// Rule 1: each reference gets its own lifetime
fn first(s: &str) -> &str {
    // elided: fn first(s: &str) -> &str
    s.split_whitespace().next().unwrap()
}

// Rule 2: if there is one input lifetime, it is assigned to all outputs
fn first_word(s: &str) -> &str {
    s.split_whitespace().next().unwrap_or("")
}

// Rule 3: methods with &self, the lifetime of self is assigned to all outputs
struct Parser {
    input: String,
}

impl Parser {
    fn next_token(&self) -> &str {
        self.input.split_whitespace().next().unwrap()
    }
}

fn main() {
    let s = String::from("hello world");
    println!("first: {}", first(&s));
    println!("word: {}", first_word(&s));

    let p = Parser { input: s };
    println!("token: {}", p.next_token());
}