Sums digits of a given i32

digit_sum.rs

fn main() {
    let args: Vec<String> = std::env::args().collect();
    let input = match args[1].parse::<i32>() {
        Ok(num) => num,
        Err(error) => panic!("The number provided is too large for an i32: {:?}", error)
    };
    println!("The sum of all digits is {}", sum_digits(&input));
}

fn sum_digits(num: &i32) -> i32 {
    let mut digit_sum = 0;
    let mut running_num = *num;
    while running_num != 0 {
        digit_sum += running_num % 10;
        running_num /= 10;
    }
    digit_sum
}

#[test]
fn check_digit_sum() {
    assert_eq!(10, sum_digits(&1234));
    assert_eq!(15, sum_digits(&12345));
}