ty_utils: AsyncDropGlueCtor allowlist misses UnsafeBinder (and ty::Pat)

[SebTardif]

Bug

When resolving async_drop_in_place (LangItem::AsyncDropInPlace), nontrivial async drop glue requires the type kind to be in an allowlist. That list includes Closure, CoroutineClosure, Coroutine, Tuple, Adt, Dynamic, Array, and Slice, but not UnsafeBinder or ty::Pat.

Sync DropGlue resolution for the same structural cases does allow UnsafeBinder (and still omits Pat).

So for an UnsafeBinder type with needs_async_drop == true, instance resolution returns Ok(None) instead of building AsyncDropGlueCtor, while sync drop glue would accept the binder.

needs_drop_components / async drop component logic treats UnsafeBinder as a component type (Ok(smallvec![ty])) and peels Pat to its inner type, so the allowlist is inconsistent with the needs-drop decomposition.

Affected code

compiler/rustc_ty_utils/src/instance.rs (resolve_instance_raw, AsyncDropInPlace branch)

Impact

Wrong/missing async-drop instance for unsafe-binder types that need async drop (I-wrong), vs sync drop. Unstable async_drop / unsafe_binder interaction.

ty::Pat note (latent): Pattern types currently focus on integer ranges (Copy, typically no drop). When Pat can wrap drop-bearing inners, both DropGlue and AsyncDropGlueCtor omit Pat from the nontrivial allowlist while needs_drop can be true via peeled components. Prefer adding Pat alongside UnsafeBinder for consistency with Array/Slice compound handling, or document intentional deferral.

Origin

Lines	Commit	Date	Author
AsyncDropInPlace allowlist	80c0b7e90fd0 / async-drop shim work	2024-04-17	Daria Sukhonina
UnsafeBinder added to DropGlue only	later UnsafeBinder wiring

Suggested fix

Add | ty::UnsafeBinder(..) to the AsyncDropGlueCtor nontrivial match (mirror DropGlue). Optionally add | ty::Pat(..) to both DropGlue and AsyncDropGlueCtor allowlists for consistency with component peeling.

Related

• Sync DropGlue already has UnsafeBinder in the same file
• tech-debt: remaining UnsafeBinder todo!/FIXME cluster (const_eval, mangling, rustc_public) #567 UnsafeBinder cluster (different sites)