Passing ownership to another thread is not the same as forgetting/leaking.

The point of !Forget is ensuring that once the owner goes out of scope the destructor must be guaranteed to run. An infinite loop is not a problem, cause the new thread will never leave its scope. Ref-cycles are a problem, cause you can create a ref-cycle. Then the program flow leaves the scope which will run the drop on all RC's but not the drop on the inner type.