Auto merge of #47752 - mark-i-m:at-most-once-rep, r=nikomatsakis

Implement `?` macro repetition

See rust-lang/rfcs#2298 (with disposition merge)

src/doc/unstable-book/src/language-features/macro-at-most-once-rep.md

@@ -0,0 +1,17 @@
+# `macro_at_most_once_rep`
+
+The tracking issue for this feature is: TODO(mark-i-m)
+
+With this feature gate enabled, one can use `?` as a Kleene operator meaning "0
+or 1 repetitions" in a macro definition. Previously only `+` and `*` were allowed.
+
+For example:
+```rust
+macro_rules! foo {
+ (something $(,)?) // `?` indicates `,` is "optional"...
+ => {}
+}
+```
+
+------------------------
+

src/libsyntax/ext/tt/macro_parser.rs

@@ -181,6 +181,8 @@ struct MatcherPos {
  match_hi: usize,
 
  // Specifically used if we are matching a repetition. If we aren't both should be `None`.
+ /// The KleeneOp of this sequence if we are in a repetition.
+ seq_op: Option<quoted::KleeneOp>,
  /// The separator if we are in a repetition
  sep: Option<Token>,
  /// The "parent" matcher position if we are in a repetition. That is, the matcher position just
@@ -263,6 +265,7 @@ fn initial_matcher_pos(ms: Vec<TokenTree>, lo: BytePos) -> Box<MatcherPos> {
  stack: vec![],
 
  // Haven't descended into any sequences, so both of these are `None`.
+ seq_op: None,
  sep: None,
  up: None,
  })
@@ -466,8 +469,8 @@ fn inner_parse_loop(
  }
  }
  // We don't need a separator. Move the "dot" back to the beginning of the matcher
- // and try to match again.
- else {
+ // and try to match again UNLESS we are only allowed to have _one_ repetition.
+ else if item.seq_op != Some(quoted::KleeneOp::ZeroOrOne) {
  item.match_cur = item.match_lo;
  item.idx = 0;
  cur_items.push(item);
@@ -486,8 +489,10 @@ fn inner_parse_loop(
  match item.top_elts.get_tt(idx) {
  // Need to descend into a sequence
  TokenTree::Sequence(sp, seq) => {
- if seq.op == quoted::KleeneOp::ZeroOrMore {
- // Examine the case where there are 0 matches of this sequence
+ // Examine the case where there are 0 matches of this sequence
+ if seq.op == quoted::KleeneOp::ZeroOrMore
+ || seq.op == quoted::KleeneOp::ZeroOrOne
+ {
  let mut new_item = item.clone();
  new_item.match_cur += seq.num_captures;
  new_item.idx += 1;
@@ -497,11 +502,11 @@ fn inner_parse_loop(
  cur_items.push(new_item);
  }
 
- // Examine the case where there is at least one match of this sequence
  let matches = create_matches(item.matches.len());
  cur_items.push(Box::new(MatcherPos {
  stack: vec![],
  sep: seq.separator.clone(),
+ seq_op: Some(seq.op),
  idx: 0,
  matches,
  match_lo: item.match_cur,

src/libsyntax/ext/tt/macro_rules.rs

@@ -237,7 +237,8 @@ pub fn compile(sess: &ParseSess, features: &RefCell<Features>, def: &ast::Item)
  s.iter().map(|m| {
  if let MatchedNonterminal(ref nt) = *m {
  if let NtTT(ref tt) = **nt {
- let tt = quoted::parse(tt.clone().into(), true, sess).pop().unwrap();
+ let tt = quoted::parse(tt.clone().into(), true, sess, features, &def.attrs)
+ .pop().unwrap();
  valid &= check_lhs_nt_follows(sess, features, &def.attrs, &tt);
  return tt;
  }
@@ -253,7 +254,8 @@ pub fn compile(sess: &ParseSess, features: &RefCell<Features>, def: &ast::Item)
  s.iter().map(|m| {
  if let MatchedNonterminal(ref nt) = *m {
  if let NtTT(ref tt) = **nt {
- return quoted::parse(tt.clone().into(), false, sess).pop().unwrap();
+ return quoted::parse(tt.clone().into(), false, sess, features, &def.attrs)
+ .pop().unwrap();
  }
  }
  sess.span_diagnostic.span_bug(def.span, "wrong-structured lhs")

src/libsyntax/ext/tt/quoted.rs

@@ -8,14 +8,17 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use ast;
+use {ast, attr};
 use ext::tt::macro_parser;
+use feature_gate::{self, emit_feature_err, Features, GateIssue};
 use parse::{token, ParseSess};
 use print::pprust;
 use symbol::keywords;
 use syntax_pos::{BytePos, Span, DUMMY_SP};
 use tokenstream;
 
+use std::cell::RefCell;
+use std::iter::Peekable;
 use std::rc::Rc;
 
 /// Contains the sub-token-trees of a "delimited" token tree, such as the contents of `(`. Note
@@ -78,6 +81,7 @@ pub enum KleeneOp {
  ZeroOrMore,
  /// Kleene plus (`+`) for one or more repetitions
  OneOrMore,
+ ZeroOrOne,
 }
 
 /// Similar to `tokenstream::TokenTree`, except that `$i`, `$i:ident`, and `$(...)`
@@ -169,6 +173,8 @@ impl TokenTree {
 /// `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.
 ///
 /// # Returns
 ///
@@ -177,18 +183,19 @@ pub fn parse(
  input: tokenstream::TokenStream,
  expect_matchers: bool,
  sess: &ParseSess,
+ features: &RefCell<Features>,
+ attrs: &[ast::Attribute],
 ) -> 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();
+ let mut trees = input.trees().peekable();
  while let Some(tree) = trees.next() {
- let tree = parse_tree(tree, &mut trees, expect_matchers, sess);
-
  // 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);
  match tree {
  TokenTree::MetaVar(start_sp, ident) if expect_matchers => {
  let span = match trees.next() {
@@ -237,11 +244,15 @@ pub fn parse(
 /// 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<I>(
  tree: tokenstream::TokenTree,
- trees: &mut I,
+ trees: &mut Peekable<I>,
  expect_matchers: bool,
  sess: &ParseSess,
+ features: &RefCell<Features>,
+ attrs: &[ast::Attribute],
 ) -> TokenTree
 where
  I: Iterator<Item = tokenstream::TokenTree>,
@@ -260,9 +271,9 @@ where
  sess.span_diagnostic.span_err(span, &msg);
  }
  // Parse the contents of the sequence itself
- let sequence = parse(delimited.tts.into(), expect_matchers, sess);
+ let sequence = parse(delimited.tts.into(), expect_matchers, sess, features, attrs);
  // Get the Kleene operator and optional separator
- let (separator, op) = parse_sep_and_kleene_op(trees, span, sess);
+ let (separator, op) = parse_sep_and_kleene_op(trees, span, sess, features, attrs);
  // Count the number of captured "names" (i.e. named metavars)
  let name_captures = macro_parser::count_names(&sequence);
  TokenTree::Sequence(
@@ -315,12 +326,46 @@ where
  span,
  Rc::new(Delimited {
  delim: delimited.delim,
- tts: parse(delimited.tts.into(), expect_matchers, sess),
+ tts: parse(delimited.tts.into(), expect_matchers, sess, features, attrs),
  }),
  ),
  }
 }
 
+/// Takes a token and returns `Some(KleeneOp)` if the token is `+` `*` or `?`. Otherwise, return
+/// `None`.
+fn kleene_op(token: &token::Token) -> Option<KleeneOp> {
+ match *token {
+ 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)) 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<I>(
+ input: &mut I,
+ span: Span,
+) -> Result<Result<KleeneOp, (token::Token, Span)>, Span>
+where
+ I: Iterator<Item = tokenstream::TokenTree>,
+{
+ match input.next() {
+ Some(tokenstream::TokenTree::Token(span, tok)) => match kleene_op(&tok) {
+ Some(op) => Ok(Ok(op)),
+ None => Ok(Err((tok, span))),
+ },
+ 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
@@ -334,55 +379,121 @@ where
 /// 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<I>(
- input: &mut I,
+ input: &mut Peekable<I>,
  span: Span,
  sess: &ParseSess,
+ features: &RefCell<Features>,
+ attrs: &[ast::Attribute],
 ) -> (Option<token::Token>, KleeneOp)
 where
  I: Iterator<Item = tokenstream::TokenTree>,
 {
- fn kleene_op(token: &token::Token) -> Option<KleeneOp> {
- match *token {
- token::BinOp(token::Star) => Some(KleeneOp::ZeroOrMore),
- token::BinOp(token::Plus) => Some(KleeneOp::OneOrMore),
- _ => None,
+ // 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
+ //
+ // `?` is ambiguous: it could be a separator or a Kleene::ZeroOrOne, so we need to look
+ // ahead one more token to be sure.
+ Ok(Ok(op)) if op != KleeneOp::ZeroOrOne => return (None, op),
+
+ // #1 is `?` token, but it could be a Kleene::ZeroOrOne without a separator or it could
+ // be a `?` separator followed by any Kleene operator. We need to look ahead 1 token to
+ // find out which.
+ Ok(Ok(op)) => {
+ assert_eq!(op, KleeneOp::ZeroOrOne);
+
+ // Lookahead at #2. If it is a KleenOp, then #1 is a separator.
+ let is_1_sep = if let Some(&tokenstream::TokenTree::Token(_, ref tok2)) = input.peek() {
+ kleene_op(tok2).is_some()
+ } else {
+ false
+ };
+
+ if is_1_sep {
+ // #1 is a separator and #2 should be a KleepeOp::*
+ // (N.B. We need to advance the input iterator.)
+ match parse_kleene_op(input, span) {
+ // #2 is a KleeneOp (this is the only valid option) :)
+ Ok(Ok(op)) if op == KleeneOp::ZeroOrOne => {
+ if !features.borrow().macro_at_most_once_rep
+ && !attr::contains_name(attrs, "allow_internal_unstable")
+ {
+ let explain = feature_gate::EXPLAIN_MACRO_AT_MOST_ONCE_REP;
+ emit_feature_err(
+ sess,
+ "macro_at_most_once_rep",
+ span,
+ GateIssue::Language,
+ explain,
+ );
+ }
+ return (Some(token::Question), op);
+ }
+ Ok(Ok(op)) => return (Some(token::Question), op),
+
+ // #2 is a random token (this is an error) :(
+ Ok(Err((_, span))) => span,
+
+ // #2 is not even a token at all :(
+ Err(span) => span,
+ }
+ } else {
+ if !features.borrow().macro_at_most_once_rep
+ && !attr::contains_name(attrs, "allow_internal_unstable")
+ {
+ let explain = feature_gate::EXPLAIN_MACRO_AT_MOST_ONCE_REP;
+ emit_feature_err(
+ sess,
+ "macro_at_most_once_rep",
+ span,
+ GateIssue::Language,
+ explain,
+ );
+ }
+
+ // #2 is a random tree and #1 is KleeneOp::ZeroOrOne
+ return (None, op);
+ }
  }
- }
 
- // We attempt to look at the next two token trees in `input`. I will call the first #1 and the
- // second #2. If #1 and #2 don't match a valid KleeneOp with/without separator, that is an
- // error, and we should emit an error on the most specific span possible.
- let span = match input.next() {
- // #1 is a token
- Some(tokenstream::TokenTree::Token(span, tok)) => match kleene_op(&tok) {
- // #1 is a KleeneOp with no separator
- Some(op) => return (None, op),
-
- // #1 is not a KleeneOp, but may be a separator... need to look at #2
- None => match input.next() {
- // #2 is a token
- Some(tokenstream::TokenTree::Token(span, tok2)) => match kleene_op(&tok2) {
- // #2 is a KleeneOp, so #1 must be a separator
- Some(op) => return (Some(tok), op),
-
-  // #2 is not a KleeneOp... error
-  None => span,
-  },
-
-  // #2 is not a token at all... error
-  tree => tree.as_ref()
- .map(tokenstream::TokenTree::span)
-  .unwrap_or(span),
- },
+  // #1 is a separator followed by #2, a KleeneOp
+  Ok(Err((tok, span))) => match parse_kleene_op(input, span) {
+  // #2 is a KleeneOp :D
+  Ok(Ok(op)) if op == KleeneOp::ZeroOrOne => {
+  if !features.borrow().macro_at_most_once_rep
+  && !attr::contains_name(attrs, "allow_internal_unstable")
+  {
+  let explain = feature_gate::EXPLAIN_MACRO_AT_MOST_ONCE_REP;
+ emit_feature_err(
+  sess,
+  "macro_at_most_once_rep",
+  span,
+  GateIssue::Language,
+  explain,
+ );
+ }
+ return (Some(tok), op);
+ }
+ Ok(Ok(op)) => return (Some(tok), op),
+
+ // #2 is a random token :(
+ Ok(Err((_, span))) => span,
+
+ // #2 is not a token at all :(
+ Err(span) => span,
  },
 
- // #1 is not a token at all... error
- tree => tree.as_ref()
- .map(tokenstream::TokenTree::span)
- .unwrap_or(span),
+ // #1 is not a token
+ Err(span) => span,
  };
 
- // Error...
- sess.span_diagnostic.span_err(span, "expected `*` or `+`");
+ if !features.borrow().macro_at_most_once_rep
+ && !attr::contains_name(attrs, "allow_internal_unstable")
+ {
+ sess.span_diagnostic
+ .span_err(span, "expected one of: `*`, `+`, or `?`");
+ } else {
+ sess.span_diagnostic.span_err(span, "expected `*` or `+`");
+ }
  (None, KleeneOp::ZeroOrMore)
 }