Maybe allocator.dispose could nullify its parameter?

[Yuxuan Shui via Digitalmars-d]

Looks like a reasonable thing to do, e.g.:

auto p = Mallocator.instance.make!ulong(0);
Mallocator.instance.dispose(p);
assert(p is null);

Proof of concept:

void disposeX(Allocator, T)(auto ref Allocator a, ref B b) {
        a.dispose(b);
        b = null;
}