> 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>.

By doing the same as with `Sized`: Automatically including the `Forget` bound on generic parameters and letting methods that don't need to be able to forget them opt out. That way existing code continues to compile and existing unsafe code doesn't become unsound.