fix: Check associated type bounds from supertraits for trait object well-formedness#153518
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…ormedness When forming a trait object like `dyn Sub<Assoc = String>` where `trait Sub: Super<Assoc: Copy>`, the compiler was not verifying that `String: Copy`. This allowed unsound code to compile, potentially leading to use-after-free vulnerabilities. The root cause was that both the old and new trait solvers used `explicit_super_predicates_of` which only yields `Self: SuperTrait` predicates, but not the associated type bounds from those supertraits. For example, `trait Sub: Super<Assoc: Copy>` expands to both `Self: Super` and `<Self as Super>::Assoc: Copy`, but only the first was being checked. This change switches to `explicit_implied_predicates_of` in both: - The old solver's `confirm_object_candidate` - The new solver's `predicates_for_object_candidate` This ensures that associated type bounds from supertraits are properly verified when constructing trait objects.
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Summary
Fixes #152607
When forming a trait object like
dyn Sub<Assoc = String>wheretrait Sub: Super<Assoc: Copy>, the compiler was not verifying thatString: Copy. This allowed unsound code to compile, potentially leading to use-after-free vulnerabilities.Root Cause
Both the old and new trait solvers used
explicit_super_predicates_ofwhich only yieldsSelf: SuperTraitpredicates (e.g.,Self: Super), but not the associated type bounds from those supertraits (e.g.,<Self as Super>::Assoc: Copy).The Fix
Changed both solvers to use
explicit_implied_predicates_ofinstead, which yields both supertrait predicates AND their associated type bounds:compiler/rustc_trait_selection/src/traits/select/confirmation.rs)compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs)Test Plan
tests/ui/traits/object/supertrait-assoc-type-bounds.rsthat now correctly fails with "the trait boundString: Copyis not satisfied"tests/ui/traits/object/supertrait-assoc-type-bounds-pass.rsconfirming valid cases still compile