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Lifetimes in structs

A struct that holds a reference needs a lifetime parameter. It is a declaration that instances of the struct cannot outlive the data they borrow.

This is how zero-copy parsers work: a `Parser<'a>` holds `&'a str` slices into the original input instead of allocating a new `String` for every token.

If keeping that relationship straight starts to hurt, that is usually a hint that the struct should own its data instead — `String` rather than `&str`. Borrowing is an optimisation, not an obligation.

A struct that outlives its data

struct Holder<'a> { text: &'a str }

fn main() {
    let holder;
    {
        let temporary = String::from("gone soon");
        holder = Holder { text: &temporary }; // error[E0597]
    }
    println!("{}", holder.text);
}

error[E0597]: `temporary` does not live long enough

Either keep the borrowed data alive as long as the struct, or store an owned `String` in the field so the struct does not depend on anyone else.

Run it and change it

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