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Rollup merge of #139421 - compiler-errors:upcast-no-principal-with-proj, r=oli-obk

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Fix trait upcasting to dyn type with no principal when there are projections

#126660 (which I had originally authored, lol) had a subtle bug that is the moral equivalent of #114036, which is that when upcasting from `dyn Principal<Projection = Ty> + AutoTrait` to `dyn AutoTrait`, we were dropping the trait ref for `Principal` but not its projections (if there were any).

With debug assertions enabled, this triggers the assertion I luckily added in a2a0cfe825, but even without debug assertions this is a logical bug since we had a dyn type with just a projection bound but no principal, so it caused a type mismatch.

This does not need an FCP because this should've been covered by the FCP in #126660, but we just weren't testing a case when casting from a `dyn` type with projections 😸

Fixes #139418

r? ````@oli-obk```` (or anyone)
This commit is contained in:
Stuart Cook 2025-04-08 20:55:09 +10:00 • committed by GitHub
commit 056756c7c4
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2 changed files with 37 additions and 14 deletions
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38
compiler/rustc_trait_selection/src/traits/select/confirmation.rs
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@ -1090,26 +1090,36 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
{
// See `assemble_candidates_for_unsizing` for more info.
// We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
let iter = data_a
.principal()
.filter(|_| {
// optionally drop the principal, if we're unsizing to no principal
data_b.principal().is_some()
})
.map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
.into_iter()
.chain(
let existential_predicates = if data_b.principal().is_some() {
tcx.mk_poly_existential_predicates_from_iter(
data_a
.projection_bounds()
.map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
.principal()
.map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
.into_iter()
.chain(
data_a
.projection_bounds()
.map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
)
.chain(
data_b
.auto_traits()
.map(ty::ExistentialPredicate::AutoTrait)
.map(ty::Binder::dummy),
),
)
.chain(
} else {
// If we're unsizing to a dyn type that has no principal, then drop
// the principal and projections from the type. We use the auto traits
// from the RHS type since as we noted that we've checked for auto
// trait compatibility during unsizing.
tcx.mk_poly_existential_predicates_from_iter(
data_b
.auto_traits()
.map(ty::ExistentialPredicate::AutoTrait)
.map(ty::Binder::dummy),
);
let existential_predicates = tcx.mk_poly_existential_predicates_from_iter(iter);
)
};
let source_trait = Ty::new_dynamic(tcx, existential_predicates, r_b, dyn_a);
// Require that the traits involved in this upcast are **equal**;