That general technique pushes the problem around, where an implementation error will still result in a use-after-free (where you access a dead/reused array elem

That general technique pushes the problem around, where an implementation error will still result in a use-after-free (where you access a dead/reused array element). This could be fixed by paying for a runtime version check. Also, because it uses a Vec, it doesn't have O(1) performance, either. This could also be fixed by using a different underlying data structure.