Lifetime annotations: telling the borrow checker how references relate

Functions that return references cause compiler errors because the borrow checker cannot determine whether the returned reference lives long enough.

Add lifetime annotations to link the lifetimes of input and output references:

// Without lifetime annotation — compiler cannot infer
// fn longest(x: &str, y: &str) -> &str { ... } // ERROR

// With lifetime annotation
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() { x } else { y }
}

// Struct holding a reference needs a lifetime
struct Important<'a> {
    content: &'a str,
}

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

Lifetime annotations do not change how long references live — they tell the compiler the relationship between lifetimes so it can verify safety. The annotation 'a means both inputs and the output share the same lifetime constraint.