Lifetime

Lifetimes such as 'a never change how long data lives; they only let the compiler check that a reference does not outlive what it points to. Most are inferred (elision), and you write them explicitly when a function returns a reference tied to one of several inputs.

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