That's entirely the point; he/she isn't expecting to call deinit on an i32, but is expecting to call deinit on an ArrayList(i32) if said ArrayList(i32) is in fact an element of an ArrayList(ArrayList(i32)). And calling deinit on an ArrayList(i32) is of course valid, nontrivial and necessary, since such a value needs to own a heap allocation that must be free()d.