I have UI tests, but that's a kinda blunt weapon.
Ideally I'd be able to spawn some MIR from source and then assert certain properties hold before/after certain MIR passes.
Rustc has some nice infra for this: https://github.com/rust-lang/rust/blob/a7c37f9f0ee5e93396106e9de20f4a86b4154e63/tests/ui/async-await/async-await.rs#L1-L9