Knowing that they usually are at the end doesn't help since you still need to write a general runtime check that expresses "are all the bits for this type zero

Knowing that they usually are at the end doesn't help since you still need to write a general runtime check that expresses "are all the bits for this type zero or uninit". Afaik currently there is no way to do this without causing UB. This would need some new intrinsic.