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Short-circuit DefIdForest::intersection()

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If the forest is already empty, there is no point in intersecting
further.

Also handle the first element separately, so we don't compute an
unnecessary intersection between the full forest and the first
element, which is always equal to the first element.
This commit is contained in:
Nikita Popov 2018-12-22 12:41:17 +01:00
parent 9689ada838
commit f93cbf6170
1 changed files with 12 additions and 1 deletions
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13
src/librustc/ty/inhabitedness/def_id_forest.rs
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@ -74,10 +74,21 @@ impl<'a, 'gcx, 'tcx> DefIdForest {
iter: I) -> DefIdForest
where I: IntoIterator<Item=DefIdForest>
{
let mut ret = DefIdForest::full(tcx);
let mut iter = iter.into_iter();
let mut ret = if let Some(first) = iter.next() {
first
} else {
return DefIdForest::full(tcx);
};
let mut next_ret = SmallVec::new();
let mut old_ret: SmallVec<[DefId; 1]> = SmallVec::new();
for next_forest in iter {
// No need to continue if the intersection is already empty.
if ret.is_empty() {
break;
}
for id in ret.root_ids.drain() {
if next_forest.contains(tcx, id) {
next_ret.push(id);