compiler/rustc_expand/src/mbe/quoted.rs - toolchain/rustc - Git at Google

 use crate::mbe::macro_parser::count_metavar_decls;
 use crate::mbe::{Delimited, KleeneOp, KleeneToken, MetaVarExpr, SequenceRepetition, TokenTree};

 use rustc_ast::token::{self, Delimiter, Token};
 use rustc_ast::{tokenstream, NodeId};
 use rustc_ast_pretty::pprust;
 use rustc_feature::Features;
 use rustc_session::parse::{feature_err, ParseSess};
 use rustc_span::symbol::{kw, sym, Ident};

 use rustc_span::edition::Edition;
 use rustc_span::Span;

 const VALID_FRAGMENT_NAMES_MSG: &str = "valid fragment specifiers are \
                                         `ident`, `block`, `stmt`, `expr`, `pat`, `ty`, `lifetime`, \
                                         `literal`, `path`, `meta`, `tt`, `item` and `vis`";

 /// 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.
 /// - `parsing_patterns`: `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.
 ///   - Metavariable expressions are only valid in the "body", not the "pattern".
 /// - `sess`: the parsing session. Any errors will be emitted to this session.
 /// - `node_id`: the NodeId of the macro we are parsing.
 /// - `features`: language features so we can do feature gating.
 ///
 /// # Returns
 ///
 /// A collection of `self::TokenTree`. There may also be some errors emitted to `sess`.
 pub(super) fn parse(
     input: &tokenstream::TokenStream,
     parsing_patterns: bool,
     sess: &ParseSess,
     node_id: NodeId,
     features: &Features,
     edition: Edition,
 ) -> 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();
     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, parsing_patterns, sess, node_id, features, edition);
         match tree {
             TokenTree::MetaVar(start_sp, ident) if parsing_patterns => {
                 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((frag, _)) => {
                                     let span = token.span.with_lo(start_sp.lo());

                                     let kind =
                                         token::NonterminalKind::from_symbol(frag.name, || {
                                             // FIXME(#85708) - once we properly decode a foreign
                                             // crate's `SyntaxContext::root`, then we can replace
                                             // this with just `span.edition()`. A
                                             // `SyntaxContext::root()` from the current crate will
                                             // have the edition of the current crate, and a
                                             // `SyntaxContext::root()` from a foreign crate will
                                             // have the edition of that crate (which we manually
                                             // retrieve via the `edition` parameter).
                                             if span.ctxt().is_root() {
                                                 edition
                                             } else {
                                                 span.edition()
                                             }
                                         })
                                         .unwrap_or_else(
                                             || {
                                                 let msg = format!(
                                                     "invalid fragment specifier `{}`",
                                                     frag.name
                                                 );
                                                 sess.span_diagnostic
                                                     .struct_span_err(span, msg)
                                                     .help(VALID_FRAGMENT_NAMES_MSG)
                                                     .emit();
                                                 token::NonterminalKind::Ident
                                             },
                                         );
                                     result.push(TokenTree::MetaVarDecl(span, ident, Some(kind)));
                                     continue;
                                 }
                                 _ => token.span,
                             },
                             tree => tree.map_or(span, tokenstream::TokenTree::span),
                         }
                     }
                     tree => tree.map_or(start_sp, tokenstream::TokenTree::span),
                 };

                 result.push(TokenTree::MetaVarDecl(span, ident, None));
             }

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

 /// Asks for the `macro_metavar_expr` feature if it is not already declared
 fn maybe_emit_macro_metavar_expr_feature(features: &Features, sess: &ParseSess, span: Span) {
     if !features.macro_metavar_expr {
         let msg = "meta-variable expressions are unstable";
         feature_err(&sess, sym::macro_metavar_expr, span, msg).emit();
     }
 }

 /// 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.
 /// - `outer_trees`: an iterator over trees. We may need to read more tokens from it in order to finish
 ///   converting `tree`
 /// - `parsing_patterns`: same as [parse].
 /// - `sess`: the parsing session. Any errors will be emitted to this session.
 /// - `features`: language features so we can do feature gating.
 fn parse_tree<'a>(
     tree: &'a tokenstream::TokenTree,
     outer_trees: &mut impl Iterator<Item = &'a tokenstream::TokenTree>,
     parsing_patterns: bool,
     sess: &ParseSess,
     node_id: NodeId,
     features: &Features,
     edition: Edition,
 ) -> 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 }, _) => {
             // FIXME: Handle `Invisible`-delimited groups in a more systematic way
             // during parsing.
             let mut next = outer_trees.next();
             let mut trees: Box<dyn Iterator<Item = &tokenstream::TokenTree>>;
             if let Some(tokenstream::TokenTree::Delimited(_, Delimiter::Invisible, tts)) = next {
                 trees = Box::new(tts.trees());
                 next = trees.next();
             } else {
                 trees = Box::new(outer_trees);
             }

             match next {
                 // `tree` is followed by a delimited set of token trees.
                 Some(&tokenstream::TokenTree::Delimited(delim_span, delim, ref tts)) => {
                     if parsing_patterns {
                         if delim != Delimiter::Parenthesis {
                             span_dollar_dollar_or_metavar_in_the_lhs_err(
                                 sess,
                                 &Token { kind: token::OpenDelim(delim), span: delim_span.entire() },
                             );
                         }
                     } else {
                         match delim {
                             Delimiter::Brace => {
                                 // The delimiter is `{`. This indicates the beginning
                                 // of a meta-variable expression (e.g. `${count(ident)}`).
                                 // Try to parse the meta-variable expression.
                                 match MetaVarExpr::parse(&tts, delim_span.entire(), sess) {
                                     Err(mut err) => {
                                         err.emit();
                                         // Returns early the same read `$` to avoid spanning
                                         // unrelated diagnostics that could be performed afterwards
                                         return TokenTree::token(token::Dollar, span);
                                     }
                                     Ok(elem) => {
                                         maybe_emit_macro_metavar_expr_feature(
                                             features,
                                             sess,
                                             delim_span.entire(),
                                         );
                                         return TokenTree::MetaVarExpr(delim_span, elem);
                                     }
                                 }
                             }
                             Delimiter::Parenthesis => {}
                             _ => {
                                 let tok = pprust::token_kind_to_string(&token::OpenDelim(delim));
                                 let msg = format!("expected `(` or `{{`, found `{tok}`");
                                 sess.span_diagnostic.span_err(delim_span.entire(), msg);
                             }
                         }
                     }
                     // If we didn't find a metavar expression above, then we must have a
                     // repetition sequence in the macro (e.g. `$(pat)*`). Parse the
                     // contents of the sequence itself
                     let sequence = parse(tts, parsing_patterns, sess, node_id, features, edition);
                     // Get the Kleene operator and optional separator
                     let (separator, kleene) =
                         parse_sep_and_kleene_op(&mut trees, delim_span.entire(), sess);
                     // Count the number of captured "names" (i.e., named metavars)
                     let num_captures =
                         if parsing_patterns { count_metavar_decls(&sequence) } else { 0 };
                     TokenTree::Sequence(
                         delim_span,
                         SequenceRepetition { tts: sequence, separator, kleene, num_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 another `$`. This is an escaped `$`.
                 Some(&tokenstream::TokenTree::Token(Token { kind: token::Dollar, span }, _)) => {
                     if parsing_patterns {
                         span_dollar_dollar_or_metavar_in_the_lhs_err(
                             sess,
                             &Token { kind: token::Dollar, span },
                         );
                     } else {
                         maybe_emit_macro_metavar_expr_feature(features, sess, span);
                     }
                     TokenTree::token(token::Dollar, span)
                 }

                 // `tree` is followed by some other 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, Ident::empty())
                 }

                 // 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.clone()),

         // `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, ref tts) => TokenTree::Delimited(
             span,
             Delimited {
                 delim,
                 tts: parse(tts, parsing_patterns, sess, node_id, features, edition),
             },
         ),
     }
 }

 /// 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<'a>(
     input: &mut impl Iterator<Item = &'a 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.clone())),
         },
         tree => Err(tree.map_or(span, tokenstream::TokenTree::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<'a>(
     input: &mut impl Iterator<Item = &'a tokenstream::TokenTree>,
     span: Span,
     sess: &ParseSess,
 ) -> (Option<Token>, KleeneToken) {
     // 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, span))) => return (None, KleeneToken::new(op, span)),

         // #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, span))) => {
                 // Error!
                 sess.span_diagnostic.span_err(
                     token.span,
                     "the `?` macro repetition operator does not take a separator",
                 );

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

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

             // #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, KleeneToken::new(KleeneOp::ZeroOrMore, span))
 }

 // `$$` or a meta-variable is the lhs of a macro but shouldn't.
 //
 // For example, `macro_rules! foo { ( ${length()} ) => {} }`
 fn span_dollar_dollar_or_metavar_in_the_lhs_err(sess: &ParseSess, token: &Token) {
     sess.span_diagnostic
         .span_err(token.span, format!("unexpected token: {}", pprust::token_to_string(token)));
     sess.span_diagnostic.span_note_without_error(
         token.span,
         "`$$` and meta-variable expressions are not allowed inside macro parameter definitions",
     );
 }
	use crate::mbe::macro_parser::count_metavar_decls;
	use crate::mbe::{Delimited, KleeneOp, KleeneToken, MetaVarExpr, SequenceRepetition, TokenTree};

	use rustc_ast::token::{self, Delimiter, Token};
	use rustc_ast::{tokenstream, NodeId};
	use rustc_ast_pretty::pprust;
	use rustc_feature::Features;
	use rustc_session::parse::{feature_err, ParseSess};
	use rustc_span::symbol::{kw, sym, Ident};

	use rustc_span::edition::Edition;
	use rustc_span::Span;

	const VALID_FRAGMENT_NAMES_MSG: &str = "valid fragment specifiers are \
	`ident`, `block`, `stmt`, `expr`, `pat`, `ty`, `lifetime`, \
	`literal`, `path`, `meta`, `tt`, `item` and `vis`";

	/// 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.
	/// - `parsing_patterns`: `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.
	/// - Metavariable expressions are only valid in the "body", not the "pattern".
	/// - `sess`: the parsing session. Any errors will be emitted to this session.
	/// - `node_id`: the NodeId of the macro we are parsing.
	/// - `features`: language features so we can do feature gating.
	///
	/// # Returns
	///
	/// A collection of `self::TokenTree`. There may also be some errors emitted to `sess`.
	pub(super) fn parse(
	input: &tokenstream::TokenStream,
	parsing_patterns: bool,
	sess: &ParseSess,
	node_id: NodeId,
	features: &Features,
	edition: Edition,
	) -> 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();
	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, parsing_patterns, sess, node_id, features, edition);
	match tree {
	TokenTree::MetaVar(start_sp, ident) if parsing_patterns => {
	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((frag, _)) => {
	let span = token.span.with_lo(start_sp.lo());

	let kind =
	token::NonterminalKind::from_symbol(frag.name, \|\| {
	// FIXME(#85708) - once we properly decode a foreign
	// crate's `SyntaxContext::root`, then we can replace
	// this with just `span.edition()`. A
	// `SyntaxContext::root()` from the current crate will
	// have the edition of the current crate, and a
	// `SyntaxContext::root()` from a foreign crate will
	// have the edition of that crate (which we manually
	// retrieve via the `edition` parameter).
	if span.ctxt().is_root() {
	edition
	} else {
	span.edition()
	}
	})
	.unwrap_or_else(
	\|\| {
	let msg = format!(
	"invalid fragment specifier `{}`",
	frag.name
	);
	sess.span_diagnostic
	.struct_span_err(span, msg)
	.help(VALID_FRAGMENT_NAMES_MSG)
	.emit();
	token::NonterminalKind::Ident
	},
	);
	result.push(TokenTree::MetaVarDecl(span, ident, Some(kind)));
	continue;
	}
	_ => token.span,
	},
	tree => tree.map_or(span, tokenstream::TokenTree::span),
	}
	}
	tree => tree.map_or(start_sp, tokenstream::TokenTree::span),
	};

	result.push(TokenTree::MetaVarDecl(span, ident, None));
	}

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

	/// Asks for the `macro_metavar_expr` feature if it is not already declared
	fn maybe_emit_macro_metavar_expr_feature(features: &Features, sess: &ParseSess, span: Span) {
	if !features.macro_metavar_expr {
	let msg = "meta-variable expressions are unstable";
	feature_err(&sess, sym::macro_metavar_expr, span, msg).emit();
	}
	}

	/// 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.
	/// - `outer_trees`: an iterator over trees. We may need to read more tokens from it in order to finish
	/// converting `tree`
	/// - `parsing_patterns`: same as [parse].
	/// - `sess`: the parsing session. Any errors will be emitted to this session.
	/// - `features`: language features so we can do feature gating.
	fn parse_tree<'a>(
	tree: &'a tokenstream::TokenTree,
	outer_trees: &mut impl Iterator<Item = &'a tokenstream::TokenTree>,
	parsing_patterns: bool,
	sess: &ParseSess,
	node_id: NodeId,
	features: &Features,
	edition: Edition,
	) -> 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 }, _) => {
	// FIXME: Handle `Invisible`-delimited groups in a more systematic way
	// during parsing.
	let mut next = outer_trees.next();
	let mut trees: Box<dyn Iterator<Item = &tokenstream::TokenTree>>;
	if let Some(tokenstream::TokenTree::Delimited(_, Delimiter::Invisible, tts)) = next {
	trees = Box::new(tts.trees());
	next = trees.next();
	} else {
	trees = Box::new(outer_trees);
	}

	match next {
	// `tree` is followed by a delimited set of token trees.
	Some(&tokenstream::TokenTree::Delimited(delim_span, delim, ref tts)) => {
	if parsing_patterns {
	if delim != Delimiter::Parenthesis {
	span_dollar_dollar_or_metavar_in_the_lhs_err(
	sess,
	&Token { kind: token::OpenDelim(delim), span: delim_span.entire() },
	);
	}
	} else {
	match delim {
	Delimiter::Brace => {
	// The delimiter is `{`. This indicates the beginning
	// of a meta-variable expression (e.g. `${count(ident)}`).
	// Try to parse the meta-variable expression.
	match MetaVarExpr::parse(&tts, delim_span.entire(), sess) {
	Err(mut err) => {
	err.emit();
	// Returns early the same read `$` to avoid spanning
	// unrelated diagnostics that could be performed afterwards
	return TokenTree::token(token::Dollar, span);
	}
	Ok(elem) => {
	maybe_emit_macro_metavar_expr_feature(
	features,
	sess,
	delim_span.entire(),
	);
	return TokenTree::MetaVarExpr(delim_span, elem);
	}
	}
	}
	Delimiter::Parenthesis => {}
	_ => {
	let tok = pprust::token_kind_to_string(&token::OpenDelim(delim));
	let msg = format!("expected `(` or `{{`, found `{tok}`");
	sess.span_diagnostic.span_err(delim_span.entire(), msg);
	}
	}
	}
	// If we didn't find a metavar expression above, then we must have a
	// repetition sequence in the macro (e.g. `$(pat)*`). Parse the
	// contents of the sequence itself
	let sequence = parse(tts, parsing_patterns, sess, node_id, features, edition);
	// Get the Kleene operator and optional separator
	let (separator, kleene) =
	parse_sep_and_kleene_op(&mut trees, delim_span.entire(), sess);
	// Count the number of captured "names" (i.e., named metavars)
	let num_captures =
	if parsing_patterns { count_metavar_decls(&sequence) } else { 0 };
	TokenTree::Sequence(
	delim_span,
	SequenceRepetition { tts: sequence, separator, kleene, num_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 another `$`. This is an escaped `$`.
	Some(&tokenstream::TokenTree::Token(Token { kind: token::Dollar, span }, _)) => {
	if parsing_patterns {
	span_dollar_dollar_or_metavar_in_the_lhs_err(
	sess,
	&Token { kind: token::Dollar, span },
	);
	} else {
	maybe_emit_macro_metavar_expr_feature(features, sess, span);
	}
	TokenTree::token(token::Dollar, span)
	}

	// `tree` is followed by some other 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, Ident::empty())
	}

	// 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.clone()),

	// `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, ref tts) => TokenTree::Delimited(
	span,
	Delimited {
	delim,
	tts: parse(tts, parsing_patterns, sess, node_id, features, edition),
	},
	),
	}
	}

	/// 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<'a>(
	input: &mut impl Iterator<Item = &'a 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.clone())),
	},
	tree => Err(tree.map_or(span, tokenstream::TokenTree::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<'a>(
	input: &mut impl Iterator<Item = &'a tokenstream::TokenTree>,
	span: Span,
	sess: &ParseSess,
	) -> (Option<Token>, KleeneToken) {
	// 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, span))) => return (None, KleeneToken::new(op, span)),

	// #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, span))) => {
	// Error!
	sess.span_diagnostic.span_err(
	token.span,
	"the `?` macro repetition operator does not take a separator",
	);

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

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

	// #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, KleeneToken::new(KleeneOp::ZeroOrMore, span))
	}

	// `$$` or a meta-variable is the lhs of a macro but shouldn't.
	//
	// For example, `macro_rules! foo { ( ${length()} ) => {} }`
	fn span_dollar_dollar_or_metavar_in_the_lhs_err(sess: &ParseSess, token: &Token) {
	sess.span_diagnostic
	.span_err(token.span, format!("unexpected token: {}", pprust::token_to_string(token)));
	sess.span_diagnostic.span_note_without_error(
	token.span,
	"`$$` and meta-variable expressions are not allowed inside macro parameter definitions",
	);
	}