fix: Detect E0804 when casting raw ptr-to-dyn adds auto traits#21699
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ShoyuVanilla merged 1 commit intorust-lang:masterfrom Feb 27, 2026
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the `check_ptr_ptr_cast` function had an early return when the source and
destination dyn trait objects shared the same principal trait def id:
if src_principal == dst_principal {
return Ok(());
}
this bypassed all three remaining checks that the code comment explicitly
listed as required: generic argument equality, projection equality, and
the auto-trait superset check. as a result, casts like
`*mut dyn Trait as *mut (dyn Trait + Send)` were silently accepted
instead of emitting E0804.
the fix removes the early return. `CastError::PtrPtrAddingAutoTraits` and
its diagnostic were already implemented — they just couldn't be reached.
the supertrait case (`trait Trait: Send`) continues to work correctly
because the auto-trait check already accounts for implied auto traits via
supertrait elaboration.
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@ChayimFriedman2 hey, can you check this out? |
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the
check_ptr_ptr_castfunction had an early return when the source and destination dyn trait objects shared the same principal trait def id:this bypassed all three remaining checks that the code comment explicitly listed as required: generic argument equality, projection equality and the auto-trait superset check. as a resultcasts like
*mut dyn Trait as *mut (dyn Trait + Send)were silently accepted instead of emitting e0804.the fix removes the early return.
CastError::PtrPtrAddingAutoTraitsand its diagnostic were already implemented they just couldnt be reached. the supertrait case (trait Trait: Send) continues to work correctly because the auto trait check already accounts for implied auto traits via supertrait elaboration.