Make Reflect impls unsafe (Reflect::any must return self) (#1679)

Fixes #1100 

Implementors must make sure that `Reflect::any` and `Reflect::any_mut` both return the `self` reference passed in (both for logical correctness and downcast safety).

crates/bevy_reflect/bevy_reflect_derive/src/lib.rs

@@ -261,7 +261,8 @@ fn impl_struct(
             }
         }
 
-        impl #impl_generics #bevy_reflect_path::Reflect for #struct_name#ty_generics #where_clause {
+        // SAFE: any and any_mut both return self
+        unsafe impl #impl_generics #bevy_reflect_path::Reflect for #struct_name#ty_generics #where_clause {
             #[inline]
             fn type_name(&self) -> &str {
                 std::any::type_name::<Self>()
@@ -382,7 +383,8 @@ fn impl_tuple_struct(
             }
         }
 
-        impl #impl_generics #bevy_reflect_path::Reflect for #struct_name#ty_generics {
+        // SAFE: any and any_mut both return self
+        unsafe impl #impl_generics #bevy_reflect_path::Reflect for #struct_name#ty_generics {
             #[inline]
             fn type_name(&self) -> &str {
                 std::any::type_name::<Self>()
@@ -457,7 +459,8 @@ fn impl_value(
     TokenStream::from(quote! {
         #get_type_registration_impl
 
-        impl #impl_generics #bevy_reflect_path::Reflect for #type_name#ty_generics #where_clause  {
+        // SAFE: any and any_mut both return self
+        unsafe impl #impl_generics #bevy_reflect_path::Reflect for #type_name#ty_generics #where_clause  {
             #[inline]
             fn type_name(&self) -> &str {
                 std::any::type_name::<Self>()

crates/bevy_reflect/src/impls/smallvec.rs

@@ -45,7 +45,8 @@ where
     }
 }
 
-impl<T: Array + Send + Sync + 'static> Reflect for SmallVec<T>
+// SAFE: any and any_mut both return self
+unsafe impl<T: Array + Send + Sync + 'static> Reflect for SmallVec<T>
 where
     T::Item: Reflect + Clone,
 {

crates/bevy_reflect/src/impls/std.rs

@@ -65,7 +65,8 @@ impl<T: Reflect> List for Vec<T> {
     }
 }
 
-impl<T: Reflect> Reflect for Vec<T> {
+// SAFE: any and any_mut both return self
+unsafe impl<T: Reflect> Reflect for Vec<T> {
     fn type_name(&self) -> &str {
         std::any::type_name::<Self>()
     }
@@ -160,7 +161,8 @@ impl<K: Reflect + Clone + Eq + Hash, V: Reflect + Clone> Map for HashMap<K, V> {
     }
 }
 
-impl<K: Reflect + Clone + Eq + Hash, V: Reflect + Clone> Reflect for HashMap<K, V> {
+// SAFE: any and any_mut both return self
+unsafe impl<K: Reflect + Clone + Eq + Hash, V: Reflect + Clone> Reflect for HashMap<K, V> {
     fn type_name(&self) -> &str {
         std::any::type_name::<Self>()
     }
@@ -227,7 +229,8 @@ where
     }
 }
 
-impl Reflect for Cow<'static, str> {
+// SAFE: any and any_mut both return self
+unsafe impl Reflect for Cow<'static, str> {
     fn type_name(&self) -> &str {
         std::any::type_name::<Self>()
     }

crates/bevy_reflect/src/list.rs

@@ -80,7 +80,8 @@ impl List for DynamicList {
     }
 }
 
-impl Reflect for DynamicList {
+// SAFE: any and any_mut both return self
+unsafe impl Reflect for DynamicList {
     #[inline]
     fn type_name(&self) -> &str {
         self.name.as_str()

crates/bevy_reflect/src/map.rs

@@ -98,7 +98,8 @@ impl Map for DynamicMap {
     }
 }
 
-impl Reflect for DynamicMap {
+// SAFE: any and any_mut both return self
+unsafe impl Reflect for DynamicMap {
     fn type_name(&self) -> &str {
         &self.name
     }

crates/bevy_reflect/src/reflect.rs

@@ -22,7 +22,11 @@ pub enum ReflectMut<'a> {
 }
 
 /// A reflected rust type.
-pub trait Reflect: Any + Send + Sync {
+///
+/// # Safety
+/// Implementors _must_ ensure that [Reflect::any] and [Reflect::any_mut] both return the `self` value passed in
+/// If this is not done, [Reflect::downcast] will be UB (and also just logically broken).
+pub unsafe trait Reflect: Any + Send + Sync {
     fn type_name(&self) -> &str;
     fn any(&self) -> &dyn Any;
     fn any_mut(&mut self) -> &mut dyn Any;

crates/bevy_reflect/src/struct_trait.rs

@@ -167,7 +167,8 @@ impl Struct for DynamicStruct {
     }
 }
 
-impl Reflect for DynamicStruct {
+// SAFE: any and any_mut both return self
+unsafe impl Reflect for DynamicStruct {
     #[inline]
     fn type_name(&self) -> &str {
         &self.name

crates/bevy_reflect/src/tuple.rs

@@ -139,7 +139,8 @@ impl Tuple for DynamicTuple {
     }
 }
 
-impl Reflect for DynamicTuple {
+// SAFE: any and any_mut both return self
+unsafe impl Reflect for DynamicTuple {
     #[inline]
     fn type_name(&self) -> &str {
         self.name()
@@ -274,7 +275,8 @@ macro_rules! impl_reflect_tuple {
             }
         }
 
-        impl<$($name: Reflect),*> Reflect for ($($name,)*) {
+        // SAFE: any and any_mut both return self
+        unsafe impl<$($name: Reflect),*> Reflect for ($($name,)*) {
             fn type_name(&self) -> &str {
                 std::any::type_name::<Self>()
             }

crates/bevy_reflect/src/tuple_struct.rs

@@ -123,7 +123,8 @@ impl TupleStruct for DynamicTupleStruct {
     }
 }
 
-impl Reflect for DynamicTupleStruct {
+// SAFE: any and any_mut both return self
+unsafe impl Reflect for DynamicTupleStruct {
     #[inline]
     fn type_name(&self) -> &str {
         self.name.as_str()