Fix clippy::flat_map_identity

src/iter/extend.rs

@@ -287,7 +287,7 @@ impl<'a> ParallelExtend<&'a char> for String {
 fn string_reserve<T: AsRef<str>>(string: &mut String, list: &LinkedList<Vec<T>>) {
     let len = list
         .iter()
-        .flat_map(|vec| vec)
+        .flatten()
         .map(T::as_ref)
         .map(str::len)
         .sum();