E0106: missing lifetime specifier

A reference is only valid while the thing it points at is alive. When a function returns a reference, the compiler needs to know which input it borrows from so it can guarantee the caller never holds a dangling pointer.

Lifetimes like `'a` are not runtime values and cost nothing. They are annotations that let the compiler check that relationship.

Code that triggers it

fn longest(a: &str, b: &str) -> &str { // error[E0106]
    if a.len() > b.len() { a } else { b }
}

Code that compiles

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

fn main() {
    println!("{}", longest("ownership", "borrowing"));
}

Introduce a lifetime parameter `'a` and tie the inputs and the return value together. If the returned data does not actually come from an input, return an owned `String` instead.

Try both versions

The broken version is loaded below so you can see the real compiler message, then fix it yourself.