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fix perf issue in macro parser

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For a fuller description of the performance issue fixed by this:

https://github.com/rust-lang/rust/issues/51754#issuecomment-403242159
This commit is contained in:
Michael Lamparski 2018-07-07 17:11:20 -04:00
parent 9fd3d7899a
commit 191e76c5b8
1 changed files with 8 additions and 1 deletions
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9
src/libsyntax/ext/tt/macro_parser.rs
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@ -696,10 +696,17 @@ pub fn parse(
} else {
return Failure(parser.span, token::Eof);
}
} else {
// Performance hack: eof_items may share matchers via Rc with other things that we want
// to modify. Dropping eof_items now may drop these refcounts to 1, preventing an
// unnecessary implicit clone later in Rc::make_mut.
drop(eof_items);
}
// Another possibility is that we need to call out to parse some rust nonterminal
// (black-box) parser. However, if there is not EXACTLY ONE of these, something is wrong.
else if (!bb_items.is_empty() && !next_items.is_empty()) || bb_items.len() > 1 {
assert!(!token_name_eq(&parser.token, &token::Eof));
if (!bb_items.is_empty() && !next_items.is_empty()) || bb_items.len() > 1 {
let nts = bb_items
.iter()
.map(|item| match item.top_elts.get_tt(item.idx) {