Reduce amount of allocations in RustLsp label handling (#35786)

There can be a lot of completions after all


Release Notes:

- N/A

crates/diagnostics/src/diagnostics_tests.rs

@@ -876,7 +876,7 @@ async fn test_random_diagnostics_with_inlays(cx: &mut TestAppContext, mut rng: S
                             vec![Inlay::edit_prediction(
                                 post_inc(&mut next_inlay_id),
                                 snapshot.buffer_snapshot.anchor_before(position),
-                                format!("Test inlay {next_inlay_id}"),
+                                Rope::from_iter(["Test inlay ", "next_inlay_id"]),
                             )],
                             cx,
                         );