added ability to get Res<T> from World with World::get_resource_ref (#11561)

# Objective

It's sometimes desirable to get a `Res<T>` rather than `&T` from
`World::get_resource`.
Alternative to #9940, partly adresses #9926

## Solution

added additional methods to `World` and `UnsafeWorldCell` to retrieve a
resource wrapped in a `Res`.
- `UnsafeWorldCell::get_resource_ref`
- `World::get_resource_ref`
- `World::resource_ref`

I can change it so `World::resource_mut` returns `ResMut` instead of
`Mut` as well if that's desired, but that could also be added later in a
seperate pr.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Mike <mike.hsu@gmail.com>
Co-authored-by: MinerSebas <66798382+MinerSebas@users.noreply.github.com>

crates/bevy_ecs/src/world/mod.rs

@@ -28,7 +28,7 @@ use crate::{
     removal_detection::RemovedComponentEvents,
     schedule::{Schedule, ScheduleLabel, Schedules},
     storage::{ResourceData, Storages},
-    system::Resource,
+    system::{Res, Resource},
     world::error::TryRunScheduleError,
 };
 use bevy_ptr::{OwningPtr, Ptr};
@@ -1302,6 +1302,30 @@ impl World {
         }
     }
 
+    /// Gets a reference to the resource of the given type
+    ///
+    /// # Panics
+    ///
+    /// Panics if the resource does not exist.
+    /// Use [`get_resource_ref`](World::get_resource_ref) instead if you want to handle this case.
+    ///
+    /// If you want to instead insert a value if the resource does not exist,
+    /// use [`get_resource_or_insert_with`](World::get_resource_or_insert_with).
+    #[inline]
+    #[track_caller]
+    pub fn resource_ref<R: Resource>(&self) -> Res<R> {
+        match self.get_resource_ref() {
+            Some(x) => x,
+            None => panic!(
+                "Requested resource {} does not exist in the `World`.
+                Did you forget to add it using `app.insert_resource` / `app.init_resource`?
+                Resources are also implicitly added via `app.add_event`,
+                and can be added by plugins.",
+                std::any::type_name::<R>()
+            ),
+        }
+    }
+
     /// Gets a mutable reference to the resource of the given type
     ///
     /// # Panics
@@ -1335,6 +1359,15 @@ impl World {
         unsafe { self.as_unsafe_world_cell_readonly().get_resource() }
     }
 
+    /// 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>> {
+        // SAFETY:
+        // - `as_unsafe_world_cell_readonly` gives permission to access everything immutably
+        // - `&self` ensures nothing in world is borrowed mutably
+        unsafe { self.as_unsafe_world_cell_readonly().get_resource_ref() }
+    }
+
     /// Gets a mutable reference to the resource of the given type if it exists
     #[inline]
     pub fn get_resource_mut<R: Resource>(&mut self) -> Option<Mut<'_, R>> {
@@ -1875,7 +1908,7 @@ impl World {
     }
 
     /// Runs both [`clear_entities`](Self::clear_entities) and [`clear_resources`](Self::clear_resources),
-    /// invalidating all [`Entity`] and resource fetches such as [`Res`](crate::system::Res), [`ResMut`](crate::system::ResMut)
+    /// invalidating all [`Entity`] and resource fetches such as [`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::{
     prelude::Component,
     removal_detection::RemovedComponentEvents,
     storage::{Column, ComponentSparseSet, Storages},
-    system::Resource,
+    system::{Res, Resource},
 };
 use bevy_ptr::Ptr;
 use std::{any::TypeId, cell::UnsafeCell, fmt::Debug, marker::PhantomData};
@@ -342,6 +342,30 @@ impl<'w> UnsafeWorldCell<'w> {
         }
     }
 
+    /// Gets a reference including change detection to the resource of the given type if it exists.
+    ///
+    /// # Safety
+    /// It is the callers responsibility to ensure that
+    /// - 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>> {
+        let component_id = self.components().get_resource_id(TypeId::of::<R>())?;
+
+        // SAFETY: caller ensures `self` has permission to access the resource
+        // caller also ensure that no mutable reference to the resource exists
+        let (ptr, ticks) = unsafe { self.get_resource_with_ticks(component_id)? };
+
+        // SAFETY: `component_id` was obtained from the type ID of `R`
+        let value = unsafe { ptr.deref::<R>() };
+
+        // SAFETY: caller ensures that no mutable reference to the resource exists
+        let ticks =
+            unsafe { Ticks::from_tick_cells(ticks, self.last_change_tick(), self.change_tick()) };
+
+        Some(Res { value, ticks })
+    }
+
     /// Gets a pointer to the resource with the id [`ComponentId`] if it exists.
     /// The returned pointer must not be used to modify the resource, and must not be
     /// dereferenced after the borrow of the [`World`] ends.