bevy_reflect: ReflectFromPtr to create &dyn Reflect

crates/bevy_reflect/bevy_reflect_derive/src/lib.rs

@@ -719,6 +719,7 @@ fn impl_get_type_registration(
         impl #impl_generics #bevy_reflect_path::GetTypeRegistration for #type_name #ty_generics #where_clause {
             fn get_type_registration() -> #bevy_reflect_path::TypeRegistration {
                 let mut registration = #bevy_reflect_path::TypeRegistration::of::<#type_name #ty_generics>();
+                registration.insert::<#bevy_reflect_path::ReflectFromPtr>(#bevy_reflect_path::FromType::<#type_name #ty_generics>::from_type());
                 #(registration.insert::<#registration_data>(#bevy_reflect_path::FromType::<#type_name #ty_generics>::from_type());)*
                 registration
             }

crates/bevy_reflect/src/impls/std.rs

@@ -1,4 +1,4 @@
-use crate as bevy_reflect;
+use crate::{self as bevy_reflect, ReflectFromPtr};
 use crate::{
     map_partial_eq, serde::Serializable, DynamicMap, FromReflect, FromType, GetTypeRegistration,
     List, ListIter, Map, MapIter, Reflect, ReflectDeserialize, ReflectMut, ReflectRef,
@@ -148,6 +148,7 @@ impl<T: FromReflect + for<'de> Deserialize<'de>> GetTypeRegistration for Vec<T>
     fn get_type_registration() -> TypeRegistration {
         let mut registration = TypeRegistration::of::<Vec<T>>();
         registration.insert::<ReflectDeserialize>(FromType::<Vec<T>>::from_type());
+        registration.insert::<ReflectFromPtr>(FromType::<Vec<T>>::from_type());
         registration
     }
 }
@@ -270,6 +271,7 @@ where
     fn get_type_registration() -> TypeRegistration {
         let mut registration = TypeRegistration::of::<Self>();
         registration.insert::<ReflectDeserialize>(FromType::<Self>::from_type());
+        registration.insert::<ReflectFromPtr>(FromType::<Self>::from_type());
         registration
     }
 }
@@ -355,6 +357,7 @@ impl GetTypeRegistration for Cow<'static, str> {
     fn get_type_registration() -> TypeRegistration {
         let mut registration = TypeRegistration::of::<Cow<'static, str>>();
         registration.insert::<ReflectDeserialize>(FromType::<Cow<'static, str>>::from_type());
+        registration.insert::<ReflectFromPtr>(FromType::<Cow<'static, str>>::from_type());
         registration
     }
 }

crates/bevy_reflect/src/type_registry.rs

@@ -3,7 +3,7 @@ use bevy_utils::{HashMap, HashSet};
 use downcast_rs::{impl_downcast, Downcast};
 use parking_lot::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 use serde::Deserialize;
-use std::{any::TypeId, fmt::Debug, sync::Arc};
+use std::{any::TypeId, fmt::Debug, marker::PhantomData, sync::Arc};
 
 /// A registry of reflected types.
 #[derive(Default)]
@@ -350,11 +350,105 @@ impl<T: for<'a> Deserialize<'a> + Reflect> FromType<T> for ReflectDeserialize {
     }
 }
 
+#[derive(Clone)]
+pub struct ReflectFromPtr {
+    type_id: TypeId,
+    to_reflect: for<'a> unsafe fn(*const (), lt: PhantomData<&'a ()>) -> &'a dyn Reflect,
+    to_reflect_mut: for<'a> unsafe fn(*mut (), lt: PhantomData<&'a ()>) -> &'a mut dyn Reflect,
+}
+
+impl ReflectFromPtr {
+    /// Safety:
+    /// - `val` must be a pointer to a value of the type that the [`ReflectFromPtr`] was constructed for
+    /// - the lifetime `'a` of the return type can be arbitrarily chosen by the caller, who must ensure that during
+    /// that lifetime, `val` is valid
+    pub unsafe fn to_reflect_ptr<'a>(&self, val: *const ()) -> &'a dyn Reflect {
+        (self.to_reflect)(val, PhantomData)
+    }
+
+    /// Safety:
+    /// - `val` must be a pointer to a value of the type that the [`ReflectFromPtr`] was constructed for
+    /// - the lifetime `'a` of the return type can be arbitrarily chosen by the caller, who must ensure that during
+    /// that lifetime, `val` is valid and not aliased
+    pub unsafe fn to_reflect_ptr_mut<'a>(&self, val: *mut ()) -> &'a mut dyn Reflect {
+        (self.to_reflect_mut)(val, PhantomData)
+    }
+
+    pub fn to_reflect<'a, T: 'static>(&self, val: &'a T) -> &'a dyn Reflect {
+        assert_eq!(self.type_id, std::any::TypeId::of::<T>());
+        // SAFE: the lifetime of `val` is the same as the lifetime of the return value
+        // and the type of `val` has been checked to be the same correct one
+        unsafe { self.to_reflect_ptr(val as *const _ as *const ()) }
+    }
+
+    pub fn to_reflect_mut<'a, T: 'static>(&self, val: &'a mut T) -> &'a mut dyn Reflect {
+        assert_eq!(self.type_id, std::any::TypeId::of::<T>());
+        // SAFE: the lifetime of `val` is the same as the lifetime of the return value
+        // and the type of `val` has been checked to be the same correct one
+        unsafe { self.to_reflect_ptr_mut(val as *mut _ as *mut ()) }
+    }
+}
+
+impl<T: Reflect + 'static> FromType<T> for ReflectFromPtr {
+    fn from_type() -> Self {
+        ReflectFromPtr {
+            type_id: std::any::TypeId::of::<T>(),
+            to_reflect: |ptr, _lt| {
+                // SAFE: can only be called by `to_reflect_ptr` where the caller promises the safety requirements
+                // or `to_reflect` which is typed and checks that the correct type is used.
+                let val: &T = unsafe { &*ptr.cast::<T>() };
+                let val = val as &dyn Reflect;
+                val
+            },
+            to_reflect_mut: |ptr, _lt| {
+                // SAFE: can only be called by `to_reflect_ptr_mut` where the caller promises the safety requirements
+                // or `to_reflect_mut` which is typed and checks that the correct type is used.
+                let val: &mut T = unsafe { &mut *ptr.cast::<T>() };
+                let val = val as &mut dyn Reflect;
+                val
+            },
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
-    use crate::TypeRegistration;
+    use crate::{GetTypeRegistration, ReflectFromPtr, TypeRegistration};
     use bevy_utils::HashMap;
 
+    use crate as bevy_reflect;
+    use crate::Reflect;
+
+    #[test]
+    fn test_reflect_from_ptr() {
+        #[derive(Reflect)]
+        struct Foo {
+            a: f32,
+        }
+
+        let foo_registration = <Foo as GetTypeRegistration>::get_type_registration();
+        let reflect_from_ptr = foo_registration.data::<ReflectFromPtr>().unwrap();
+
+        let mut value = Foo { a: 1.0 };
+
+        let dyn_reflect = reflect_from_ptr.to_reflect_mut({ &mut value }); // make sure the mutable borrow is only alive on this line
+        match dyn_reflect.reflect_mut() {
+            bevy_reflect::ReflectMut::Struct(strukt) => {
+                strukt.field_mut("a").unwrap().apply(&2.0f32)
+            }
+            _ => panic!("invalid reflection"),
+        }
+
+        let dyn_reflect = reflect_from_ptr.to_reflect(&value);
+        match dyn_reflect.reflect_ref() {
+            bevy_reflect::ReflectRef::Struct(strukt) => {
+                let a = strukt.field("a").unwrap().downcast_ref::<f32>().unwrap();
+                assert_eq!(*a, 2.0);
+            }
+            _ => panic!("invalid reflection"),
+        }
+    }
+
     #[test]
     fn test_get_short_name() {
         assert_eq!(