Allow access to non-send resource through World::resource_scope (#6113)

# Objective

Relaxes the trait bound for `World::resource_scope` to allow non-send resources. Fixes #6037.

## Solution

No big changes in code had to be made. Added a check so that the non-send resources won't be accessed from a different thread.

---

## Changelog
 - `World::resource_scope` accepts non-send resources now
 - `World::resource_scope` verifies non-send access if the resource is non-send
 - Two new tests are added, one for valid use of `World::resource_scope` with a non-send resource, and one for invalid use (calling it from a different thread, resulting in panic)

Co-authored-by: Dawid Piotrowski <41804418+Pietrek14@users.noreply.github.com>

crates/bevy_ecs/src/lib.rs

@@ -65,6 +65,7 @@ mod tests {
     use bevy_tasks::{ComputeTaskPool, TaskPool};
     use std::{
         any::TypeId,
+        marker::PhantomData,
         sync::{
             atomic::{AtomicUsize, Ordering},
             Arc, Mutex,
@@ -78,6 +79,9 @@ mod tests {
     #[derive(Component, Debug, PartialEq, Eq, Clone, Copy)]
     struct C;
 
+    #[derive(Default)]
+    struct NonSendA(usize, PhantomData<*mut ()>);
+
     #[derive(Component, Clone, Debug)]
     struct DropCk(Arc<AtomicUsize>);
     impl DropCk {
@@ -1262,6 +1266,38 @@ mod tests {
         assert_eq!(world.resource::<A>().0, 1);
     }
 
+    #[test]
+    fn non_send_resource_scope() {
+        let mut world = World::default();
+        world.insert_non_send_resource(NonSendA::default());
+        world.resource_scope(|world: &mut World, mut value: Mut<NonSendA>| {
+            value.0 += 1;
+            assert!(!world.contains_resource::<NonSendA>());
+        });
+        assert_eq!(world.non_send_resource::<NonSendA>().0, 1);
+    }
+
+    #[test]
+    #[should_panic(
+        expected = "attempted to access NonSend resource bevy_ecs::tests::NonSendA off of the main thread"
+    )]
+    fn non_send_resource_scope_from_different_thread() {
+        let mut world = World::default();
+        world.insert_non_send_resource(NonSendA::default());
+
+        let thread = std::thread::spawn(move || {
+            // Accessing the non-send resource on a different thread
+            // Should result in a panic
+            world.resource_scope(|_: &mut World, mut value: Mut<NonSendA>| {
+                value.0 += 1;
+            });
+        });
+
+        if let Err(err) = thread.join() {
+            std::panic::resume_unwind(err);
+        }
+    }
+
     #[test]
     fn insert_overwrite_drop() {
         let (dropck1, dropped1) = DropCk::new_pair();

crates/bevy_ecs/src/world/mod.rs

@@ -1149,7 +1149,13 @@ impl World {
     /// });
     /// assert_eq!(world.get_resource::<A>().unwrap().0, 2);
     /// ```
-    pub fn resource_scope<R: Resource, U>(&mut self, f: impl FnOnce(&mut World, Mut<R>) -> U) -> U {
+    pub fn resource_scope<
+        R: 'static, /* The resource doesn't need to be Send nor Sync. */
+        U,
+    >(
+        &mut self,
+        f: impl FnOnce(&mut World, Mut<R>) -> U,
+    ) -> U {
         let last_change_tick = self.last_change_tick();
         let change_tick = self.change_tick();
 
@@ -1157,6 +1163,13 @@ impl World {
             .components
             .get_resource_id(TypeId::of::<R>())
             .unwrap_or_else(|| panic!("resource does not exist: {}", std::any::type_name::<R>()));
+
+        // If the resource isn't send and sync, validate that we are on the main thread, so that we can access it.
+        let component_info = self.components().get_info(component_id).unwrap();
+        if !component_info.is_send_and_sync() {
+            self.validate_non_send_access::<R>();
+        }
+
         let (ptr, mut ticks) = {
             let resource_archetype = self.archetypes.resource_mut();
             let unique_components = resource_archetype.unique_components_mut();