Add register_resource_with_descriptor (#15501)

# Objective

- Fixes #15448.

## Solution

- Add `World::register_resource_with_descriptor` and
`Components::register_resource_with_descriptor`.

## Testing

- Added a test `dynamic_resource`.

crates/bevy_ecs/src/component.rs

@@ -836,7 +836,7 @@ pub struct Components {
 }
 
 impl Components {
-    /// Registers a component of type `T` with this instance.
+    /// Registers a [`Component`] of type `T` with this instance.
     /// If a component of this type has already been registered, this will return
     /// the ID of the pre-existing component.
     ///
@@ -876,9 +876,9 @@ impl Components {
 
     /// Registers a component described by `descriptor`.
     ///
-    /// ## Note
+    /// # Note
     ///
-    /// If this method is called multiple times with identical descriptors, a distinct `ComponentId`
+    /// If this method is called multiple times with identical descriptors, a distinct [`ComponentId`]
     /// will be created for each one.
     ///
     /// # See also
@@ -1034,6 +1034,7 @@ impl Components {
     /// # See also
     ///
     /// * [`Components::resource_id()`]
+    /// * [`Components::register_resource_with_descriptor()`]
     #[inline]
     pub fn register_resource<T: Resource>(&mut self) -> ComponentId {
         // SAFETY: The [`ComponentDescriptor`] matches the [`TypeId`]
@@ -1044,6 +1045,24 @@ impl Components {
         }
     }
 
+    /// Registers a [`Resource`] described by `descriptor`.
+    ///
+    /// # Note
+    ///
+    /// If this method is called multiple times with identical descriptors, a distinct [`ComponentId`]
+    /// will be created for each one.
+    ///
+    /// # See also
+    ///
+    /// * [`Components::resource_id()`]
+    /// * [`Components::register_resource()`]
+    pub fn register_resource_with_descriptor(
+        &mut self,
+        descriptor: ComponentDescriptor,
+    ) -> ComponentId {
+        Components::register_resource_inner(&mut self.components, descriptor)
+    }
+
     /// Registers a [non-send resource](crate::system::NonSend) of type `T` with this instance.
     /// If a resource of this type has already been registered, this will return
     /// the ID of the pre-existing resource.
@@ -1069,12 +1088,20 @@ impl Components {
         let components = &mut self.components;
         *self.resource_indices.entry(type_id).or_insert_with(|| {
             let descriptor = func();
-            let component_id = ComponentId(components.len());
+            Components::register_resource_inner(components, descriptor)
-            components.push(ComponentInfo::new(component_id, descriptor));
-            component_id
         })
     }
 
+    #[inline]
+    fn register_resource_inner(
+        components: &mut Vec<ComponentInfo>,
+        descriptor: ComponentDescriptor,
+    ) -> ComponentId {
+        let component_id = ComponentId(components.len());
+        components.push(ComponentInfo::new(component_id, descriptor));
+        component_id
+    }
+
     /// Gets an iterator over all components registered with this instance.
     pub fn iter(&self) -> impl Iterator<Item = &ComponentInfo> + '_ {
         self.components.iter()

crates/bevy_ecs/src/world/mod.rs

@@ -1320,6 +1320,23 @@ impl World {
             .map(Into::into)
     }
 
+    /// Registers a new [`Resource`] type and returns the [`ComponentId`] created for it.
+    ///
+    /// This enables the dynamic registration of new [`Resource`] definitions at runtime for
+    /// advanced use cases.
+    ///
+    /// # Note
+    ///
+    /// Registering a [`Resource`] does not insert it into [`World`]. For insertion, you could use
+    /// [`World::insert_resource_by_id`].
+    pub fn register_resource_with_descriptor(
+        &mut self,
+        descriptor: ComponentDescriptor,
+    ) -> ComponentId {
+        self.components
+            .register_resource_with_descriptor(descriptor)
+    }
+
     /// Initializes a new resource and returns the [`ComponentId`] created for it.
     ///
     /// If the resource already exists, nothing happens.
@@ -3248,6 +3265,39 @@ mod tests {
         );
     }
 
+    #[test]
+    fn dynamic_resource() {
+        let mut world = World::new();
+
+        let descriptor = ComponentDescriptor::new_resource::<TestResource>();
+
+        let component_id = world.register_resource_with_descriptor(descriptor);
+
+        let value = 0;
+        OwningPtr::make(value, |ptr| {
+            // SAFETY: value is valid for the layout of `TestResource`
+            unsafe {
+                world.insert_resource_by_id(
+                    component_id,
+                    ptr,
+                    #[cfg(feature = "track_change_detection")]
+                    panic::Location::caller(),
+                );
+            }
+        });
+
+        // SAFETY: We know that the resource is of type `TestResource`
+        let resource = unsafe {
+            world
+                .get_resource_by_id(component_id)
+                .unwrap()
+                .deref::<TestResource>()
+        };
+        assert_eq!(resource.0, 0);
+
+        assert!(world.remove_resource_by_id(component_id).is_some());
+    }
+
     #[test]
     fn custom_resource_with_layout() {
         static DROP_COUNT: AtomicU32 = AtomicU32::new(0);
@@ -3268,7 +3318,7 @@ mod tests {
             )
         };
 
-        let component_id = world.register_component_with_descriptor(descriptor);
+        let component_id = world.register_resource_with_descriptor(descriptor);
 
         let value: [u8; 8] = [0, 1, 2, 3, 4, 5, 6, 7];
         OwningPtr::make(value, |ptr| {