Fix Rust Error E0621 by adding explicit lifetime parameters to function signatures to define how long references remain valid.
Fix Error E0621 by adding an explicit lifetime parameter to the function signature and all reference arguments and return types involved.
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
if x.len() > y.len() { x } else { y }
}
This tells the compiler that the returned reference lives as long as the shortest of the two input references.
Error E0621 occurs because Rust doesn't know how long the data you are returning will stay valid. You need to explicitly tell the compiler that the output reference cannot outlive the input references you are using. Think of it like a library book: you must promise the book will be returned before the library closes, so the librarian (compiler) knows it is safe to lend it to you.