Although, Rust only knows about certain obvious niches. It can reason that it can fit None in the same space as NonZeroU64 or a reference &T but it doesn't see how to do this with a hypothetical EvenU64 as I understand it because that "niche" is in the wrong place.

For anyone not sure, here is the proof.

https://godbolt.org/z/4zhWM9hTP