An underscore _ character has been used as the identifier for a lifetime.

Erroneous code example:

fn longest<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
         //^^ `'_` is a reserved lifetime name
    if str1.len() > str2.len() {
        str1
    } else {
        str2
    }
}

'_, cannot be used as a lifetime identifier because it is a reserved for the anonymous lifetime. To fix this, use a lowercase letter such as 'a, or a series of lowercase letters such as 'foo. For more information, see the book. For more information on using the anonymous lifetime in rust nightly, see the nightly book.

Corrected example:

fn longest<'a>(str1: &'a str, str2: &'a str) -> &'a str {
    if str1.len() > str2.len() {
        str1
    } else {
        str2
    }
}