[Merged by Bors] - bevy_reflect: ReflectFromPtr to create &dyn Reflect from a *const ()

crates/bevy_reflect/Cargo.toml

@@ -16,6 +16,7 @@ bevy = ["glam", "smallvec"]
 # bevy
 bevy_reflect_derive = { path = "bevy_reflect_derive", version = "0.8.0-dev" }
 bevy_utils = { path = "../bevy_utils", version = "0.8.0-dev" }
+bevy_ptr = { path = "../bevy_ptr", version = "0.8.0-dev" }
 
 # other
 erased-serde = "0.3"

crates/bevy_reflect/bevy_reflect_derive/src/registration.rs

@@ -17,6 +17,7 @@ pub(crate) 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, Array, ArrayIter, DynamicMap, FromReflect, FromType,
     GetTypeRegistration, List, Map, MapIter, Reflect, ReflectDeserialize, ReflectMut, ReflectRef,
@@ -162,6 +162,7 @@ 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
     }
 }
@@ -292,6 +293,7 @@ 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
     }
 }
@@ -531,6 +533,7 @@ 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

@@ -1,9 +1,10 @@
 use crate::Reflect;
+use bevy_ptr::{Ptr, PtrMut};
 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, ptr::NonNull, sync::Arc};
 
 /// A registry of reflected types.
 #[derive(Default)]
@@ -379,11 +380,133 @@ impl<T: for<'a> Deserialize<'a> + Reflect> FromType<T> for ReflectDeserialize {
     }
 }
 
+/// [`Reflect`] values are commonly used in situations where the actual types of values
+/// are not known at runtime. In such situations you might have access to a `*const ()` pointer
+/// that you know implements [`Reflect`], but have no way of turning it into a `&dyn Reflect`.
+///
+/// This is where [`ReflectFromPtr`] comes in, when creating a [`ReflectFromPtr`] for a given type `T: Reflect`.
+/// Internally, this saves a concrete function `*const T -> const dyn Reflect` which lets you create a trait object of [`Reflect`]
+/// from a pointer.
+///
+/// # Example
+/// ```rust
+/// use bevy_reflect::{TypeRegistry, Reflect, ReflectFromPtr};
+/// use bevy_ptr::Ptr;
+/// use std::ptr::NonNull;
+///
+/// #[derive(Reflect)]
+/// struct Reflected(String);
+///
+/// let mut type_registry = TypeRegistry::default();
+/// type_registry.register::<Reflected>();
+///
+/// let value = Reflected("Hello world!".to_string());
+///
+/// let reflect_data = type_registry.get(std::any::TypeId::of::<Reflected>()).unwrap();
+/// let reflect_from_ptr = reflect_data.data::<ReflectFromPtr>().unwrap();
+/// // SAFE: `value` is of type `Reflected`, which the `ReflectFromPtr` was created for
+/// let value = unsafe { reflect_from_ptr.as_reflect(&value) };
+///
+/// assert_eq!(value.downcast_ref::<Reflected>().unwrap().0, "Hello world!");
+/// ```
+#[derive(Clone)]
+pub struct ReflectFromPtr {
+    type_id: TypeId,
+    to_reflect: for<'a> unsafe fn(Ptr<'a>) -> &'a dyn Reflect,
+    to_reflect_mut: for<'a> unsafe fn(PtrMut<'a>) -> &'a mut dyn Reflect,
+}
+
+impl ReflectFromPtr {
+    /// Returns the [`TypeId`] that the [`ReflectFromPtr`] was constructed for
+    pub fn type_id(&self) -> TypeId {
+        self.type_id
+    }
+
+    /// # Safety
+    ///
+    /// `val` must be a pointer to value of the type that the [`ReflectFromPtr`] was constructed for.
+    /// This can be verified by checking that the type id returned by [`ReflectFromPtr::type_id`] is the expected one.
+    pub unsafe fn as_reflect_ptr<'a>(&self, val: Ptr<'a>) -> &'a dyn Reflect {
+        (self.to_reflect)(val)
+    }
+
+    /// # Safety
+    ///
+    /// `val` must be a pointer to a value of the type that the [`ReflectFromPtr`] was constructed for
+    /// This can be verified by checking that the type id returned by [`ReflectFromPtr::type_id`] is the expected one.
+    pub unsafe fn as_reflect_ptr_mut<'a>(&self, val: PtrMut<'a>) -> &'a mut dyn Reflect {
+        (self.to_reflect_mut)(val)
+    }
+
+    pub fn as_reflect<'a, T: Reflect>(&self, val: &'a T) -> &'a dyn Reflect {
+        assert_eq!(self.type_id, std::any::TypeId::of::<T>());
+        // SAFE: `val` is of type `T`
+        unsafe { &*self.as_reflect_ptr(Ptr::new(NonNull::from(val).cast())) }
+    }
+
+    pub fn as_reflect_mut<'a, T: Reflect>(&self, val: &'a mut T) -> &'a mut dyn Reflect {
+        assert_eq!(self.type_id, std::any::TypeId::of::<T>());
+        // SAFE: `val` is of type `T`
+        unsafe { &mut *self.as_reflect_ptr_mut(PtrMut::new(NonNull::from(val).cast())) }
+    }
+}
+
+impl<T: Reflect> FromType<T> for ReflectFromPtr {
+    fn from_type() -> Self {
+        ReflectFromPtr {
+            type_id: std::any::TypeId::of::<T>(),
+            to_reflect: |ptr| {
+                // SAFE: only called from `as_reflect`, where the `ptr` is guaranteed to be of type `T`,
+                // and `as_reflect_ptr`, where the caller promises to call it with type `T`
+                unsafe { ptr.deref::<T>() as &dyn Reflect }
+            },
+            to_reflect_mut: |ptr| {
+                // SAFE: only called from `as_reflect_mut`, where the `ptr` is guaranteed to be of type `T`,
+                // and `as_reflect_ptr_mut`, where the caller promises to call it with type `T`
+                unsafe { ptr.deref_mut::<T>() as &mut dyn Reflect }
+            },
+        }
+    }
+}
+
 #[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.as_reflect_mut(&mut value);
+        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.as_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!(