> mem::forget isn’t the only way you can safely leak a value, you can do it with reference cycles too, right? And there is no way for the compiler to detect that?

But there's an easy solution for that: you make the reference-counted smart pointers require their pointee type to be Forget. It will be like how Arc<T> doesn't implement Send unless <T: Sync>.

An alternative way to "forget" a value is to hand it off to another thread that then loops infinitely.

Could of course be plugged by saying `!Forget : !Send`, but wouldn't that preclude legitimate useful scenarios for `!Forget`?