Rust Lifetimes

A lifetime tells the compiler how long a reference stays valid. Most of the time Rust works it out, but when a function returns a reference you may need to name it with 'a.

ExampleRust

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

struct Excerpt<'a> {
    part: &'a str,
}

fn main() {
    let first = String::from("a long sentence");
    let result;
    {
        let second = String::from("short");
        result = longest(first.as_str(), second.as_str()).to_string();
    }
    println!("{}", result);

    let text = String::from("Call me Ishmael. Some years ago...");
    let excerpt = Excerpt { part: text.split('.').next().unwrap() };
    println!("{}", excerpt.part);
}

Output

a long sentence
Call me Ishmael

The annotation <'a> says: the returned reference lives no longer than the two inputs. This is how Rust prevents dangling references.

Quick check

What problem do lifetimes prevent?

References that outlive the data they point toSlow codeInteger overflow

Finish the quick check (and any code exercise) to complete this lesson, or mark it done yourself.

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