To expand on the sibling answer: sort of! Rust will do niche optimisation, but for references and NonNull pointers this is limited to "the value 0 is invalid and can thus be used as a niche". But Rust does not (currently) take advantage of alignment niches in pointers. Nor does it use high bit on architectures where you know your whole theoretical address space isn't actually in use.

Is doing that manually worth it? Usually not, but for some core types (classical example is strings) or in language runtimes it can be.

Would it be awesome if this could be done automatically? Absolutely, but I understand it is a large change, and the plan is to later build upon the pattern types that are currently work in progress (and would allow you to specify custom ranged integer typed).