When do you need explicit lifetime annotations

You need explicit lifetime annotations when the compiler cannot infer how long a reference lives, typically when a function returns a reference derived from one of its arguments or when multiple references with different scopes interact. Add the 'a lifetime parameter to the function signature and the references to tell the compiler that the returned reference is tied to the input reference's lifetime.

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

Explicit lifetime annotations are needed when the compiler can't automatically figure out how long a piece of data is valid. Think of it like telling a librarian exactly how long you plan to keep a borrowed book so they know when to expect it back. You use them to prevent your program from accidentally using data that has already been deleted or changed.