ReverseMapper::fold_ty missing CoroutineClosure arm; fold_const skips super_fold_with

[SebTardif]

Bug

In compiler/rustc_middle/src/ty/opaque_types.rs, ReverseMapper::fold_ty (line 146) handles Closure, Coroutine, and CoroutineWitness with fold_closure_args (which suppresses region-not-found errors for parent lifetimes), but CoroutineClosure (async closures, stable since Rust 1.85) falls through to the _ => ty.super_fold_with(self) catch-all. This calls fold_kind_normally on ALL args including parent args, causing spurious compilation errors for unused parent lifetimes.

Additionally, fold_const (line 192) returns non-Param consts unchanged (_ => ct) without calling ct.super_fold_with(self), so nested types and regions inside const expressions are not properly remapped.

Impact

Async closures inside opaque types (impl Trait) with unused enclosing lifetimes will hit a false "lifetime is part of concrete type but not used in parameter list" error.

Fix

PR #476

[SebTardif]

Upstream context and cross-reference

This file was created by @oli-obk in commit 9eb69e8 ("Move ReverseMapper logic onto OpaqueHiddenType", 2022-09-27). The fold_ty method originally handled Closure, Generator (renamed to Coroutine), and GeneratorWitness (renamed to CoroutineWitness), all using fold_closure_args to suppress region-not-found errors for parent lifetimes.

ty::CoroutineClosure was added to TyKind by @compiler-errors in commit c567edd ("Add CoroutineClosure to TyKind", PR rust-lang#120361), but ReverseMapper::fold_ty was not updated to handle it. The omission means CoroutineClosure falls through to _ => ty.super_fold_with(self), which applies fold_kind_normally to ALL args including parent args, producing spurious lifetime errors.

Separately, fold_const (also from @oli-obk's original commit 9eb69e8) returns non-Param consts unchanged (_ => ct) without calling ct.super_fold_with(self). Compare with fold_ty's catch-all which does call ty.super_fold_with(self). This means nested types/regions inside const expressions are not remapped.

Async closures were stabilized in Rust 1.85. Any async closure inside impl Trait with an unused enclosing lifetime could trigger the spurious error.

Fix with both corrections: PR #476