There's two things going on in the exercise: 1) some introductory Rust programming; 2) some introductory math and crypto.

Maybe it's just me but I think it's better to separate the two. If you're going to do a prime number generation exercise, it will be easier in (e.g.) Python since the bignum arithmetic is built in, you don't have all the memory management headache, etc. If you're going to do a Rust exercise, imho it is better to focus on Rust stuff.