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Fix macro parser quadratic complexity in small repeating groups #52145

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merged 2 commits into from
Jul 17, 2018
Merged

Fix macro parser quadratic complexity in small repeating groups #52145

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191e76c
fix perf issue in macro parser
ExpHP Jul 7, 2018
0467ae0
cleanup unnecessary else
ExpHP Jul 16, 2018
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7 changes: 6 additions & 1 deletion src/libsyntax/ext/tt/macro_parser.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -697,9 +697,14 @@ pub fn parse(
return Failure(parser.span, token::Eof);
}
}
// 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 {
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) {
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