Rust: "borrowed value does not live long enough" when returning a &str from a function

fn first_word(s: String) -> &str {
    s.split(' ').next().unwrap()
}

The compiler says the borrowed value does not live long enough. I just want the first word.

s is owned by the function and dropped when it returns, so a &str pointing into it would dangle. Either borrow the input:

fn first_word(s: &str) -> &str {
    s.split(' ').next().unwrap_or("")
}

or return an owned String:

fn first_word(s: String) -> String {
    s.split(' ').next().unwrap_or("").to_string()
}