That Rust was in fact always unsound if it would cause problems to core::mem::drop(a); and the `become` call just drops things so it's the same.

Safe-but-undesirable outcomes are acceptable. For example maybe our tail call ends up reverting a database transaction and we wish it were otherwise. But if the code did compile but wasn't memory safe as a result of this new drop then it was always unsound and shouldn't have existed.

Just as the guts of some STL classes are very complicated in order to deliver the promised exception safety promises, the guts of unsafe Rust code are often tricky for similar reasons, you are mandated to deliver safety, it's not up to you to say "That's stupid, don't do that" either ensure it won't compile or safely cope.