Make WinitWindows non send (#4027)

# Objective

- Fixes #4010, as well as any similar issues in this class.
- Winit functions used outside of the main thread can cause the application to unexpectedly hang.

## Solution

- Make the `WinitWindows` resource `!Send`.
- This ensures that any systems that use `WinitWindows` must either be exclusive (run on the main thread), or the resource is explicitly marked with the `NonSend` parameter in user systems.

crates/bevy_winit/src/lib.rs

@@ -32,7 +32,7 @@ pub struct WinitPlugin;
 
 impl Plugin for WinitPlugin {
     fn build(&self, app: &mut App) {
-        app.init_resource::<WinitWindows>()
+        app.init_non_send_resource::<WinitWindows>()
             .set_runner(winit_runner)
             .add_system_to_stage(CoreStage::PostUpdate, change_window.exclusive_system());
         let event_loop = EventLoop::new();
@@ -43,7 +43,7 @@ impl Plugin for WinitPlugin {
 
 fn change_window(world: &mut World) {
     let world = world.cell();
-    let winit_windows = world.get_resource::<WinitWindows>().unwrap();
+    let winit_windows = world.get_non_send::<WinitWindows>().unwrap();
     let mut windows = world.get_resource_mut::<Windows>().unwrap();
 
     for bevy_window in windows.iter_mut() {
@@ -264,7 +264,7 @@ pub fn winit_runner_with(mut app: App) {
                 ..
             } => {
                 let world = app.world.cell();
-                let winit_windows = world.get_resource_mut::<WinitWindows>().unwrap();
+                let winit_windows = world.get_non_send_mut::<WinitWindows>().unwrap();
                 let mut windows = world.get_resource_mut::<Windows>().unwrap();
                 let window_id =
                     if let Some(window_id) = winit_windows.get_window_id(winit_window_id) {
@@ -525,7 +525,7 @@ fn handle_create_window_events(
     create_window_event_reader: &mut ManualEventReader<CreateWindow>,
 ) {
     let world = world.cell();
-    let mut winit_windows = world.get_resource_mut::<WinitWindows>().unwrap();
+    let mut winit_windows = world.get_non_send_mut::<WinitWindows>().unwrap();
     let mut windows = world.get_resource_mut::<Windows>().unwrap();
     let create_window_events = world.get_resource::<Events<CreateWindow>>().unwrap();
     let mut window_created_events = world.get_resource_mut::<Events<WindowCreated>>().unwrap();
@@ -544,7 +544,7 @@ fn handle_create_window_events(
 
 fn handle_initial_window_events(world: &mut World, event_loop: &EventLoop<()>) {
     let world = world.cell();
-    let mut winit_windows = world.get_resource_mut::<WinitWindows>().unwrap();
+    let mut winit_windows = world.get_non_send_mut::<WinitWindows>().unwrap();
     let mut windows = world.get_resource_mut::<Windows>().unwrap();
     let mut create_window_events = world.get_resource_mut::<Events<CreateWindow>>().unwrap();
     let mut window_created_events = world.get_resource_mut::<Events<WindowCreated>>().unwrap();

crates/bevy_winit/src/winit_windows.rs

@@ -9,6 +9,10 @@ pub struct WinitWindows {
     pub windows: HashMap<winit::window::WindowId, winit::window::Window>,
     pub window_id_to_winit: HashMap<WindowId, winit::window::WindowId>,
     pub winit_to_window_id: HashMap<winit::window::WindowId, WindowId>,
+    // Some winit functions, such as `set_window_icon` can only be used from the main thread. If
+    // they are used in another thread, the app will hang. This marker ensures `WinitWindows` is
+    // only ever accessed with bevy's non-send functions and in NonSend systems.
+    _not_send_sync: core::marker::PhantomData<*const ()>,
 }
 
 impl WinitWindows {