The reason the struct is avoided here is so the array can be typed to its element type (rather than casting to and from `void*`).

With a struct we would need one struct for each element type - at least prior to C23 which provides a better approach where we can declare the same struct multiple times in a translation unit.

    #define Array(T) struct array_##T { size_t len; T *elems; }

We can use `Array(int)` in multiple places in the same TU - but in C11 or earlier, this is an error.