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Use is_global in candidate_should_be_dropped_in_favor_of

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This manifistated in #90195 with compiler being unable to keep
one candidate for a trait impl, if where is a global impl and more
than one trait bound in the where clause.

Before #87280 `candidate_should_be_dropped_in_favor_of` was using
`TypeFoldable::is_global()` that was enough to discard the two
`ParamCandidate`s. But #87280 changed it to use
`TypeFoldable::is_known_global()` instead, which is pessimistic, so
now the compiler drops the global impl instead (because
`is_known_global` is not sure) and then can't decide between the
two `ParamCandidate`s.

Switching it to use `is_global` again solves the issue.

Fixes #90195.
This commit is contained in:
Ilya Yanok 2021-10-28 12:49:46 +00:00
parent 4e0d3973fa
commit 6c61db4407
1 changed files with 1 additions and 1 deletions
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2
compiler/rustc_trait_selection/src/traits/select/mod.rs
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@ -1548,7 +1548,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
// the param_env so that it can be given the lowest priority. See
// #50825 for the motivation for this.
let is_global =
|cand: &ty::PolyTraitRef<'_>| cand.is_known_global() && !cand.has_late_bound_regions();
|cand: &ty::PolyTraitRef<'tcx>| cand.is_global(self.infcx.tcx) && !cand.has_late_bound_regions();
// (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
// and `DiscriminantKindCandidate` to anything else.