remove ReflectMut in favor of Mut<dyn Reflect> (#5630)

# Objective

- `ReflectMut` served no purpose that wasn't met by `Mut<dyn Reflect>` which is easier to understand since you have to deal with fewer types
- there is another `ReflectMut` type that could be confused with this one

## Solution/Changelog

- relax `T: ?Sized` bound in `Mut<T>`
- replace all instances of `ReflectMut` with `Mut<dyn Reflect>`

crates/bevy_ecs/src/change_detection.rs

@@ -2,7 +2,6 @@
 
 use crate::{component::ComponentTicks, ptr::PtrMut, system::Resource};
 #[cfg(feature = "bevy_reflect")]
-use bevy_reflect::Reflect;
 use std::ops::{Deref, DerefMut};
 
 /// The (arbitrarily chosen) minimum number of world tick increments between `check_tick` scans.
@@ -230,7 +229,7 @@ impl<'a, T: 'static> From<NonSendMut<'a, T>> for Mut<'a, T> {
 }
 
 /// Unique mutable borrow of an entity's component
-pub struct Mut<'a, T> {
+pub struct Mut<'a, T: ?Sized> {
     pub(crate) value: &'a mut T,
     pub(crate) ticks: Ticks<'a>,
 }
@@ -239,18 +238,6 @@ change_detection_impl!(Mut<'a, T>, T,);
 impl_into_inner!(Mut<'a, T>, T,);
 impl_debug!(Mut<'a, T>,);
 
-/// Unique mutable borrow of a reflected component or resource
-#[cfg(feature = "bevy_reflect")]
-pub struct ReflectMut<'a> {
-    pub(crate) value: &'a mut dyn Reflect,
-    pub(crate) ticks: Ticks<'a>,
-}
-
-#[cfg(feature = "bevy_reflect")]
-change_detection_impl!(ReflectMut<'a>, dyn Reflect,);
-#[cfg(feature = "bevy_reflect")]
-impl_into_inner!(ReflectMut<'a>, dyn Reflect,);
-
 /// Unique mutable borrow of resources or an entity's component.
 ///
 /// Similar to [`Mut`], but not generic over the component type, instead

crates/bevy_ecs/src/reflect.rs

@@ -1,7 +1,7 @@
 //! Types that enable reflection support.
 
-pub use crate::change_detection::ReflectMut;
 use crate::{
+    change_detection::Mut,
     component::Component,
     entity::{Entity, EntityMap, MapEntities, MapEntitiesError},
     system::Resource,
@@ -23,7 +23,7 @@ pub struct ReflectComponent {
     apply_or_insert: fn(&mut World, Entity, &dyn Reflect),
     remove: fn(&mut World, Entity),
     reflect: fn(&World, Entity) -> Option<&dyn Reflect>,
-    reflect_mut: unsafe fn(&World, Entity) -> Option<ReflectMut>,
+    reflect_mut: unsafe fn(&World, Entity) -> Option<Mut<dyn Reflect>>,
     copy: fn(&World, &mut World, Entity, Entity),
 }
 
@@ -70,7 +70,11 @@ impl ReflectComponent {
     }
 
     /// Gets the value of this [`Component`] type from the entity as a mutable reflected reference.
-    pub fn reflect_mut<'a>(&self, world: &'a mut World, entity: Entity) -> Option<ReflectMut<'a>> {
+    pub fn reflect_mut<'a>(
+        &self,
+        world: &'a mut World,
+        entity: Entity,
+    ) -> Option<Mut<'a, dyn Reflect>> {
         // SAFETY: unique world access
         unsafe { (self.reflect_mut)(world, entity) }
     }
@@ -85,7 +89,7 @@ impl ReflectComponent {
         &self,
         world: &'a World,
         entity: Entity,
-    ) -> Option<ReflectMut<'a>> {
+    ) -> Option<Mut<'a, dyn Reflect>> {
         (self.reflect_mut)(world, entity)
     }
 
@@ -155,7 +159,7 @@ impl<C: Component + Reflect + FromWorld> FromType<C> for ReflectComponent {
                     world
                         .get_entity(entity)?
                         .get_unchecked_mut::<C>(world.last_change_tick(), world.read_change_tick())
-                        .map(|c| ReflectMut {
+                        .map(|c| Mut {
                             value: c.value as &mut dyn Reflect,
                             ticks: c.ticks,
                         })
@@ -176,7 +180,7 @@ pub struct ReflectResource {
     apply_or_insert: fn(&mut World, &dyn Reflect),
     remove: fn(&mut World),
     reflect: fn(&World) -> Option<&dyn Reflect>,
-    reflect_unchecked_mut: unsafe fn(&World) -> Option<ReflectMut>,
+    reflect_unchecked_mut: unsafe fn(&World) -> Option<Mut<dyn Reflect>>,
     copy: fn(&World, &mut World),
 }
 
@@ -211,7 +215,7 @@ impl ReflectResource {
     }
 
     /// 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<ReflectMut<'a>> {
+    pub fn reflect_mut<'a>(&self, world: &'a mut World) -> Option<Mut<'a, dyn Reflect>> {
         // SAFETY: unique world access
         unsafe { (self.reflect_unchecked_mut)(world) }
     }
@@ -222,7 +226,10 @@ impl ReflectResource {
     /// * Only call this method in an exclusive system to avoid sharing across threads (or use a
     ///   scheduler that enforces safe memory access).
     /// * Don't call this method more than once in the same scope for a given [`Resource`].
-    pub unsafe fn reflect_unchecked_mut<'a>(&self, world: &'a World) -> Option<ReflectMut<'a>> {
+    pub unsafe fn reflect_unchecked_mut<'a>(
+        &self,
+        world: &'a World,
+    ) -> Option<Mut<'a, dyn Reflect>> {
         // SAFETY: caller promises to uphold uniqueness guarantees
         (self.reflect_unchecked_mut)(world)
     }
@@ -266,12 +273,10 @@ impl<C: Resource + Reflect + FromWorld> FromType<C> for ReflectResource {
                 // 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
                 unsafe {
-                    world
-                        .get_resource_unchecked_mut::<C>()
-                        .map(|res| ReflectMut {
-                            value: res.value as &mut dyn Reflect,
-                            ticks: res.ticks,
-                        })
+                    world.get_resource_unchecked_mut::<C>().map(|res| Mut {
+                        value: res.value as &mut dyn Reflect,
+                        ticks: res.ticks,
+                    })
                 }
             },
             copy: |source_world, destination_world| {