sum_tree: Replace rayon with futures (#41586)

Release Notes:

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

Co-authored by: Kate <kate@zed.dev>

crates/gpui/src/app/async_context.rs

@@ -260,6 +260,19 @@ impl AsyncApp {
     }
 }
 
+impl sum_tree::BackgroundSpawn for BackgroundExecutor {
+    type Task<R>
+        = Task<R>
+    where
+        R: Send + Sync;
+    fn background_spawn<R>(&self, future: impl Future<Output = R> + Send + 'static) -> Self::Task<R>
+    where
+        R: Send + Sync + 'static,
+    {
+        self.spawn(future)
+    }
+}
+
 /// A cloneable, owned handle to the application context,
 /// composed with the window associated with the current task.
 #[derive(Clone, Deref, DerefMut)]

crates/gpui/src/app/test_context.rs

@@ -393,6 +393,11 @@ impl TestAppContext {
         }
     }
 
+    /// Returns the background executor for this context.
+    pub fn background_executor(&self) -> &BackgroundExecutor {
+        &self.background_executor
+    }
+
     /// Wait until there are no more pending tasks.
     pub fn run_until_parked(&mut self) {
         self.background_executor.run_until_parked()

crates/gpui/src/executor.rs

@@ -342,7 +342,7 @@ impl BackgroundExecutor {
     /// for all of them to complete before returning.
     pub async fn scoped<'scope, F>(&self, scheduler: F)
     where
-        F: FnOnce(&mut Scope<'scope>),
+        F: for<'a> FnOnce(&'a mut Scope<'scope>),
     {
         let mut scope = Scope::new(self.clone());
         (scheduler)(&mut scope);