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Michael Goulet 2025-05-07 16:55:21 +00:00 • committed by Pietro Albini
parent 2f71d0a243
commit d800540c12
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2 changed files with 21 additions and 6 deletions
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25
compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs
View file

@ -146,7 +146,14 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
let principal_trait = regular_traits.into_iter().next();
let mut needed_associated_types = vec![];
// A stable ordering of associated types from the principal trait and all its
// supertraits. We use this to ensure that different substitutions of a trait
// don't result in `dyn Trait` types with different projections lists, which
// can be unsound: <https://github.com/rust-lang/rust/pull/136458>.
// We achieve a stable ordering by walking over the unsubstituted principal
// trait ref.
let mut ordered_associated_types = vec![];
if let Some((principal_trait, ref spans)) = principal_trait {
let principal_trait = principal_trait.map_bound(|trait_pred| {
assert_eq!(trait_pred.polarity, ty::PredicatePolarity::Positive);
@ -171,7 +178,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
// FIXME(negative_bounds): Handle this correctly...
let trait_ref =
tcx.anonymize_bound_vars(bound_predicate.rebind(pred.trait_ref));
needed_associated_types.extend(
ordered_associated_types.extend(
tcx.associated_items(pred.trait_ref.def_id)
.in_definition_order()
// We only care about associated types.
@ -249,15 +256,23 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
}
}
// We compute the list of projection bounds taking the ordered associated types,
// and check if there was an entry in the collected `projection_bounds`. Those
// are computed by first taking the user-written associated types, then elaborating
// the principal trait ref, and only using those if there was no user-written.
// See note below about how we handle missing associated types with `Self: Sized`,
// which are not required to be provided, but are still used if they are provided.
let mut missing_assoc_types = FxIndexSet::default();
let projection_bounds: Vec<_> = needed_associated_types
let projection_bounds: Vec<_> = ordered_associated_types
.into_iter()
.filter_map(|key| {
if let Some(assoc) = projection_bounds.get(&key) {
Some(*assoc)
} else {
// If the associated type has a `where Self: Sized` bound,
// we do not need to constrain the associated type.
// If the associated type has a `where Self: Sized` bound, then
// we do not need to provide the associated type. This results in
// a `dyn Trait` type that has a different number of projection
// bounds, which may lead to type mismatches.
if !tcx.generics_require_sized_self(key.0) {
missing_assoc_types.insert(key);
}

2
tests/ui/traits/object/constrain-via-unnecessary-bound.rs
View file

@ -13,7 +13,7 @@ pub trait Trait {
}
impl Other for dyn Trait {}
// `dyn Trait<Assoc = ()>` is a different "nominal type" than `dyn Traiat`.
// `dyn Trait<Assoc = ()>` is a different "nominal type" than `dyn Trait`.
impl Other for dyn Trait<Assoc = ()> {}
//~^ WARN unnecessary associated type bound for dyn-incompatible associated type