Providing an implementation when both trait and type are not in this crate [duplicate]

Answer 1

You should use a newtype struct to do this:

pub struct U8(pub u8)

impl ToHex for U8 {
    fn to_hex(&self) -> String {
        let U8(x) = *self;
        x.to_str_radix(16)
    }
}

This does mean, however, that you should wrap u8 into U8 where you need to perform this conversion:

let x: u8 = 127u8

// println!("{}", x.to_hex());   // does not compile
println!("{}", U8(x).to_hex());

This is absolutely free in terms of performance.