I would like to self nominate Typekin, it is a newtype creator, but with concept of c++'s friend classes baked in, so the constructor's validator can be fully bypassed case-by-case and what you'll get at the end in the binary is again plain integers (currently it supports integers only).
Code: Koosha / Typekin · GitLab
I also have a non-procmacro version attemp in a private repo with mountain of commits and experimentation, but I gave up on the idea as the macro rules were turning into a turing complete rust syntax parser, and a nightmare to debug. I'll try to polish it and put in an alternate branch at some point.