tech-debt: remaining UnsafeBinder todo!/FIXME cluster (const_eval, mangling, rustc_public)

[SebTardif]

Scope

PR for #525 addresses ICE-prone paths in pointer_kind, ptr_metadata_ty_or_tail, and trait projection for DiscriminantKind/Pointee.

Still open (grep FIXME(unsafe_binder) / todo! on UnsafeBinder):

Area	File
const_eval validity	compiler/rustc_const_eval/src/interpret/validity.rs
symbol mangling v0	compiler/rustc_symbol_mangling/src/v0.rs
rustc_public stable convert	compiler/rustc_public/src/unstable/convert/stable/ty.rs (+ mir Wrap/Unwrap)
trait select notes	compiler/rustc_trait_selection/src/traits/select/mod.rs, candidate_assembly.rs
normalizes_to pointee	compiler/rustc_next_trait_solver/src/solve/normalizes_to.rs
ty_utils consts	compiler/rustc_ty_utils/src/consts.rs

Track incremental removal as the unsafe_binder feature stabilizes paths. Not blocking normal compilation.

[SebTardif]

Incremental progress this session:

• ty: handle UnsafeBinder in pointer_kind, metadata, and projections #565 / PR ty: handle UnsafeBinder in pointer_kind, metadata, and projections #565: pointer_kind, ptr_metadata, is_trivially_not_async_drop, project DiscriminantKind/Pointee
• const_eval: validity todo!() ICE on ty::UnsafeBinder #570 / PR const_eval: treat UnsafeBinder as compound in validity checks #572: const_eval validity compound treatment
• rustc_public: CoroutineClosure types todo! in stable SMIR conversion #575 / PR rustc_public: lower CoroutineClosure types to SMIR #578: rustc_public CoroutineClosure SMIR (related async_closures gap)
• New: symbol_mangling v0 UnsafeBinder (issue/PR pair)
• New: ty_utils consts unsafe binder recurse (issue/PR pair)

Still open in cluster: rustc_public Wrap/UnwrapUnsafeBinder MIR convert, select/mod notes, normalizes_to pointee.

[SebTardif]

R73 incremental progress:

• rustc_public MIR Wrap/UnwrapUnsafeBinder (new issue/PR pair)
• normalizes_to Pointee/DiscriminantKind on UnsafeBinder (new issue/PR pair)
• rustc_public ty convert for UnsafeBinder (+ CoroutineClosure arm; may overlap rustc_public: CoroutineClosure types todo! in stable SMIR conversion #575/rustc_public: lower CoroutineClosure types to SMIR #578)

Remaining in cluster: trait select notes/FIXME comments (non-ICE), any other specialized solvers.