borrowck: is_closure_fn_mut ignores DefiningTy::CoroutineClosure

[SebTardif]

Summary

compiler/rustc_borrowck/src/diagnostics/region_errors.rs is_closure_fn_mut only inspects DefiningTy::Closure when the error region is a ClosureEnv late-param region. For async/coroutine-closure MIR (DefiningTy::CoroutineClosure), it always returns false even when args.as_coroutine_closure().kind() == FnMut.

That can change region-error wording/suggestions that branch on is_closure_fn_mut for coroutine closures.

Origin

Audit of incomplete CoroutineClosure handling in borrowck region diagnostics, SebTardif/rust fork of rust-lang/rust.

Affected code (upstream tip at audit time)

https://github.com/rust-lang/rust/blob/f28ac764c36/compiler/rustc_borrowck/src/diagnostics/region_errors.rs#L177-L188

&& let DefiningTy::Closure(_, args) = self.regioncx.universal_regions().defining_ty
{
    return args.as_closure().kind() == ty::ClosureKind::FnMut;
}

Suggested fix

Match DefiningTy::CoroutineClosure(_, args) and compare args.as_coroutine_closure().kind() to FnMut (same ClosureEnv late-param region shape as normal closures).

Impact

Low. Diagnostic quality for async/coroutine-closure region errors only.

Related

• diagnostics: missing CoroutineClosure in fulfillment span adjust and explain_borrow destructor #569 / borrowck: handle CoroutineClosure in describe_field and moved-closure notes #576 (other CoroutineClosure diagnostic gaps)