Lifetime Annotations

Explicit lifetime annotations tell the compiler how reference parameters and return values relate. They appear in function signatures, structs, enums, and trait impls.

Recipe

fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}
 
fn main() {
    let s = String::from("abcd");
    let slice = longest(&s, "xy");
    println!("{slice}");
}

When to reach for this: Public APIs returning references, structs holding borrows, and resolving lifetime errors the compiler cannot elide.

Working Example

struct Parser<'a> {
    input: &'a str,
}
 
impl<'a> Parser<'a> {
    fn new(input: &'a str) -> Self {
        Self { input }
    }
 
    fn peek_word(&self) -> Option<&'a str> {
        self.input.split_whitespace().next()
    }
}
 
fn main() {
    let text = String::from("hello world");
    let p = Parser::new(&text);
    println!("{:?}", p.peek_word());
}

What this demonstrates:

• impl<'a> repeats lifetime on impl block
• Methods return &'a str tied to parser's input
• Parser cannot outlive text
• Same lifetime on all tied references

Deep Dive

Function Annotations

fn split<'a>(s: &'a str, at: usize) -> (&'a str, &'a str) {
    s.split_at(at)
}

Struct with Multiple Lifetimes

struct Pair<'a, 'b> {
    first: &'a str,
    second: &'b str,
}

Each field may borrow from different sources.

Lifetime Bounds

fn copy_str<'a: 'b, 'b>(x: &'a str, y: &'b mut &str) {
    *y = x;
}

'a: 'b means 'a outlives 'b.

impl Lifetimes

Lifetime params on impl must match struct/trait definition when methods return borrowed inner data.

Gotchas

• Annotating only return lifetime - Inputs need connection. Fix: Add 'a to all related refs.
• Unrelated lifetimes forced equal - Over-constrains API. Fix: Use separate 'a, 'b when sources differ.
• Missing lifetime on impl struct with refs - Compile error on methods. Fix: impl<'a> Struct<'a>.
• Lifetime on fn inside impl without need - Noise. Fix: Use elision when compiler accepts.
• Thinking 'a sets duration - You declare relationships; compiler verifies callsites. Fix: Read errors as constraint failures.

Alternatives

FAQs

Related

• Lifetime Elision - when to omit
• Lifetimes Explained - conceptual model
• 'static & Bounds - outlives bounds
• Lifetime Mismatch Scenarios - fixes

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.