generic factorial function in rust

generic-factorial.rs

use num::{One, BigUint}; // 0.2.0
use std::ops::{MulAssign, AddAssign};


fn fac<T>(n: T) -> T 
    where T: One + AddAssign + Ord,
          for <'a> T : MulAssign<&'a T> {
    let mut r = T::one();
    let mut i = T::one();
    while i <= n {
        r *= &i;
        i += T::one();
    }
    r
}

fn main() {
    let n : u32 = 999;
    let r = fac(BigUint::from(n));
    println!("fac({}) = {}", n, r);
}