Rust — Lifetimes

Lifetime annotations

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

fn main() {
    let result;
    let s1 = String::from("long string");
    {
        let s2 = String::from("xyz");
        result = longest(&s1, &s2);
    }
    // Error: s2 doesn't live long enough
}

Struct lifetimes

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

fn main() {
    let novel = String::from("Call me Ishmael. Some years ago...");
    let first_sentence = novel.split('.').next().unwrap();
    let excerpt = ImportantExcerpt {
        part: first_sentence,
    };
    println!("{}", excerpt.part);
}

Lifetime elision

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

Static lifetime

fn main() {
    let s: &'static str = "I live forever";
    println!("{}", s);
}

Generic type parameters, trait bounds, and lifetimes

use std::fmt::Display;

fn longest_with_announcement<'a, T>(
    x: &'a str,
    y: &'a str,
    ann: T,
) -> &'a str
where
    T: Display,
{
    println!("Announcement: {}", ann);
    if x.len() > y.len() { x } else { y }
}

Mini Practice

Write Rust code that:

1. Uses lifetime annotations in a function
2. Creates a struct with lifetime parameters
3. Demonstrates lifetime elision
4. Uses 'static lifetime

Up Next

In the next lesson, you'll learn about Error Handling — Result and Option.