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typeck::check::coercion - roll back failed unsizing type vars

This wraps unsizing coercions within an additional level of
`commit_if_ok`, which rolls back type variables if the unsizing coercion
fails. This prevents a large amount of type-variables from accumulating
while type-checking a large function, e.g. shaving 2GB off one of the
4GB peaks in #36799.
This commit is contained in:
Ariel Ben-Yehuda 2017-09-21 17:54:39 +03:00
parent acb73dbe8b
commit 9d6b9d62ba

View file

@ -187,7 +187,11 @@ impl<'f, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
}
// Consider coercing the subtype to a DST
let unsize = self.coerce_unsized(a, b);
//
// NOTE: this is wrapped in a `commit_if_ok` because it creates
// a "spurious" type variable, and we don't want to have that
// type variable in memory if the coercion fails.
let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
if unsize.is_ok() {
debug!("coerce: unsize successful");
return unsize;