simple password gen in rust as learning exercise

password.rs

fn password_generate(len: i32) -> String {
    let mut rng = rand::thread_rng();
    let mut password = String::from("");

    let lower_chars = String::from("abcdefghijklmopqrstuvwxyz");
    let upper_chars = String::from("QWERTYUIOPASDFGHJKLZXCVBNM");
    let number_chars = String::from("0987654321");
    let symbols_chars = String::from("$!@?%+,().-[];:><\\~#&|/_*");

    let mut chars_coll: Vec<char> = lower_chars.chars().collect();
    chars_coll.append(&mut upper_chars.chars().collect());
    chars_coll.append(&mut number_chars.chars().collect());
    chars_coll.append(&mut symbols_chars.chars().collect());

    for _ in 0..len {
        password.push_str(&chars_coll[rng.gen_range(0..chars_coll.len())].to_string());
    }

    return password;
}