以rustbook中的代码为例

fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() {
        x
    } else {
        y
    }
}

我们有上面的函数，现在假设我们有main：

// can't compile , i understand this part, just post here for reference
fn main1(){
       let string1 = String::from("abcd");
        let result;
        {
            let string2 = String::from("xyz"); // we will change this to &str in next example
            result = longest(string1.as_str(), string2.as_str());
                                                ^^^^^^ doesn't compile , string2 doesn't live long enough
        }
        println!("The longest string is {}", result);
}

但是如果我们稍微修改一下下面的代码，把string2修改成一个字符串片，这段代码实际上是编译的，我不太清楚是怎么回事，“xyz”仍然算作有效内存吗？

//This compiles. but why? shouldn't ```longest()``` return a smaller lifetime and refuses to compile?
fn main2(){
    let string1 = String::from("abcd");
    let result;
    {
        let string2 = "xyz"; // <------ changed
        result = longest(string1.as_str(), string2);
    }
    println!("The longest string is {}", result);

}