Function understand_lifetime

pub fn understand_lifetime()

理解普通生命周期参数：

说明： 生命周期参数：late bound vs early bound

示例1:

fn return_str<'a>() -> &'a str {
    let mut s = "Rust".to_string();
    for i in 0..3 {
        s.push_str("Good ");
    }
    &s[..]                   //"Rust Good Good Good"
}
fn main() {
    let x = return_str();
}

示例2:

fn foo<'a>(x: &'a str, y: &'a str) -> &'a str {
    let result = String::from("really long string");
    // error
    result.as_str()
}

fn main() {
    let x = "hello";
    let y = "rust";
    foo(x, y);
}

示例3:

fn the_longest(s1: &str, s2: &str) -> &str {
    if s1.len() > s2.len() { s1 } else { s2 }
}
fn main() {
    let s1 = String::from("Rust");
    let s1_r = &s1;
    {
        let s2 = String::from("C");
        let res = the_longest(s1_r, &s2);
    println!("{} is the longest", res);
}

示例4:

fn the_longest<'a>(s1: &'a str, s2: &'a str) -> &'a str {
    if s1.len() > s2.len() { s1 } else { s2}
}
fn main() {
    let s1 = String::from("Rust");
    let s1_r = &s1;
    {
        let s2 = String::from("C");
        let res = the_longest(s1_r, &s2);
    println!("{} is the longest", res); // Rust is the longest
}