diff options
Diffstat (limited to 'compiler/rustc_hir_analysis')
| -rw-r--r-- | compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs | 30 |
1 files changed, 23 insertions, 7 deletions
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs index 88f74589204..f6e5149bd2b 100644 --- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs +++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs @@ -172,7 +172,14 @@ 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); @@ -197,16 +204,13 @@ 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. .filter(|item| item.is_type()) // No RPITITs -- they're not dyn-compatible for now. .filter(|item| !item.is_impl_trait_in_trait()) - // If the associated type has a `where Self: Sized` bound, - // we do not need to constrain the associated type. - .filter(|item| !tcx.generics_require_sized_self(item.def_id)) .map(|item| (item.def_id, trait_ref)), ); } @@ -278,14 +282,26 @@ 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 { - missing_assoc_types.insert(key); + // 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); + } None } }) |