Commaify an integer in Rust by the English rules without using a crate or traits or unsafe reads of the locale

commaify.rs

// Based on https://github.com/Totobird-Creations/Lrinser-Laser-Etcher-Edition-2/blob/81efa5a7355ba3df38e756f62c177d391d8e9060/src/helper.rs
//
/// Commaify a number by the British English rules (not the American English
/// rules because we don't want the '1' in 1000 to align with a comma in 10,000).
pub fn commaify_i64(number: i64) -> String {
    // Fast path
    if number > -1000 && number < 1000 {
        return number.to_string();
    }
    let string = number.abs().to_string();
    let mut result = String::with_capacity(5);
    let chars = string.chars().rev().collect::<Vec<char>>();
    for (i, char) in chars.into_iter().enumerate() {
        if i != 0 && i % 3 == 0 {
            result.push(',');
        }
        result.push(char);
    }
    if number < 0 {
        result.push('-');
    }
    result.chars().rev().collect::<String>()
}