Any generic data type with a private ArrayList(T) member is an example. Unless you also expect callers to manually run destructors for elements of a type's private members, and their private members, etc. And it's not just about destroying the ArrayList entirely. Any function which just removes elements from an ArrayList needs to know how to destroy elements or else pass the buck for half its purpose to the caller. When I call shrink on an ArrayList(ArrayList(i32)) I'm supposed to preloop over the shrunken-over elements and call deinit on them before I call shrink? When I call the function that removes elements satisfying a predicate I'm supposed to preloop over the elements and call deinit on the ones I'm about to remove? Obviously not.

I already mentioned that hack. All you're doing there is introducing the ad-hoc notion of a destructors as a method named deinit. Again, in order for that to work in generic code that convention needs to be blessed by the language.