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Auto merge of #88139 - lcnr:marker-trait-attr, r=nikomatsakis

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marker_traits: require `EvaluatedToOk` during winnowing

closes #84955, while it doesn't really fix it in a way that makes me happy it should prevent the issue for now and this
test can't be reproduced anyways, so it doesn't make much sense to keep it open.

fixes #84917 as only one of the impls depends on regions, so we now drop the ambiguous one instead of the correct one.

cc 247899832

r? `@nikomatsakis`
This commit is contained in:
bors 2021-08-22 10:44:38 +00:00
commit 1eb187c16e
6 changed files with 93 additions and 3 deletions
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13
compiler/rustc_trait_selection/src/traits/select/mod.rs
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@ -1586,12 +1586,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
// See if we can toss out `victim` based on specialization.
// This requires us to know *for sure* that the `other` impl applies
// i.e., `EvaluatedToOk`.
//
// FIXME(@lcnr): Using `modulo_regions` here seems kind of scary
// to me but is required for `std` to compile, so I didn't change it
// for now.
let tcx = self.tcx();
if other.evaluation.must_apply_modulo_regions() {
let tcx = self.tcx();
if tcx.specializes((other_def, victim_def)) {
return true;
}
return match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
}
if other.evaluation.must_apply_considering_regions() {
match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
Some(ty::ImplOverlapKind::Permitted { marker: true }) => {
// Subtle: If the predicate we are evaluating has inference
// variables, do *not* allow discarding candidates due to
@ -1636,7 +1643,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
}
Some(_) => true,
None => false,
};
}
} else {
false
}