Modded Example.rs

main.rs

extern crate num;
use num::{BigUint, Zero, One};
use std::mem::replace;

fn fib(n: usize) -> usize {
    let mut f0: BigUint = Zero::zero();
    let mut f1: BigUint = One::one();
    for _ in (0..n) {
        let f2 = f0 + &f1;
        // This is a low cost way of swapping f0 with f1 and f1 with f2.
        f0 = replace(&mut f1, f2);
    }
    f0.to_string().len()
}

fn main() {
println!("fib(10000) = {}", fib(4784));
}