The final element of the struct is a zero length array of elements of size N bytes. So that element isn't padding, it has size 0! It's pure hinting. I'm not sure why or how this works under the hood I'm afraid. I used it successfully to call a library with pretty strict alignment requirements (Intel Embree).

The zero length array still must be aligned correctly. You can create a pointer to it (e.g. by taking &something._alignment[..] to create a slice) and pointers must have correct alignment, so it follows that a zero length array has the same alignment requirements as a longer array. So padding must be inserted in your struct so that the address of the zero-length array is correctly aligned.

Hmm, I don't understand (sorry). The pointer to the internal array must be aligned. But how does the element size play into the padding. If we take these two examples?

   [repr(C)]
   struct StructA
   {
      pub foo: f32,
      _alignment: [SixteenBytes, 0]
   }

   [repr(C)]
   struct StructB
   {
      pub foo: f32,
      _alignment: [ThirtytwoBytes, 0]
   }

Are both just padded with 16-4 and 32-4 bytes respectively, so they are equivalent to making a padding like this in the first case?

   [repr(C)]
   struct StructAPadded
   {
      pub foo: f32,
      _padding : TwelveBytes;
   }