What’s where [(); SIZE + Kind::EXTRA]:?

This bound is essentially used to declare that your const computation (SIZE + Kind::EXTRA) can be evaluated. In this particular case, it means that the addition should not overflow usize. It also can be used to declare that types created with the const computation result have a proper size (i.e. less than isize::MAX bytes) by using u8 instead of () in the bound.

In other words, the bound prevents potential monomorphization errors (which are, unfortunately, already exist in Rust for some corner cases...).

daniel-pfeiffer:

is this temporary, until some prerequisite gets implemented?

IIRC, yes. Though I can not link exact proposals out of my head.