请看下面两段代码

fn strcmp<'a>(a: &'a str, b: &'a str) -> &'a str {
    if a.len() > b.len() {
        a
    } else {
        b
    }
}

fn main() {
    let a = "abc";
    let result;
    {
        let b = "abc";
        result = strcmp(&a, &b);
    }   //             compile successful
    let a : String;
    println!("{}", result);
}

上面这一段代码可以编译成功

fn strcmp<'a>(a: &'a str, b: &'a str) -> &'a str {
    if a.len() > b.len() {
        a
    } else {
        b
    }
}

fn main() {
    let a = String::from("abc");
    let result;
    {
        let b = String::from("abc");
        result = strcmp(&a, &b);
    }   //                  ^^ borrowed value does not live long enough      

    println!("{}", result);
}

这一段代码编译失败，提示生命周期错误。

这两种情况让我非常困惑，希望各位大佬指教。谢谢。