Lifetime annotations for the Rust compiler

Question

In the Rust book, such an example for lifetime annotation (LA) is given as below.

It uses LA for a valid reason, because neither the compiler nor we know whether a or b will be returned in the run time.

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

However, if this function simply returns y or even a static &str, the compiler still expects a named LA parameter in the return. Why can't it read the function content when compiling and make the right decision for us?

fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    y
    // "str slice" <-- or return a static value

}

score 0 · Answer 1 · edited Jun 08 '23 at 08:27

The compiler requires the lifetime annotation in the return type of the longest function, even if it always returns y or a static value because the lifetime annotation (named LA parameter) indicates the relationship between the references in the input parameters and the reference being returned.

In the first example you provided, the function conditionally returns either x or y, so the lifetime annotation 'a is necessary because the returned reference is tied to the lifetime of the shorter of the two input strings. The compiler needs this information to ensure that the returned reference is always valid and doesn't reference memory that has been deallocated.

In the second example, the function unconditionally returns y or a static value, and the lifetime annotation may seem unnecessary since there isn't any conditional logic affecting the lifetime of the returned reference. However, the Rust compiler currently does not perform an in-depth analysis of the function body to determine if the lifetime annotation can be omitted.

Lifetime annotations for the Rust compiler

1 Answers1