change return type of World::resource_ref to Ref (#15263)

# Objective

Closes #11825

## Solution

Change return type of `get_resource_ref` and `resource_ref` from `Res`
to `Ref` and implement `From Res<T> for Ref<T>`.

crates/bevy_ecs/src/change_detection.rs

@@ -587,6 +587,19 @@ impl<'w, T: Resource> From<ResMut<'w, T>> for Res<'w, T> {
     }
 }
 
+impl<'w, T: Resource> From<Res<'w, T>> for Ref<'w, T> {
+    /// Convert a `Res` into a `Ref`. This allows keeping the change-detection feature of `Ref`
+    /// while losing the specificity of `Res` for resources.
+    fn from(res: Res<'w, T>) -> Self {
+        Self {
+            value: res.value,
+            ticks: res.ticks,
+            #[cfg(feature = "track_change_detection")]
+            changed_by: res.changed_by,
+        }
+    }
+}
+
 impl<'w, 'a, T: Resource> IntoIterator for &'a Res<'w, T>
 where
     &'a T: IntoIterator,

crates/bevy_ecs/src/world/mod.rs

@@ -40,7 +40,7 @@ use crate::{
     removal_detection::RemovedComponentEvents,
     schedule::{Schedule, ScheduleLabel, Schedules},
     storage::{ResourceData, Storages},
-    system::{Commands, Res, Resource},
+    system::{Commands, Resource},
     world::{command_queue::RawCommandQueue, error::TryRunScheduleError},
 };
 use bevy_ptr::{OwningPtr, Ptr};
@@ -1612,7 +1612,7 @@ impl World {
     /// use [`get_resource_or_insert_with`](World::get_resource_or_insert_with).
     #[inline]
     #[track_caller]
-    pub fn resource_ref<R: Resource>(&self) -> Res<R> {
+    pub fn resource_ref<R: Resource>(&self) -> Ref<R> {
         match self.get_resource_ref() {
             Some(x) => x,
             None => panic!(
@@ -1660,7 +1660,7 @@ impl World {
 
     /// Gets a reference including change detection to the resource of the given type if it exists.
     #[inline]
-    pub fn get_resource_ref<R: Resource>(&self) -> Option<Res<R>> {
+    pub fn get_resource_ref<R: Resource>(&self) -> Option<Ref<R>> {
         // SAFETY:
         // - `as_unsafe_world_cell_readonly` gives permission to access everything immutably
         // - `&self` ensures nothing in world is borrowed mutably
@@ -2400,7 +2400,7 @@ impl World {
     }
 
     /// Runs both [`clear_entities`](Self::clear_entities) and [`clear_resources`](Self::clear_resources),
-    /// invalidating all [`Entity`] and resource fetches such as [`Res`], [`ResMut`](crate::system::ResMut)
+    /// invalidating all [`Entity`] and resource fetches such as [`Res`](crate::system::Res), [`ResMut`](crate::system::ResMut)
     pub fn clear_all(&mut self) {
         self.clear_entities();
         self.clear_resources();

crates/bevy_ecs/src/world/unsafe_world_cell.rs

@@ -14,7 +14,7 @@ use crate::{
     query::{DebugCheckedUnwrap, ReadOnlyQueryData},
     removal_detection::RemovedComponentEvents,
     storage::{ComponentSparseSet, Storages, Table},
-    system::{Res, Resource},
+    system::Resource,
     world::RawCommandQueue,
 };
 use bevy_ptr::Ptr;
@@ -353,7 +353,7 @@ impl<'w> UnsafeWorldCell<'w> {
     /// - the [`UnsafeWorldCell`] has permission to access the resource
     /// - no mutable reference to the resource exists at the same time
     #[inline]
-    pub unsafe fn get_resource_ref<R: Resource>(self) -> Option<Res<'w, R>> {
+    pub unsafe fn get_resource_ref<R: Resource>(self) -> Option<Ref<'w, R>> {
         let component_id = self.components().get_resource_id(TypeId::of::<R>())?;
 
         // SAFETY: caller ensures `self` has permission to access the resource
@@ -371,7 +371,7 @@ impl<'w> UnsafeWorldCell<'w> {
         #[cfg(feature = "track_change_detection")]
         let caller = unsafe { _caller.deref() };
 
-        Some(Res {
+        Some(Ref {
             value,
             ticks,
             #[cfg(feature = "track_change_detection")]