Running out of memory and killing the OS I would guess unless the OS kills misbehaving process first.

So pretty much completely safe, then?

Destructors do more than just free memory.

I've specifically said leaking memory? Then again, destructor that didn't run would be an application level error not "safety in rust" error.

You were responding to my comment, which had scope broader than just leaking memory. So, to suggest it is only about leaking memory is not really responsive.