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Clarify idx handling in sequences.

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By adding comments, and improving an assertion. I finally fully
understand this part!
This commit is contained in:
Nicholas Nethercote 2022-03-31 07:00:07 +11:00
parent 2e423c7fd0
commit 048bd67d51
1 changed files with 18 additions and 9 deletions
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27
compiler/rustc_expand/src/mbe/macro_parser.rs
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@ -122,7 +122,9 @@ struct MatcherPos<'tt> {
/// submatcher, this is just the contents of that submatcher.
tts: &'tt [TokenTree],
/// The "dot" position within the current submatcher, i.e. the index into `tts`.
/// The "dot" position within the current submatcher, i.e. the index into `tts`. Can go one or
/// two positions past the final elements in `tts` when dealing with sequences, see
/// `parse_tt_inner` for details.
idx: usize,
/// This vector ends up with one element per metavar in the *top-level* matcher, even when this
@ -540,12 +542,17 @@ impl<'tt> TtParser<'tt> {
}
} else if let Some(sequence) = &mp.sequence {
// We are past the end of a sequence.
debug_assert!(idx <= len + 1);
// - If it has no separator, we must be only one past the end.
// - If it has a separator, we may be one past the end, in which case we must
// look for a separator. Or we may be two past the end, in which case we have
// already dealt with the separator.
debug_assert!(idx == len || idx == len + 1 && sequence.seq.separator.is_some());
if idx == len {
// Add all matches from the sequence to `parent`, and move the "dot" past the
// sequence in `parent`. This allows for the case where the sequence matching
// is finished.
// Sequence matching may have finished: move the "dot" past the sequence in
// `parent`. This applies whether a separator is used or not. If sequence
// matching hasn't finished, this `new_mp` will fail quietly when it is
// processed next time around the loop.
let mut new_mp = sequence.parent.clone();
new_mp.matches = mp.matches.clone();
new_mp.match_cur = mp.match_cur;
@ -553,7 +560,8 @@ impl<'tt> TtParser<'tt> {
self.cur_mps.push(new_mp);
}
if idx == len && sequence.seq.separator.is_some() {
if sequence.seq.separator.is_some() && idx == len {
// Look for the separator.
if sequence
.seq
.separator
@ -566,9 +574,10 @@ impl<'tt> TtParser<'tt> {
self.next_mps.push(mp);
}
} else if sequence.seq.kleene.op != mbe::KleeneOp::ZeroOrOne {
// We don't need a separator. Move the "dot" back to the beginning of the
// matcher and try to match again UNLESS we are only allowed to have _one_
// repetition.
// We don't need to look for a separator: either this sequence doesn't have
// one, or it does and we've already handled it. Also, we are allowed to have
// more than one repetition. Move the "dot" back to the beginning of the
// matcher and try to match again.
mp.match_cur -= sequence.seq.num_captures;
mp.idx = 0;
self.cur_mps.push(mp);