The whole argument that you always know when Zig allocates is just wrong, as this shows. They “fixed” the hidden allocation by making it explicit in the append function but now you can pass in the wrong allocator. Not great is it? It is just all so fragile and relying on convention rather than hard checks. People sometimes pursue a bad idea to incredible lengths and then everything else is justifiable based on that initial, bad idea that just causes much more pain than it solves.

If the array list has enough capacity, you don't know if it allocated or not.