How to Use Lifetimes in Struct Definitions

You use lifetimes in struct definitions by annotating reference fields with a lifetime parameter to tell the compiler how long the struct can outlive the data it references.

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

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

Lifetimes act like a safety contract ensuring that any data a struct points to stays alive as long as the struct itself exists. Think of it like a rental agreement: the struct cannot exist longer than the data it is borrowing. You use this whenever a struct holds references instead of owning the data directly.