The test case src/test/ui/lint/must_not_suspend/ref.rs fails when -Zdrop-tracking is enabled.

We are not recognizing the Umm type as held across a borrow. Here's the revelant code from the test:

In a way, that's correct. We aren't holding Umm across the borrow, we are holding &mut Umm. The question is whether this should trigger the lint? If not, we need some way to annotate &mut Umm as also not being able to be held across the borrow.

It looks like in check_must_not_suspend_ty, we already recurse through a few types, so maybe we should look through references too?