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bail out of selection when there are multiple surviving candidates

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In some cases (e.g. <[int-var] as Add<[int-var]>>), selection can turn up
a large number of candidates. Bailing out early avoids O(n^2) performance.

This improves item-type checking time by quite a bit, resulting in ~2% of total
time-to-typeck.
This commit is contained in:
Ariel Ben-Yehuda 2017-04-22 15:05:18 +03:00 • committed by Ariel Ben-Yehuda
parent a0f145ba8c
commit a660ad84b3
1 changed files with 7 additions and 7 deletions
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14
src/librustc/traits/select.rs
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@ -943,17 +943,17 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
debug!("Retaining candidate #{}/{}: {:?}", debug!("Retaining candidate #{}/{}: {:?}",
i, candidates.len(), candidates[i]); i, candidates.len(), candidates[i]);
i += 1; i += 1;
// If there are *STILL* multiple candidates, give up
// and report ambiguity.
if i > 1 {
debug!("multiple matches, ambig");
return Ok(None);
}
} }
} }
} }
// If there are *STILL* multiple candidates, give up and
// report ambiguity.
if candidates.len() > 1 {
debug!("multiple matches, ambig");
return Ok(None);
}
// If there are *NO* candidates, then there are no impls -- // If there are *NO* candidates, then there are no impls --
// that we know of, anyway. Note that in the case where there // that we know of, anyway. Note that in the case where there
// are unbound type variables within the obligation, it might // are unbound type variables within the obligation, it might