Rust tip: if you implement `Borrow<B>` and `Hash` for a type `Ty`, then for all `x: Ty`, `hash(x) == hash(x.borrow())` must be true. If this property does not hold, then objects of type `Ty` won't work properly with hashmaps.

I may have spent several hours debugging this yesterday :-(

sounds like a job for a borrow test macro or some such to assert the invariant