Lesson 02

Lifetimes for returned data

Return borrowed data only when the lifetime is tied to an input, instead of returning references to local values.

lifetimes
borrowing
returns

Good Code

src/review_lookup.rs

pub fn first_approved<'a>(reviews: &'a [Review]) -> Option<&'a Review> {
    // The returned reference can only live as long as the input slice.
    reviews.iter().find(|review| review.approved)
}

Bad Code

review_lookup.rs

pub fn first_approved_title(reviews: &[Review]) -> &str {
    let title = reviews
        .iter()
        .find(|review| review.approved)
        .map(|review| review.title.clone())
        .unwrap_or_default();

    // Returning a reference to local storage cannot outlive this function.
    title.as_str()
}

Review Notes

What to review

Good Code

The good version returns a reference to an element already owned by the input slice. The lifetime in the signature documents that relationship.

Bad Code

The bad version builds a local String and tries to return a borrowed &str from it. That reference would point to storage dropped at the end of the function.

Takeaways

A returned reference should point to data owned outside the function, with a lifetime that makes that relationship visible.