string push_stuff(char[] buf, int x) {
    if (x == 1) {
        buf ~= 'A';
        buf ~= 'B';
        buf ~= 'C';
        return cast(string) buf[0 .. 3];
    }
    else {
        buf ~= 'A';
        buf ~= 'B';
        return cast(string) buf[0 .. 2];
    }
}

void foo() {
    {
        char[2] buf;
        string result = push_stuff(buf, 1);
        assert(buf.ptr != result.ptr);
    }

    {
        char[2] buf;
        string result = push_stuff(buf, 0);
        assert(buf.ptr == result.ptr); // <-- this assert fails
    }
}