Rust: How do you work with lifetimes?

1fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
2    if x.len() > y.len() { x } else { y }
3}

1struct Excerpt<'a> {
2    part: &'a str,
3}
4
5impl<'a> Excerpt<'a> {
6    fn level(&self) -> i32 { 3 }
7}

1fn first_word<'a>(s: &'a str) -> &'a str { &s[..1] }