I feel like the separate crate with UniqueNumber(u32) is a bad idea. There's nothing unique about these, you're gas lighting me. Like a PrimeNumber(u32) that's just a thin wrapper and expects somebody else to ensure they're prime - no, I know the compiler doesn't understand English, but these aren't prime (or unique) numbers, you're bad at your job.

One of my favourite decisions about Rust which could have gone the other way was their choice to define &str (and String) to be actual text. &str is basically &[u8] but with an extra requirement that this is valid UTF-8 text, likewise then String is basically Vec<u8> but requiring UTF-8.

I feel like UniqueNumber is the same, if you call yourself UniqueNumber don't just be a thin wrapper around an integer, you need to own that uniqueness problem.