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Rollup merge of #92917 - jackh726:issue-91762-2, r=nikomatsakis

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Don't constrain projection predicates with inference vars in GAT substs

cc #91762

Not a fix, but a mitigation to prevent a backwards-compatible hazard where we normalize using a predicate only because it's the only one available, but shouldn't. This would constrain an inference variable which didn't really want. We already do this when selecting a projection candidate, which isn't always correct. But changing that is a problem for a different day.

Also found out that a suggestion for `await`ing a future was using the wrong substs.

r? ``@nikomatsakis``
This commit is contained in:
Matthias Krüger 2022-02-08 16:40:48 +01:00 • committed by GitHub
commit a11e2b1f75
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6 changed files with 60 additions and 3 deletions
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2
compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
View file

@ -2473,7 +2473,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
// `T`
substs: self.tcx.mk_substs_trait(
trait_pred.self_ty().skip_binder(),
self.fresh_substs_for_item(span, item_def_id),
&self.fresh_substs_for_item(span, item_def_id)[1..],
),
// `Future::Output`
item_def_id,

10
compiler/rustc_trait_selection/src/traits/project.rs
View file

@ -1073,6 +1073,16 @@ fn project<'cx, 'tcx>(
return Ok(Projected::Progress(Progress::error(selcx.tcx())));
}
// If the obligation contains any inference types or consts in associated
// type substs, then we don't assemble any candidates.
// This isn't really correct, but otherwise we can end up in a case where
// we constrain inference variables by selecting a single predicate, when
// we need to stay general. See issue #91762.
let (_, predicate_own_substs) = obligation.predicate.trait_ref_and_own_substs(selcx.tcx());
if predicate_own_substs.iter().any(|g| g.has_infer_types_or_consts()) {
return Err(ProjectionError::TooManyCandidates);
}
let mut candidates = ProjectionCandidateSet::None;
// Make sure that the following procedures are kept in order. ParamEnv