Fix the 'cannot infer an appropriate lifetime' error by adding explicit lifetime parameters to your function signature.
Add explicit lifetime parameters to your function signature to tell the compiler how long the returned reference is valid.
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
if x.len() > y.len() { x } else { y }
}
The 'a lifetime parameter ensures the returned reference lives as long as the shortest of the two input references.
The "cannot infer an appropriate lifetime" error happens when Rust cannot determine how long a reference will stay valid. You fix it by explicitly telling the compiler that the output reference must not outlive the input references it came from. Think of it like a library book: you must specify that the book you lend out cannot be kept longer than the time you borrowed it yourself.