Use-after-free checking at low runtime cost

The best that a run-time check for this sort of thing can do is turn one bug into a different kind of bug, at a considerable performance cost. While that can be useful for legacy programming languages like C (primarily as a debugging tool), it's simply the wrong approach for new languages. Modern programming language design should be focused on statically preventing bugs, and messing around with run-time checks is simply a waste of time.