fn main() {
    let mut a = vec!["a".to_string(), "b".to_string(), "c".to_string()];

    {
        let c: Vec<&mut String> = a
            .iter_mut()
            .map(|x: &mut String| {
                x.push_str("第一次");
                x
            })
            .collect();
        println!("{:?}", c);
    }

    let b: Vec<String> = a
        .iter()
        .map(|x: &String| {
            let mut s: String = x.clone();
            s.push_str("_第二次");
            s
        })
        .collect();
    println!("{:?}", b);
}