Rust 1.78 (#11314)

Notable things I've had to fix due to 1.78:
- Better detection of unused items
- New clippy lint (`assigning_clones`) that points out places where assignment operations with clone rhs could be replaced with more performant `clone_into`
Release Notes:

- N/A

crates/language/src/outline.rs

@@ -82,7 +82,7 @@ impl<T> Outline<T> {
         let mut prev_item_ix = 0;
         for mut string_match in matches {
             let outline_match = &self.items[string_match.candidate_id];
-            string_match.string = outline_match.text.clone();
+            string_match.string.clone_from(&outline_match.text);
 
             if is_path_query {
                 let prefix_len = self.path_candidate_prefixes[string_match.candidate_id];