sum_tree: Spawn less tasks in SumTree::from_iter_async (#41793)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/rope/src/rope.rs

@@ -323,21 +323,13 @@ impl Rope {
         const PARALLEL_THRESHOLD: usize = 4 * (2 * sum_tree::TREE_BASE);
 
         if new_chunks.len() >= PARALLEL_THRESHOLD {
-            let cx2 = executor.clone();
-            executor
-                .scoped(|scope| {
-                    // SAFETY: transmuting to 'static is safe because the future is scoped
-                    // and the underlying string data cannot go out of scope because dropping the scope
-                    // will wait for the task to finish
-                    let new_chunks =
-                        unsafe { std::mem::transmute::<Vec<&str>, Vec<&'static str>>(new_chunks) };
-
-                    let async_extend = self
-                        .chunks
-                        .async_extend(new_chunks.into_iter().map(Chunk::new), cx2);
-
-                    scope.spawn(async_extend);
-                })
+            // SAFETY: transmuting to 'static is sound here. We block on the future making use of this
+            // and we know that the result of this computation is not stashing the static reference
+            // away.
+            let new_chunks =
+                unsafe { std::mem::transmute::<Vec<&str>, Vec<&'static str>>(new_chunks) };
+            self.chunks
+                .async_extend(new_chunks.into_iter().map(Chunk::new), executor)
                 .await;
         } else {
             self.chunks