Implementation of count_digit() in Rust

count_digits.rs

use std::ops::DivAssign;

trait Int {
    fn ten() -> Self;
    fn zero() -> Self;
}

macro_rules! impl_int_for {
    ( $( $t:ty ) *) => {
        $( impl Int for $t {
            fn zero() -> Self { 0 }
            fn ten() -> Self { 10 }
        } )*
    };
}

impl_int_for!(u8 u16 u32 u64 u128 i8 i16 i32 i64 i128);

fn count_digit<T>(mut n: T) -> usize
where
    T: Int +PartialOrd + DivAssign + Copy,
{
    let zero = T::zero();
    if n == zero {
        return 1;
    }

    let ten = T::ten();
    let mut count: usize = 0;
    loop {
        if n == zero {
            return count;
        }
        n /= ten;
        count += 1;
    }
}

fn main() {
    println!("digits: {}", count_digit(12345));
}

#[test]
fn test_1() {
    for i in 0..10 {
        assert_eq!(count_digit(i), 1);
    }
    for i in -9..0 {
        assert_eq!(count_digit(i), 1);
    }
}

#[test]
fn test_2() {
    for i in 10..100u128 {
        assert_eq!(count_digit(i), 2);
    }
    for i in -99..-10i128 {
        assert_eq!(count_digit(i), 2);
    }
}

#[test]
fn test_types() {
    assert_eq!(count_digit(101i8), 3);
    assert_eq!(count_digit(101u8), 3);

    assert_eq!(count_digit(12345u16), 5);
    assert_eq!(count_digit(12345i16), 5);
}