Lifetime annotations in Rust

When a function returns a reference, the compiler needs to know which input it borrows from.

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

fn first_line<'a>(text: &'a str, fallback: &'a str) -> &'a str {
    text.lines().next().unwrap_or(fallback)
}

fn main() {
    let title = String::from("Lifetimes");
    println!("{}", longer(&title, "short"));
    println!("{}", first_line("one\ntwo", "empty"));
}

How it works

1. <'a> declares a lifetime, then names it on the references.
2. The return value may not outlive the inputs it came from.
3. The annotation describes the relationship; it never changes it.

Keywords and builtins used here

• String
• else
• first_line
• fn
• if
• let
• longer
• main
• str

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Step 1 of 4 in Lifetime annotations, step 1 of 11 in Lifetimes & interior mutability.

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