Coroutine state transform `check_must_not_suspend_ty` ignores allocator field of `Box`

[RalfJung]

This code here has a potential issue:

rust/compiler/rustc_mir_transform/src/coroutine.rs

Lines 1967 to 1977 in 010f394

	ty::Adt(..) if ty.is_box() => {
	let boxed_ty = ty.boxed_ty();
	let descr_pre = &format!("{}boxed ", data.descr_pre);
	check_must_not_suspend_ty(
	tcx,
	boxed_ty,
	hir_id,
	param_env,
	SuspendCheckData { descr_pre, ..data },
	)
	}

If the Box has a custom allocator, then that will not be checked at all. So whatever check_must_not_suspend_ty is used for, it ignores anything stored in custom allocators.

It seems like this is just for a lint, so this is probably not critical, but I guess should still be fixed.

Cc @cjgillot -- not sure whom else to ping for coroutine state machine transform things?

[RalfJung]

Cc @compiler-errors @tmiasko (also a regular occurrence in this file's git log)

[compiler-errors]

Lol sorry, I thought I had clicked enter on a comment but I must've not. Let me rewrite it.

It seems like this is just for a lint, so this is probably not critical, but I guess should still be fixed.

Yeah, this is approximately the level of urgency you determined. This code exists for an unstable lint, must_not_suspend. I don't expect people to be writing Box<T, MyAllocatorWhichShouldntBeHeldAcrossAwaitPoints>, but technically they could.