Format a number with thousand separator characters in Rust

The following code function snippet is an example of how to format a number (already in String/&str form) with thousand separator characters in Rust:

pub fn thousand_separate_string_number(string_val: &str) -> String {
    let mut s = String::new();
    let mut local_copy = string_val.to_string();
    
    let decimal_pos = local_copy.rfind('.');
    if let Some(pos) = decimal_pos {
        // deal with any decimal place digit and following digits first
        s.push_str(&string_val[pos..]);
        local_copy = string_val[..pos].to_string();
    }
    
    let chrs = local_copy.chars().rev().enumerate();
    for (idx, val) in chrs {
        if idx != 0 && idx % 3 == 0 {
            s.insert(0, ',');
        }
        s.insert(0, val);
    }
    s
}