How do I use integer number literals when using generic types?

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Many things are going wrong here:

  1. As Shepmaster says, 0 and 1 cannot be converted to everything implementing Integer. Use Zero::zero and One::one instead.
  2. 10 can definitely not be converted to anything implementing Integer, you need to use NumCast for that
  3. a /= b is not sugar for a = a / b but an separate trait that Integer does not require.
  4. -x is an unary operation which is not part of Integer but requires the Neg trait (since it only makes sense for signed types).

Here’s an implementation. Note that you need a bound on Neg, to make sure that it results in the same type as T

extern crate num;

use num::{Integer, NumCast};
use std::ops::Neg;

fn int_length<T>(mut x: T) -> u8
where
    T: Integer + Neg<Output = T> + NumCast,
{
    if x == T::zero() {
        return 1;
    }

    let mut length = 0;
    if x < T::zero() {
        length += 1;
        x = -x;
    }

    while x > T::zero() {
        x = x / NumCast::from(10).unwrap();
        length += 1;
    }

    length
}

fn main() {
    println!("{}", int_length(45));
    println!("{}", int_length(-45));
}

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