[dllthomas]

I don't see how the code assumes anything about the placement of the array. Indeed, it works just fine for static arrays:

    $ cat test.c
    #include <stdio.h>
    
    int arr[5];
    
    int main(int argc, char *argv[]) {
    	printf("%lu, %ld\n", sizeof(arr) / sizeof(*arr), (&arr)[1] - arr);
    }
    
    $ gcc test.c && ./a.out
    5, 5

Not saying it's "a good way to program" - it's needlessly obfuscated compared to the standard sizeof alternative. But it doesn't rely on anything tricky.