Scrapping the guarantee that destructors will be called is tantamount to scrapping RAII. thread::scoped made the assumption that destructors were called before a borrowed value was returned, and it led to a memory-safety error. If you don't use unsafe, you won't get memory-safety errors, but you can get plenty of other nastiness.

The guarantee that destructors are called when values leave scope is fantastic. It means that one can leverage memory-safety into arbitrary-resource-acquisition-safety. It violates the principle of least astonishment in a brutal way if Rust's borrow checker can statically guarantee that a value no longer exists, but can't know whether or not its destructor has been called.