub tts: Vec<TokenTree>,
	/// The optional separator
	pub separator: Option<Token>,
	/// Whether the sequence can be repeated zero (*), or one or more times (+)
	pub op: KleeneOp,
	/// The number of `Match`s that appear in the sequence (and subsequences)
	pub num_captures: usize,
	}

	/// A Kleene-style [repetition operator](http://en.wikipedia.org/wiki/Kleene_star)
	/// for token sequences.
	#[derive(Clone, PartialEq, RustcEncodable, RustcDecodable, Hash, Debug, Copy)]
	pub enum KleeneOp {
	/// Kleene star (`*`) for zero or more repetitions
	ZeroOrMore,
	/// Kleene plus (`+`) for one or more repetitions
	OneOrMore,
	/// Kleene optional (`?`) for zero or one reptitions
	ZeroOrOne,
	}

	/// Similar to `tokenstream::TokenTree`, except that `$i`, `$i:ident`, and `$(...)`
	/// are "first-class" token trees. Useful for parsing macros.
	#[derive(Debug, Clone, PartialEq, RustcEncodable, RustcDecodable)]
	pub enum TokenTree {
	Token(Token),
	Delimited(DelimSpan, Lrc<Delimited>),
	/// A kleene-style repetition sequence
	Sequence(DelimSpan, Lrc<SequenceRepetition>),
	/// e.g., `$var`
	MetaVar(Span, ast::Ident),
	/// e.g., `$var:expr`. This is only used in the left hand side of MBE macros.
	MetaVarDecl(
	Span,
	ast::Ident, /* name to bind */
	ast::Ident, /* kind of nonterminal */
	),
	}

	impl TokenTree {
	/// Return the number of tokens in the tree.
	pub fn len(&self) -> usize {
	match *self {
	TokenTree::Delimited(_, ref delimed) => match delimed.delim {
	token::NoDelim => delimed.tts.len(),
	_ => delimed.tts.len() + 2,
	},
	TokenTree::Sequence(_, ref seq) => seq.tts.len(),
	_ => 0,
	}
	}

	/// Returns `true` if the given token tree contains no other tokens. This is vacuously true for
	/// single tokens or metavar/decls, but may be false for delimited trees or sequences.
	pub fn is_empty(&self) -> bool {
	match *self {
	TokenTree::Delimited(_, ref delimed) => match delimed.delim {
	token::NoDelim => delimed.tts.is_empty(),
	_ => false,
	},
	TokenTree::Sequence(_, ref seq) => seq.tts.is_empty(),
	_ => true,
	}
	}

	/// Gets the `index`-th sub-token-tree. This only makes sense for delimited trees and sequences.
	pub fn get_tt(&self, index: usize) -> TokenTree {
	match (self, index) {
	(&TokenTree::Delimited(_, ref delimed), _) if delimed.delim == token::NoDelim => {
	delimed.tts[index].clone()
	}
	(&TokenTree::Delimited(span, ref delimed), _) => {
	if index == 0 {
	return delimed.open_tt(span.open);
	}
	if index == delimed.tts.len() + 1 {
	return delimed.close_tt(span.close);
	}
	delimed.tts[index - 1].clone()
	}
	(&TokenTree::Sequence(_, ref seq), _) => seq.tts[index].clone(),
	_ => panic!("Cannot expand a token tree"),
	}
	}

	/// Retrieves the `TokenTree`'s span.
	pub fn span(&self) -> Span {
	match *self {
	TokenTree::Token(Token { span, .. })
	\| TokenTree::MetaVar(span, _)
	\| TokenTree::MetaVarDecl(span, _, _) => span,
	TokenTree::Delimited(span, _) \| TokenTree::Sequence(span, _) => span.entire(),
	}
	}

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

	/// Takes a `tokenstream::TokenStream` and returns a `Vec<self::TokenTree>`. Specifically, this
	/// takes a generic `TokenStream`, such as is used in the rest of the compiler, and returns a
	/// collection of `TokenTree` for use in parsing a macro.
	///
	/// # Parameters
	///
	/// - `input`: a token stream to read from, the contents of which we are parsing.
	/// - `expect_matchers`: `parse` can be used to parse either the "patterns" or the "body" of a
	/// macro. Both take roughly the same form _except_ that in a pattern, metavars are declared with
	/// their "matcher" type. For example `$var:expr` or `$id:ident`. In this example, `expr` and
	/// `ident` are "matchers". They are not present in the body of a macro rule -- just in the
	/// pattern, so we pass a parameter to indicate whether to expect them or not.
	/// - `sess`: the parsing session. Any errors will be emitted to this session.
	/// - `features`, `attrs`: language feature flags and attributes so that we know whether to use
	/// unstable features or not.
	/// - `edition`: which edition are we in.
	/// - `macro_node_id`: the NodeId of the macro we are parsing.
	///
	/// # Returns
	///
	/// A collection of `self::TokenTree`. There may also be some errors emitted to `sess`.
	pub fn parse(
	input: tokenstream::TokenStream,
	expect_matchers: bool,
	sess: &ParseSess,
	features: &Features,
	attrs: &[ast::Attribute],
	edition: Edition,
	macro_node_id: NodeId,
	) -> Vec<TokenTree> {
	// Will contain the final collection of `self::TokenTree`
	let mut result = Vec::new();

	// For each token tree in `input`, parse the token into a `self::TokenTree`, consuming
	// additional trees if need be.
	let mut trees = input.trees().peekable();
	while let Some(tree) = trees.next() {
	// Given the parsed tree, if there is a metavar and we are expecting matchers, actually
	// parse out the matcher (i.e., in `$id:ident` this would parse the `:` and `ident`).
	let tree = parse_tree(
	tree,
	&mut trees,
	expect_matchers,
	sess,
	features,
	attrs,
	edition,
	macro_node_id,
	);
	match tree {
	TokenTree::MetaVar(start_sp, ident) if expect_matchers => {
	let span = match trees.next() {
	Some(tokenstream::TokenTree::Token(Token { kind: token::Colon, span })) => {
	match trees.next() {
	Some(tokenstream::TokenTree::Token(token)) => match token.ident() {
	Some((kind, _)) => {
	let span = token.span.with_lo(start_sp.lo());
	result.push(TokenTree::MetaVarDecl(span, ident, kind));
	continue;
	}
	_ => token.span,
	},
	tree => tree.as_ref().map(tokenstream::TokenTree::span).unwrap_or(span),
	}
	}
	tree => tree.as_ref().map(tokenstream::TokenTree::span).unwrap_or(start_sp),
	};
	sess.missing_fragment_specifiers.borrow_mut().insert(span);
	result.push(TokenTree::MetaVarDecl(span, ident, ast::Ident::invalid()));
	}

	// Not a metavar or no matchers allowed, so just return the tree
	_ => result.push(tree),
	}
	}
	result
	}

	/// Takes a `tokenstream::TokenTree` and returns a `self::TokenTree`. Specifically, this takes a
	/// generic `TokenTree`, such as is used in the rest of the compiler, and returns a `TokenTree`
	/// for use in parsing a macro.
	///
	/// Converting the given tree may involve reading more tokens.
	///
	/// # Parameters
	///
	/// - `tree`: the tree we wish to convert.
	/// - `trees`: an iterator over trees. We may need to read more tokens from it in order to finish
	/// converting `tree`
	/// - `expect_matchers`: same as for `parse` (see above).
	/// - `sess`: the parsing session. Any errors will be emitted to this session.
	/// - `features`, `attrs`: language feature flags and attributes so that we know whether to use
	/// unstable features or not.
	fn parse_tree(
	tree: tokenstream::TokenTree,
	trees: &mut Peekable<impl Iterator<Item = tokenstream::TokenTree>>,
	expect_matchers: bool,
	sess: &ParseSess,
	features: &Features,
	attrs: &[ast::Attribute],
	edition: Edition,
	macro_node_id: NodeId,
	) -> TokenTree {
	// Depending on what `tree` is, we could be parsing different parts of a macro
	match tree {
	// `tree` is a `$` token. Look at the next token in `trees`
	tokenstream::TokenTree::Token(Token { kind: token::Dollar, span }) => match trees.next() {
	// `tree` is followed by a delimited set of token trees. This indicates the beginning
	// of a repetition sequence in the macro (e.g. `$(pat)*`).
	Some(tokenstream::TokenTree::Delimited(span, delim, tts)) => {
	// Must have `(` not `{` or `[`
	if delim != token::Paren {
	let tok = pprust::token_kind_to_string(&token::OpenDelim(delim));
	let msg = format!("expected `(`, found `{}`", tok);
	sess.span_diagnostic.span_err(span.entire(), &msg);
	}
	// Parse the contents of the sequence itself
	let sequence = parse(
	tts.into(),
	expect_matchers,
	sess,
	features,
	attrs,
	edition,
	macro_node_id,
	);
	// Get the Kleene operator and optional separator
	let (separator, op) = parse_sep_and_kleene_op(trees, span.entire(), sess);
	// Count the number of captured "names" (i.e., named metavars)
	let name_captures = macro_parser::count_names(&sequence);
	TokenTree::Sequence(
	span,
	Lrc::new(SequenceRepetition {
	tts: sequence,
	separator,
	op,
	num_captures: name_captures,
	}),
	)
	}

	// `tree` is followed by an `ident`. This could be `$meta_var` or the `$crate` special
	// metavariable that names the crate of the invocation.
	Some(tokenstream::TokenTree::Token(token)) if token.is_ident() => {
	let (ident, is_raw) = token.ident().unwrap();
	let span = ident.span.with_lo(span.lo());
	if ident.name == kw::Crate && !is_raw {
	TokenTree::token(token::Ident(kw::DollarCrate, is_raw), span)
	} else {
	TokenTree::MetaVar(span, ident)
	}
	}

	// `tree` is followed by a random token. This is an error.
	Some(tokenstream::TokenTree::Token(token)) => {
	let msg =
	format!("expected identifier, found `{}`", pprust::token_to_string(&token),);
	sess.span_diagnostic.span_err(token.span, &msg);
	TokenTree::MetaVar(token.span, ast::Ident::invalid())
	}

	// There are no more tokens. Just return the `$` we already have.
	None => TokenTree::token(token::Dollar, span),
	},

	// `tree` is an arbitrary token. Keep it.
	tokenstream::TokenTree::Token(token) => TokenTree::Token(token),

	// `tree` is the beginning of a delimited set of tokens (e.g., `(` or `{`). We need to
	// descend into the delimited set and further parse it.
	tokenstream::TokenTree::Delimited(span, delim, tts) => TokenTree::Delimited(
	span,
	Lrc::new(Delimited {
	delim,
	tts: parse(
	tts.into(),
	expect_matchers,
	sess,
	features,
	attrs,
	edition,
	macro_node_id,
	),
	}),
	),
	}
	}

	/// Takes a token and returns `Some(KleeneOp)` if the token is `+` `*` or `?`. Otherwise, return
	/// `None`.
	fn kleene_op(token: &Token) -> Option<KleeneOp> {
	match token.kind {
	token::BinOp(token::Star) => Some(KleeneOp::ZeroOrMore),
	token::BinOp(token::Plus) => Some(KleeneOp::OneOrMore),
	token::Question => Some(KleeneOp::ZeroOrOne),
	_ => None,
	}
	}

	/// Parse the next token tree of the input looking for a KleeneOp. Returns
	///
	/// - Ok(Ok((op, span))) if the next token tree is a KleeneOp
	/// - Ok(Err(tok, span)) if the next token tree is a token but not a KleeneOp
	/// - Err(span) if the next token tree is not a token
	fn parse_kleene_op(
	input: &mut impl Iterator<Item = tokenstream::TokenTree>,
	span: Span,
	) -> Result<Result<(KleeneOp, Span), Token>, Span> {
	match input.next() {
	Some(tokenstream::TokenTree::Token(token)) => match kleene_op(&token) {
	Some(op) => Ok(Ok((op, token.span))),
	None => Ok(Err(token)),
	},
	tree => Err(tree.as_ref().map(tokenstream::TokenTree::span).unwrap_or(span)),
	}
	}

	/// Attempt to parse a single Kleene star, possibly with a separator.
	///
	/// For example, in a pattern such as `$(a),*`, `a` is the pattern to be repeated, `,` is the
	/// separator, and `*` is the Kleene operator. This function is specifically concerned with parsing
	/// the last two tokens of such a pattern: namely, the optional separator and the Kleene operator
	/// itself. Note that here we are parsing the _macro_ itself, rather than trying to match some
	/// stream of tokens in an invocation of a macro.
	///
	/// This function will take some input iterator `input` corresponding to `span` and a parsing
	/// session `sess`. If the next one (or possibly two) tokens in `input` correspond to a Kleene
	/// operator and separator, then a tuple with `(separator, KleeneOp)` is returned. Otherwise, an
	/// error with the appropriate span is emitted to `sess` and a dummy value is returned.
	fn parse_sep_and_kleene_op(
	input: &mut Peekable<impl Iterator<Item = tokenstream::TokenTree>>,
	span: Span,
	sess: &ParseSess,
	) -> (Option<Token>, KleeneOp) {
	// We basically look at two token trees here, denoted as #1 and #2 below
	let span = match parse_kleene_op(input, span) {
	// #1 is a `?`, `+`, or `*` KleeneOp
	Ok(Ok((op, _))) => return (None, op),

	// #1 is a separator followed by #2, a KleeneOp
	Ok(Err(token)) => match parse_kleene_op(input, token.span) {
	// #2 is the `?` Kleene op, which does not take a separator (error)
	Ok(Ok((KleeneOp::ZeroOrOne, _))) => {
	// Error!
	sess.span_diagnostic.span_err(
	token.span,
	"the `?` macro repetition operator does not take a separator",
	);

	// Return a dummy
	return (None, KleeneOp::ZeroOrMore);
	}

	// #2 is a KleeneOp :D
	Ok(Ok((op, _))) => return (Some(token), op),

	// #2 is a random token or not a token at all :(
	Ok(Err(Token { span, .. })) \| Err(span) => span,
	},

	// #1 is not a token
	Err(span) => span,
	};

	// If we ever get to this point, we have experienced an "unexpected token" error
	sess.span_diagnostic.span_err(span, "expected one of: `*`, `+`, or `?`");

	// Return a dummy
	(None, KleeneOp::ZeroOrMore)
	}