Outline drop glue for App to ensure that a lot of drop glue is codege…

…ned inside bevy_app

crates/bevy_app/src/app.rs

@@ -1,3 +1,5 @@
+use std::mem::ManuallyDrop;
+
 use crate::app_builder::AppBuilder;
 use bevy_ecs::{
     schedule::{Schedule, Stage},
@@ -30,21 +32,36 @@ use bevy_utils::tracing::info_span;
 /// }
 /// ```
 pub struct App {
-    pub world: World,
-    pub runner: Box<dyn Fn(App)>,
-    pub schedule: Schedule,
+    pub world: ManuallyDrop<World>,
+    pub runner: ManuallyDrop<Box<dyn Fn(App)>>,
+    pub schedule: ManuallyDrop<Schedule>,
 }
 
 impl Default for App {
     fn default() -> Self {
         Self {
             world: Default::default(),
             schedule: Default::default(),
-            runner: Box::new(run_once),
+            runner: ManuallyDrop::new(Box::new(run_once)),
         }
     }
 }
 
+impl Drop for App {
+    fn drop(&mut self) {
+        unsafe {
+            drop_app(self);
+        }
+    }
+}
+
+// Outlined to ensure that a lot of drop glue is codegened inside bevy_app instead of the game.
+unsafe fn drop_app(app: &mut App) {
+    ManuallyDrop::drop(&mut app.world);
+    ManuallyDrop::drop(&mut app.schedule);
+    ManuallyDrop::drop(&mut app.runner);
+}
+
 fn run_once(mut app: App) {
     app.update();
 }
@@ -64,7 +81,7 @@ impl App {
         #[cfg(feature = "trace")]
         let _bevy_app_run_guard = bevy_app_run_span.enter();
 
-        let runner = std::mem::replace(&mut self.runner, Box::new(run_once));
+        let runner = std::mem::replace(&mut *self.runner, Box::new(run_once));
         (runner)(self);
     }
 }

crates/bevy_app/src/app_builder.rs

@@ -58,7 +58,7 @@ impl AppBuilder {
     }
 
     pub fn set_world(&mut self, world: World) -> &mut Self {
-        self.app.world = world;
+        *self.app.world = world;
         self
     }
 
@@ -276,7 +276,7 @@ impl AppBuilder {
     }
 
     pub fn set_runner(&mut self, run_fn: impl Fn(App) + 'static) -> &mut Self {
-        self.app.runner = Box::new(run_fn);
+        *self.app.runner = Box::new(run_fn);
         self
     }