Which side is handling lifetime specifier in rust compiler?

help
8

Lifetime annotations guide the borrow checker. Consider the following function (which does not compile, because the borrow checker cannot work out what I mean):

use std::str::FromStr;

fn choose_a_string(which_one: &str, str1: &str, str2: &str) -> &str {
    let choice = bool::from_str(which_one).unwrap_or_else(|_| {
        let num = i32::from_str(which_one).unwrap_or(0);
        num == 0
    });
    if choice {
        str1
    } else {
        str2
    }
}

Without lifetime annotations, it's unclear to the borrow checker what constraints apply to the lifetimes of the references. By adding lifetime annotations, I can make it clear:

fn choose_a_string<'retval>(
    which_one: &'_ str,
    str1: &'retval str,
    str2: &'retval str,
) -> &'retval str {

(Rust Playground with lifetime annotations).

With the added annotations, the borrow checker can see that which_one's lifetime is unrelated to the lifetime of the return value, and that both str1 and str2 need to be valid while the return value is alive.

8 Likes