Is there a modulus (not remainder) function / operation?

RFC 2196 adds a couple of integer methods related to euclidian division. Specifically, the rem_euclid method (example link for i32) is what you are searching for:

println!("{}", -1i32 % 4);                // -1
println!("{}", (-21i32).rem_euclid(4));   // 3

This method is available in rustc 1.38.0 (released on 2019-09-27) and above.