fn main() {
    let mut s: String = String::from("hello");
    println!("{}", s);
    s.push_str("bbb");
    println!("{}", s);

    // 这个是不可变的
    let s1: &str = "aaa";
    println!("{}", s1);

    let mut s2: String = s1.to_string();
    println!("{}", s2);
    s2.push_str("bbb");
    println!("{}", s2);

    let f = format!("{}-{}-{}", s1, s2, s);
    println!("{}", f);
}