Challenge 2: FirstLine and Elision Rule 3 — Possible Solution
====================================================================

struct FirstLine<'a> {
    line: &'a str,
}

impl<'a> FirstLine<'a> {
    fn announce(&self) -> &str {
        self.line
    }
}

fn main() {
    let text = String::from("Hello\nWorld");
    let first = FirstLine { line: text.lines().next().unwrap() };
    println!("{}", first.announce());
}

WHY THIS WORKS AS AN ANSWER
------------------------------
The struct itself, FirstLine<'a> { line: &'a str }, needs an explicit
lifetime parameter because it HOLDS a reference — exactly this
chapter's struct-lifetime rule: the struct instance's own validity is
tied to how long the referenced string data (here, a slice of `text`)
remains valid.

announce, however, needs NO explicit lifetime annotation of its own,
despite returning a reference — this is elision RULE 3 specifically:
because announce takes &self as a parameter, the compiler
automatically assigns self's lifetime to the returned reference. Since
self is a &FirstLine<'a>, and FirstLine<'a> already carries the 'a
lifetime that line was defined with, the compiler can mechanically
work out that the returned &str shares that same 'a — without the
method needing to spell out &'a self -> &'a str explicitly.

This is precisely the situation this chapter names as the explanation
for why Course 1's own &self methods (like Chapter 5's area()) never
needed lifetime annotations even when they returned references: rule 3
was quietly doing the work in every one of those cases too.