add back all changes

crates/bevy_ecs/src/component.rs

@@ -1734,6 +1734,9 @@ pub struct Components {
     components: Vec<Option<ComponentInfo>>,
     indices: TypeIdMap<ComponentId>,
     resource_indices: TypeIdMap<ComponentId>,
+    /// A lookup for the entities on which resources are stored.
+    /// It uses ComponentIds instead of TypeIds for untyped APIs
+    pub(crate) resource_entities: HashMap<ComponentId, Entity>,
     // This is kept internal and local to verify that no deadlocks can occor.
     queued: bevy_platform::sync::RwLock<QueuedComponents>,
 }

crates/bevy_ecs/src/entity_disabling.rs

@@ -207,6 +207,7 @@ mod tests {
     use crate::{
         prelude::World,
         query::{Has, With},
+        resource::IsResource,
     };
     use alloc::{vec, vec::Vec};
 
@@ -278,6 +279,9 @@ mod tests {
         let mut world = World::new();
         world.register_disabling_component::<CustomDisabled>();
 
+        // We don't want to query resources for this test.
+        world.register_disabling_component::<IsResource>();
+
         world.spawn_empty();
         world.spawn(Disabled);
         world.spawn(CustomDisabled);

crates/bevy_ecs/src/query/builder.rs

@@ -332,6 +332,9 @@ mod tests {
     #[test]
     fn builder_or() {
         let mut world = World::new();
+        // We don't want to query resources for this test.
+        world.register_disabling_component::<IsResource>();
+
         world.spawn((A(0), B(0)));
         world.spawn(B(0));
         world.spawn(C(0));

crates/bevy_ecs/src/query/iter.rs

@@ -2659,6 +2659,7 @@ mod tests {
     use crate::component::Component;
     use crate::entity::Entity;
     use crate::prelude::World;
+    use crate::resource::IsResource;
 
     #[derive(Component, Debug, PartialEq, PartialOrd, Clone, Copy)]
     struct A(f32);
@@ -2669,6 +2670,9 @@ mod tests {
     #[test]
     fn query_iter_sorts() {
         let mut world = World::new();
+        // We don't want to query resources for this test.
+        world.register_disabling_component::<IsResource>();
+
         for i in 0..100 {
             world.spawn(A(i as f32));
             world.spawn((A(i as f32), Sparse(i)));

crates/bevy_ecs/src/query/state.rs

@@ -1850,6 +1850,9 @@ mod tests {
     #[test]
     fn can_transmute_empty_tuple() {
         let mut world = World::new();
+        // We don't want to query resources for this test.
+        world.register_disabling_component::<IsResource>();
+
         world.register_component::<A>();
         let entity = world.spawn(A(10)).id();
 
@@ -2207,6 +2210,7 @@ mod tests {
     #[test]
     fn query_default_filters_updates_is_dense() {
         let mut world = World::new();
+        let num_resources = world.components().num_resources();
         world.spawn((Table, Sparse));
         world.spawn(Table);
         world.spawn(Sparse);
@@ -2214,7 +2218,7 @@ mod tests {
         let mut query = QueryState::<()>::new(&mut world);
         // There are no sparse components involved thus the query is dense
         assert!(query.is_dense);
-        assert_eq!(3, query.iter(&world).count());
+        assert_eq!(3, query.iter(&world).count() - num_resources);
 
         let mut df = DefaultQueryFilters::empty();
         df.register_disabling_component(world.register_component::<Sparse>());

crates/bevy_ecs/src/resource.rs

@@ -1,5 +1,10 @@
 //! Resources are unique, singleton-like data types that can be accessed from systems and stored in the [`World`](crate::world::World).
 
+use crate::prelude::Component;
+use crate::prelude::ReflectComponent;
+use bevy_reflect::prelude::ReflectDefault;
+use bevy_reflect::Reflect;
+use core::marker::PhantomData;
 // The derive macro for the `Resource` trait
 pub use bevy_ecs_macros::Resource;
 
@@ -73,3 +78,89 @@ pub use bevy_ecs_macros::Resource;
     note = "consider annotating `{Self}` with `#[derive(Resource)]`"
 )]
 pub trait Resource: Send + Sync + 'static {}
+
+/// A marker component for the entity that stores the resource of type `T`.
+///
+/// This component is automatically inserted when a resource of type `T` is inserted into the world,
+/// and can be used to find the entity that stores a particular resource.
+///
+/// By contrast, the [`IsResource`] component is used to find all entities that store resources,
+/// regardless of the type of resource they store.
+///
+/// This component comes with a hook that ensures that at most one entity has this component for any given `R`:
+/// adding this component to an entity (or spawning an entity with this component) will despawn any other entity with this component.
+#[derive(Component, Debug)]
+#[require(IsResource)]
+#[cfg_attr(feature = "bevy_reflect", derive(Reflect), reflect(Component, Default))]
+pub struct ResourceEntity<R: Resource>(#[reflect(ignore)] PhantomData<R>);
+
+impl<R: Resource> Default for ResourceEntity<R> {
+    fn default() -> Self {
+        ResourceEntity(PhantomData)
+    }
+}
+
+/// A marker component for entities which store resources.
+///
+/// By contrast, the [`ResourceEntity<R>`] component is used to find the entity that stores a particular resource.
+/// This component is required by the [`ResourceEntity<R>`] component, and will automatically be added.
+#[cfg_attr(
+    feature = "bevy_reflect",
+    derive(Reflect),
+    reflect(Component, Default, Debug)
+)]
+#[derive(Component, Default, Debug)]
+pub struct IsResource;
+
+#[cfg(test)]
+#[expect(clippy::print_stdout, reason = "Allowed in tests.")]
+mod tests {
+    use crate::change_detection::MaybeLocation;
+    use crate::ptr::PtrMut;
+    use crate::resource::Resource;
+    use crate::world::World;
+    use bevy_platform::prelude::String;
+    use core::mem::ManuallyDrop;
+
+    #[test]
+    fn unique_resource_entities() {
+        #[derive(Default, Resource)]
+        struct TestResource1;
+
+        #[derive(Resource)]
+        #[expect(dead_code, reason = "field needed for testing")]
+        struct TestResource2(String);
+
+        #[derive(Resource)]
+        #[expect(dead_code, reason = "field needed for testing")]
+        struct TestResource3(u8);
+
+        let mut world = World::new();
+        let start = world.entities().len();
+        world.init_resource::<TestResource1>();
+        assert_eq!(world.entities().len(), start + 1);
+        world.insert_resource(TestResource2(String::from("Foo")));
+        assert_eq!(world.entities().len(), start + 2);
+        // like component registration, which just makes it known to the world that a component exists,
+        // registering a resource should not spawn an entity.
+        let id = world.register_resource::<TestResource3>();
+        assert_eq!(world.entities().len(), start + 2);
+        unsafe {
+            // SAFETY
+            // *
+            world.insert_resource_by_id(
+                id,
+                PtrMut::from(&mut ManuallyDrop::new(20 as u8)).promote(),
+                MaybeLocation::caller(),
+            );
+        }
+        assert_eq!(world.entities().len(), start + 3);
+        assert!(world.remove_resource_by_id(id).is_some());
+        assert_eq!(world.entities().len(), start + 2);
+        world.remove_resource::<TestResource1>();
+        assert_eq!(world.entities().len(), start + 1);
+        // make sure that trying to add a resource twice results, doesn't change the entity count
+        world.insert_resource(TestResource2(String::from("Bar")));
+        assert_eq!(world.entities().len(), start + 1);
+    }
+}

crates/bevy_ecs/src/world/mod.rs

@@ -216,12 +216,14 @@ impl World {
         &mut self.entities
     }
 
-    /// Retrieves the number of [`Entities`] in the world.
+    /// Retrieves the number of [`Entities`] in the world. This count does not include resource entities.
     ///
     /// This is helpful as a diagnostic, but it can also be used effectively in tests.
     #[inline]
-    pub fn num_entities(&self) -> u32 {
+    pub fn entity_count(&self) -> u32 {
-        self.entities.len()
+        self.entities
+            .len()
+            .saturating_sub(self.components.resource_entities.len() as u32)
     }
 
     /// Retrieves this world's [`Archetypes`] collection.

crates/bevy_scene/src/lib.rs

@@ -49,7 +49,13 @@ pub mod prelude {
 use bevy_app::prelude::*;
 
 #[cfg(feature = "serialize")]
-use {bevy_asset::AssetApp, bevy_ecs::schedule::IntoScheduleConfigs};
+use {
+    bevy_asset::AssetApp,
+    bevy_ecs::schedule::IntoScheduleConfigs,
+    bevy_ecs::{
+        entity_disabling::DefaultQueryFilters, resource::IsResource, resource::ResourceEntity,
+    },
+};
 
 /// Plugin that provides scene functionality to an [`App`].
 #[derive(Default)]
@@ -64,6 +70,8 @@ impl Plugin for ScenePlugin {
             .init_resource::<SceneSpawner>()
             .register_type::<SceneRoot>()
             .register_type::<DynamicSceneRoot>()
+            .register_type::<IsResource>()
+            .register_type::<ResourceEntity<DefaultQueryFilters>>()
             .add_systems(SpawnScene, (scene_spawner, scene_spawner_system).chain());
 
         // Register component hooks for DynamicSceneRoot

crates/bevy_scene/src/scene_spawner.rs

@@ -609,7 +609,7 @@ mod tests {
         assert_eq!(scene_component_a.y, 4.0);
         assert_eq!(
             app.world().entity(entity).get::<Children>().unwrap().len(),
-            1
+            3 // two resources-as-entities are also counted
         );
 
         // let's try to delete the scene

crates/bevy_scene/src/serde.rs

@@ -516,9 +516,11 @@ mod tests {
     };
     use bevy_ecs::{
         entity::{Entity, EntityHashMap},
+        entity_disabling::DefaultQueryFilters,
         prelude::{Component, ReflectComponent, ReflectResource, Resource, World},
         query::{With, Without},
         reflect::AppTypeRegistry,
+        resource::{IsResource, ResourceEntity},
         world::FromWorld,
     };
     use bevy_reflect::{Reflect, ReflectDeserialize, ReflectSerialize};
@@ -611,6 +613,8 @@ mod tests {
             registry.register::<MyEntityRef>();
             registry.register::<Entity>();
             registry.register::<MyResource>();
+            registry.register::<IsResource>();
+            registry.register::<ResourceEntity<DefaultQueryFilters>>();
         }
         world.insert_resource(registry);
         world
@@ -638,20 +642,20 @@ mod tests {
     ),
   },
   entities: {
-    4294967293: (
+    4294967291: (
       components: {
         "bevy_scene::serde::tests::Bar": (345),
         "bevy_scene::serde::tests::Baz": (789),
         "bevy_scene::serde::tests::Foo": (123),
       },
     ),
-    4294967294: (
+    4294967292: (
       components: {
         "bevy_scene::serde::tests::Bar": (345),
         "bevy_scene::serde::tests::Foo": (123),
       },
     ),
-    4294967295: (
+    4294967293: (
       components: {
         "bevy_scene::serde::tests::Foo": (123),
       },
@@ -757,7 +761,7 @@ mod tests {
             .write_to_world(&mut dst_world, &mut map)
             .unwrap();
 
-        assert_eq!(2, deserialized_scene.entities.len());
+        assert_eq!(4, deserialized_scene.entities.len());
         assert_scene_eq(&scene, &deserialized_scene);
 
         let bar_to_foo = dst_world
@@ -785,7 +789,7 @@ mod tests {
 
         let (scene, deserialized_scene) = roundtrip_ron(&world);
 
-        assert_eq!(1, deserialized_scene.entities.len());
+        assert_eq!(3, deserialized_scene.entities.len());
         assert_scene_eq(&scene, &deserialized_scene);
 
         let mut world = create_world();
@@ -815,10 +819,19 @@ mod tests {
 
         assert_eq!(
             vec![
-                0, 1, 255, 255, 255, 255, 15, 1, 37, 98, 101, 118, 121, 95, 115, 99, 101, 110, 101,
+                0, 3, 253, 255, 255, 255, 15, 1, 37, 98, 101, 118, 121, 95, 115, 99, 101, 110, 101,
                 58, 58, 115, 101, 114, 100, 101, 58, 58, 116, 101, 115, 116, 115, 58, 58, 77, 121,
                 67, 111, 109, 112, 111, 110, 101, 110, 116, 1, 2, 3, 102, 102, 166, 63, 205, 204,
-                108, 64, 1, 12, 72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33
+                108, 64, 1, 12, 72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33, 254, 255,
+                255, 255, 15, 1, 30, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58, 114, 101, 115,
+                111, 117, 114, 99, 101, 58, 58, 73, 115, 82, 101, 115, 111, 117, 114, 99, 101, 255,
+                255, 255, 255, 15, 2, 30, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58, 114, 101,
+                115, 111, 117, 114, 99, 101, 58, 58, 73, 115, 82, 101, 115, 111, 117, 114, 99, 101,
+                83, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58, 114, 101, 115, 111, 117, 114, 99,
+                101, 58, 58, 82, 101, 115, 111, 117, 114, 99, 101, 69, 110, 116, 105, 116, 121, 60,
+                98, 101, 118, 121, 95, 101, 99, 115, 58, 58, 101, 110, 116, 105, 116, 121, 95, 100,
+                105, 115, 97, 98, 108, 105, 110, 103, 58, 58, 68, 101, 102, 97, 117, 108, 116, 81,
+                117, 101, 114, 121, 70, 105, 108, 116, 101, 114, 115, 62
             ],
             serialized_scene
         );
@@ -830,7 +843,7 @@ mod tests {
             .deserialize(&mut postcard::Deserializer::from_bytes(&serialized_scene))
             .unwrap();
 
-        assert_eq!(1, deserialized_scene.entities.len());
+        assert_eq!(3, deserialized_scene.entities.len());
         assert_scene_eq(&scene, &deserialized_scene);
     }
 
@@ -856,11 +869,21 @@ mod tests {
 
         assert_eq!(
             vec![
-                146, 128, 129, 206, 255, 255, 255, 255, 145, 129, 217, 37, 98, 101, 118, 121, 95,
+                146, 128, 131, 206, 255, 255, 255, 253, 145, 129, 217, 37, 98, 101, 118, 121, 95,
                 115, 99, 101, 110, 101, 58, 58, 115, 101, 114, 100, 101, 58, 58, 116, 101, 115,
                 116, 115, 58, 58, 77, 121, 67, 111, 109, 112, 111, 110, 101, 110, 116, 147, 147, 1,
                 2, 3, 146, 202, 63, 166, 102, 102, 202, 64, 108, 204, 205, 129, 165, 84, 117, 112,
-                108, 101, 172, 72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33
+                108, 101, 172, 72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33, 206, 255,
+                255, 255, 254, 145, 129, 190, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58, 114,
+                101, 115, 111, 117, 114, 99, 101, 58, 58, 73, 115, 82, 101, 115, 111, 117, 114, 99,
+                101, 144, 206, 255, 255, 255, 255, 145, 130, 190, 98, 101, 118, 121, 95, 101, 99,
+                115, 58, 58, 114, 101, 115, 111, 117, 114, 99, 101, 58, 58, 73, 115, 82, 101, 115,
+                111, 117, 114, 99, 101, 144, 217, 83, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58,
+                114, 101, 115, 111, 117, 114, 99, 101, 58, 58, 82, 101, 115, 111, 117, 114, 99,
+                101, 69, 110, 116, 105, 116, 121, 60, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58,
+                101, 110, 116, 105, 116, 121, 95, 100, 105, 115, 97, 98, 108, 105, 110, 103, 58,
+                58, 68, 101, 102, 97, 117, 108, 116, 81, 117, 101, 114, 121, 70, 105, 108, 116,
+                101, 114, 115, 62, 144
             ],
             buf
         );
@@ -874,7 +897,7 @@ mod tests {
             .deserialize(&mut rmp_serde::Deserializer::new(&mut reader))
             .unwrap();
 
-        assert_eq!(1, deserialized_scene.entities.len());
+        assert_eq!(3, deserialized_scene.entities.len());
         assert_scene_eq(&scene, &deserialized_scene);
     }
 
@@ -899,13 +922,23 @@ mod tests {
 
         assert_eq!(
             vec![
-                0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 255, 255, 255, 255, 0, 0, 0, 0, 1,
+                0, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 253, 255, 255, 255, 0, 0, 0, 0, 1,
                 0, 0, 0, 0, 0, 0, 0, 37, 0, 0, 0, 0, 0, 0, 0, 98, 101, 118, 121, 95, 115, 99, 101,
                 110, 101, 58, 58, 115, 101, 114, 100, 101, 58, 58, 116, 101, 115, 116, 115, 58, 58,
                 77, 121, 67, 111, 109, 112, 111, 110, 101, 110, 116, 1, 0, 0, 0, 0, 0, 0, 0, 2, 0,
                 0, 0, 0, 0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 102, 102, 166, 63, 205, 204, 108, 64, 1,
                 0, 0, 0, 12, 0, 0, 0, 0, 0, 0, 0, 72, 101, 108, 108, 111, 32, 87, 111, 114, 108,
-                100, 33
+                100, 33, 254, 255, 255, 255, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 30, 0, 0, 0, 0, 0,
+                0, 0, 98, 101, 118, 121, 95, 101, 99, 115, 58, 58, 114, 101, 115, 111, 117, 114,
+                99, 101, 58, 58, 73, 115, 82, 101, 115, 111, 117, 114, 99, 101, 255, 255, 255, 255,
+                0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 30, 0, 0, 0, 0, 0, 0, 0, 98, 101, 118, 121, 95,
+                101, 99, 115, 58, 58, 114, 101, 115, 111, 117, 114, 99, 101, 58, 58, 73, 115, 82,
+                101, 115, 111, 117, 114, 99, 101, 83, 0, 0, 0, 0, 0, 0, 0, 98, 101, 118, 121, 95,
+                101, 99, 115, 58, 58, 114, 101, 115, 111, 117, 114, 99, 101, 58, 58, 82, 101, 115,
+                111, 117, 114, 99, 101, 69, 110, 116, 105, 116, 121, 60, 98, 101, 118, 121, 95,
+                101, 99, 115, 58, 58, 101, 110, 116, 105, 116, 121, 95, 100, 105, 115, 97, 98, 108,
+                105, 110, 103, 58, 58, 68, 101, 102, 97, 117, 108, 116, 81, 117, 101, 114, 121, 70,
+                105, 108, 116, 101, 114, 115, 62
             ],
             serialized_scene
         );
@@ -918,7 +951,7 @@ mod tests {
             bincode::serde::seed_decode_from_slice(scene_deserializer, &serialized_scene, config)
                 .unwrap();
 
-        assert_eq!(1, deserialized_scene.entities.len());
+        assert_eq!(3, deserialized_scene.entities.len());
         assert_scene_eq(&scene, &deserialized_scene);
     }