How to fix lifetime mismatch error

Fix the lifetime mismatch by explicitly annotating the function signature with a lifetime parameter that ties the input reference to the output reference. Add the 'a lifetime to the function, the input slice, and the return value to tell the compiler the output cannot outlive the input.

fn largest<'a>(list: &'a [i32]) -> &'a i32 {
    let mut largest = &list[0];
    for item in list {
        if item > largest {
            largest = item;
        }
    }
    largest
}

If the error occurs in a struct method, ensure the &self reference and the return reference share the same lifetime annotation, or use &mut self if the data is being modified.

The lifetime mismatch error occurs when the compiler cannot guarantee that a reference you are returning will stay valid after the function finishes. You fix it by adding a lifetime label to tell the compiler that the returned reference is tied to the lifetime of the input data. Think of it like a library book: you can only lend out a book (return a reference) if you promise the borrower will return it before the library closes (the input data goes out of scope).