Short, very incomplete implementation of u256 to showcase the use of `u128::overflowing_add`

u256.rs

use std::fmt::{self, Display, Formatter};
use std::ops::Add;

#[derive(Debug, Clone)]
struct u256 {
    lo: u128,
    hi: u128,
}

impl Add<u256> for u256 {
    type Output = u256;

    fn add(self, rhs: u256) -> u256 {
        let (lo, carry) = self.lo.overflowing_add(rhs.lo);
        u256 {
            lo,
            hi: self.hi + rhs.hi + if carry { 1 } else { 0 },
        }
    }
}

impl From<u128> for u256 {
    fn from(lo: u128) -> Self {
        Self { lo, hi: 0 }
    }
}

impl Display for u256 {
    fn fmt(&self, fmt: &mut Formatter) -> Result<(), fmt::Error> {
        // Doesn't work for non-hex, this is just an example
        write!(fmt, "{:x}{:x}", self.hi, self.lo)
    }
}

fn main() {
    let mut x = u256::from(0x123456789ABCDEF0123456798ABCDEF0);
    // Multiplication by 16, forgive my laziness to implement other operators
    for _ in 0..4 {
        x = x.clone() + x;
    }

    println!("{}", x);
}