Fix missing lifetime specifier by adding an explicit lifetime parameter to link input and output references.
Add an explicit lifetime parameter to the function signature to tell the compiler how long the returned reference lives relative to the input.
fn first_word(s: &str) -> &str {
// implementation
}
Change to:
fn first_word<'a>(s: &'a str) -> &'a str {
// implementation
}
This links the lifetime of the input s to the output, ensuring the reference remains valid.
A missing lifetime specifier error happens when the compiler cannot guarantee that a reference you return will still be valid after the function finishes. You fix it by explicitly telling the compiler that the returned reference lives as long as the input data does. Think of it like a library card: you can only borrow a book (return a reference) if you promise to return it before the library closes (the input goes out of scope).