trait_selection: project.rs todo! ICE on UnsafeBinder for DiscriminantKind/Pointee

[SebTardif]

Bug

Classic trait projection (assemble_candidates_from_impls in rustc_trait_selection) still has todo!("FIXME(unsafe_binder)") for ty::UnsafeBinder when assembling built-in candidates for:

1. DiscriminantKind (known-discriminant match arm)
2. Pointee / metadata (struct-tail match arm)

These are separate from the new solver path in #584 / PR #587 (rustc_next_trait_solver normalizes_to), which has its own todo! arms.

Affected code

compiler/rustc_trait_selection/src/traits/project.rs (two sites)

Impact

ICE when projecting DiscriminantKind or Pointee metadata for unsafe-binder types on the classic selection/projection path.

Origin

Lines	Commit	Date	Author
DiscriminantKind / Pointee arms	9a1c5eb5b385	2024-12-21	Michael Goulet

Suggested fix

Delegate to the erased inner type (same pattern as discriminant_ty / PR #565 for pointer_kind and ptr_metadata_ty_or_tail): instantiate bound regions with erased, then treat like the inner type for candidate assembly.

Related

• trait_solver: normalizes_to todo! on UnsafeBinder for Pointee/DiscriminantKind #584 / PR trait_solver: handle UnsafeBinder in Pointee/Discriminant normalizes_to #587 (new-solver normalizes_to only)
• hir_typeck: pointer_kind todo!() ICE on ty::UnsafeBinder #525 / PR ty: handle UnsafeBinder in pointer_kind, metadata, and projections #565 (includes the project.rs fix; filing this so the classic projection ICEs are not only implied by hir_typeck: pointer_kind todo!() ICE on ty::UnsafeBinder #525)
• tech-debt: remaining UnsafeBinder todo!/FIXME cluster (const_eval, mangling, rustc_public) #567

[SebTardif]

Fix already included in PR #565 (classic project.rs arms; distinct from #584 / PR #587 new-solver path). Updating that PR to Closes #610.