This is a temporary fix to not unsafe impl Sync + Send for backends

The problem is that when we call Context::create_viewport_sync can be on any thread because egui::Context is Sync + Send but the backend is not!
So we want the callback to the backend to be thread local!

The problem that this adds is that now you cannot create more then one egui::Context,
 because the thread local variabile is static and i don't know how to store a non static LocalKey on egui::ContextImpl

crates/eframe/src/native/run.rs

@@ -538,11 +538,6 @@ mod glow_integration {
         builders: HashMap<ViewportId, ViewportBuilder>,
     }
 
-    #[allow(unsafe_code)]
-    unsafe impl Sync for GlutinWindowContext {}
-    #[allow(unsafe_code)]
-    unsafe impl Send for GlutinWindowContext {}
-
     impl GlutinWindowContext {
         /// There is a lot of complexity with opengl creation, so prefer extensive logging to get all the help we can to debug issues.
         ///
@@ -1889,11 +1884,6 @@ mod wgpu_integration {
     #[derive(Clone)]
     pub struct Viewports(Arc<RwLock<HashMap<ViewportId, Window>>>);
 
-    #[allow(unsafe_code)]
-    unsafe impl Send for Viewports {}
-    #[allow(unsafe_code)]
-    unsafe impl Sync for Viewports {}
-
     impl std::ops::Deref for Viewports {
         type Target = Arc<RwLock<HashMap<ViewportId, Window>>>;
 

crates/egui/src/context.rs

@@ -172,6 +172,12 @@ impl Repaint {
 
 // ----------------------------------------------------------------------------
 
+thread_local! {
+    static EGUI_RENDER_SYNC: RwLock<Option<Box<ViewportRenderSyncCallback>>> = RwLock::new(None);
+}
+
+// ----------------------------------------------------------------------------
+
 #[derive(Default)]
 struct ContextImpl {
     /// `None` until the start of the first frame.
@@ -201,8 +207,6 @@ struct ContextImpl {
     viewports: HashMap<Id, Viewport>,
     viewport_commands: Vec<(ViewportId, ViewportCommand)>,
 
-    render_sync: Option<Arc<Box<ViewportRenderSyncCallback>>>,
-
     viewport_counter: u64,
     is_desktop: bool,
     force_embedding: bool,
@@ -467,9 +471,6 @@ impl Default for Context {
 
         s.write(|ctx| {
             ctx.force_embedding = true;
-            ctx.render_sync = Some(Arc::new(Box::new(move |ctx, _builder, _pair, render| {
-                render(ctx);
-            })));
         });
 
         s
@@ -2554,15 +2555,16 @@ impl Context {
     /// When a viewport sync is created will be rendered by this function
     ///
     /// Look in `crates/eframe/native/run.rs` and search for ``set_render_sync_callback`` to see for what is used!
+    #[allow(clippy::unused_self)]
     pub fn set_render_sync_callback(
         &self,
         callback: impl for<'a> Fn(&Context, ViewportBuilder, ViewportIdPair, Box<dyn FnOnce(&Context) + 'a>)
-            + Send
-            + Sync
             + 'static,
     ) {
         let callback = Box::new(callback);
-        self.write(|ctx| ctx.render_sync = Some(Arc::new(callback)));
+        EGUI_RENDER_SYNC.with(|render_sync| {
+            *render_sync.write() = Some(callback);
+        });
     }
 
     /// If this is true no other native windows will be created
@@ -2644,7 +2646,7 @@ impl Context {
     ) -> T {
         if !self.force_embedding() {
             let mut id_pair = ViewportIdPair::MAIN;
-            let render_sync = self.write(|ctx| {
+            self.write(|ctx| {
                 id_pair.parent = ctx.viewport_id();
                 if let Some(window) = ctx.viewports.get_mut(&viewport_builder.id) {
                     window.builder = viewport_builder.clone();
@@ -2666,18 +2668,20 @@ impl Context {
                         },
                     );
                 }
-
-                ctx.render_sync.clone()
             });
             let mut out = None;
             {
                 let out = &mut out;
-                render_sync.unwrap()(
-                    self,
-                    viewport_builder,
-                    id_pair,
-                    Box::new(move |context| *out = Some(func(context))),
-                );
+                EGUI_RENDER_SYNC.with(|render_sync|{
+                    let render_sync = render_sync.read();
+                    let render_sync = render_sync.as_ref().expect("No EGUI_RENDER_SYNC callback on this thread, if you try to use Context::create_viewport_sync you cannot do that in other thread! If that is not the issue your egui intrecration is invalid or do not support sync viewports!");
+                    render_sync(
+                        self,
+                        viewport_builder,
+                        id_pair,
+                        Box::new(move |context| *out = Some(func(context))),
+                    );
+                });
             }
 
             out.expect("egui backend is implemented incorrectly! Context::set_render_sync_callback")

crates/egui/src/viewport.rs

@@ -51,9 +51,8 @@ impl std::ops::Deref for ViewportIdPair {
 /// This is used to render an async viewport
 pub type ViewportRender = dyn Fn(&Context) + Sync + Send;
 
-pub type ViewportRenderSyncCallback = dyn for<'a> Fn(&Context, ViewportBuilder, ViewportIdPair, Box<dyn FnOnce(&Context) + 'a>)
-    + Send
-    + Sync;
+pub type ViewportRenderSyncCallback =
+    dyn for<'a> Fn(&Context, ViewportBuilder, ViewportIdPair, Box<dyn FnOnce(&Context) + 'a>);
 
 /// The filds in this struct should not be change directly, but is not problem tho!
 /// Every thing is wrapped in Option<> indicates that thing should not be changed!