Support using FilteredResources with ReflectResource. (#15624)

# Objective

Support accessing resources using reflection when using
`FilteredResources` in a dynamic system. This is similar to how
components can be queried using reflection when using
`FilteredEntityRef|Mut`.

## Solution

Change `ReflectResource` from taking `&World` and `&mut World` to taking
`impl Into<FilteredResources>` and `impl Into<FilteredResourcesMut>`,
similar to how `ReflectComponent` takes `impl Into<FilteredEntityRef>`
and `impl Into<FilteredEntityMut>`. There are `From` impls that ensure
code passing `&World` and `&mut World` continues to work as before.

## Migration Guide

If you are manually creating a `ReflectComponentFns` struct, the
`reflect` function now takes `FilteredResources` instead `&World`, and
there is a new `reflect_mut` function that takes `FilteredResourcesMut`.

crates/bevy_ecs/src/reflect/resource.rs

@@ -8,7 +8,7 @@ use crate::{
     change_detection::Mut,
     component::ComponentId,
     resource::Resource,
-    world::{unsafe_world_cell::UnsafeWorldCell, World},
+    world::{unsafe_world_cell::UnsafeWorldCell, FilteredResources, FilteredResourcesMut, World},
 };
 use bevy_reflect::{FromReflect, FromType, PartialReflect, Reflect, TypePath, TypeRegistry};
 
@@ -52,7 +52,9 @@ pub struct ReflectResourceFns {
     /// Function pointer implementing [`ReflectResource::remove()`].
     pub remove: fn(&mut World),
     /// Function pointer implementing [`ReflectResource::reflect()`].
-    pub reflect: fn(&World) -> Option<&dyn Reflect>,
+    pub reflect: for<'w> fn(FilteredResources<'w, '_>) -> Option<&'w dyn Reflect>,
+    /// Function pointer implementing [`ReflectResource::reflect_mut()`].
+    pub reflect_mut: for<'w> fn(FilteredResourcesMut<'w, '_>) -> Option<Mut<'w, dyn Reflect>>,
     /// Function pointer implementing [`ReflectResource::reflect_unchecked_mut()`].
     ///
     /// # Safety
@@ -111,14 +113,23 @@ impl ReflectResource {
     }
 
     /// Gets the value of this [`Resource`] type from the world as a reflected reference.
-    pub fn reflect<'a>(&self, world: &'a World) -> Option<&'a dyn Reflect> {
-        (self.0.reflect)(world)
+    ///
+    /// Note that [`&World`](World) is a valid type for `resources`.
+    pub fn reflect<'w, 's>(
+        &self,
+        resources: impl Into<FilteredResources<'w, 's>>,
+    ) -> Option<&'w dyn Reflect> {
+        (self.0.reflect)(resources.into())
     }
 
     /// Gets the value of this [`Resource`] type from the world as a mutable reflected reference.
-    pub fn reflect_mut<'a>(&self, world: &'a mut World) -> Option<Mut<'a, dyn Reflect>> {
-        // SAFETY: unique world access
-        unsafe { (self.0.reflect_unchecked_mut)(world.as_unsafe_world_cell()) }
+    ///
+    /// Note that [`&mut World`](World) is a valid type for `resources`.
+    pub fn reflect_mut<'w, 's>(
+        &self,
+        resources: impl Into<FilteredResourcesMut<'w, 's>>,
+    ) -> Option<Mut<'w, dyn Reflect>> {
+        (self.0.reflect_mut)(resources.into())
     }
 
     /// # Safety
@@ -212,7 +223,12 @@ impl<R: Resource + FromReflect + TypePath> FromType<R> for ReflectResource {
             remove: |world| {
                 world.remove_resource::<R>();
             },
-            reflect: |world| world.get_resource::<R>().map(|res| res as &dyn Reflect),
+            reflect: |world| world.get::<R>().map(|res| res.into_inner() as &dyn Reflect),
+            reflect_mut: |world| {
+                world
+                    .into_mut::<R>()
+                    .map(|res| res.map_unchanged(|value| value as &mut dyn Reflect))
+            },
             reflect_unchecked_mut: |world| {
                 // SAFETY: all usages of `reflect_unchecked_mut` guarantee that there is either a single mutable
                 // reference or multiple immutable ones alive at any given point

crates/bevy_ecs/src/system/builder.rs

@@ -715,9 +715,11 @@ mod tests {
     use crate::{
         entity::Entities,
         prelude::{Component, Query},
+        reflect::ReflectResource,
         system::{Local, RunSystemOnce},
     };
     use alloc::vec;
+    use bevy_reflect::{FromType, Reflect, ReflectRef};
 
     use super::*;
 
@@ -730,8 +732,11 @@ mod tests {
     #[derive(Component)]
     struct C;
 
-    #[derive(Resource, Default)]
-    struct R;
+    #[derive(Resource, Default, Reflect)]
+    #[reflect(Resource)]
+    struct R {
+        foo: usize,
+    }
 
     fn local_system(local: Local<u64>) -> u64 {
         *local
@@ -1071,4 +1076,31 @@ mod tests {
             .build_state(&mut world)
             .build_system(|_r: ResMut<R>, _fr: FilteredResourcesMut| {});
     }
+
+    #[test]
+    fn filtered_resource_reflect() {
+        let mut world = World::new();
+        world.insert_resource(R { foo: 7 });
+
+        let system = (FilteredResourcesParamBuilder::new(|builder| {
+            builder.add_read::<R>();
+        }),)
+            .build_state(&mut world)
+            .build_system(|res: FilteredResources| {
+                let reflect_resource = <ReflectResource as FromType<R>>::from_type();
+                let ReflectRef::Struct(reflect_struct) =
+                    reflect_resource.reflect(res).unwrap().reflect_ref()
+                else {
+                    panic!()
+                };
+                *reflect_struct
+                    .field("foo")
+                    .unwrap()
+                    .try_downcast_ref::<usize>()
+                    .unwrap()
+            });
+
+        let output = world.run_system_once(system).unwrap();
+        assert_eq!(output, 7);
+    }
 }