bevy_reflect: Get owned fields (#5728)

# Objective

Sometimes it's useful to be able to retrieve all the fields of a container type so that they may be processed separately. With reflection, however, we typically only have access to references.

The only alternative is to "clone" the value using `Reflect::clone_value`. This, however, returns a Dynamic type in most cases. The solution there would be to use `FromReflect` instead, but this also has a problem in that it means we need to add `FromReflect` as an additional bound.

## Solution

Add a `drain` method to all container traits. This returns a `Vec<Box<dyn Reflect>>` (except for `Map` which returns `Vec<(Box<dyn Reflect>, Box<dyn Reflect>)>`).

This allows us to do things a lot simpler. For example, if we finished processing a struct and just need a particular value:

```rust
// === OLD === //
/// May or may not return a Dynamic*** value (even if `container` wasn't a `DynamicStruct`)
fn get_output(container: Box<dyn Struct>, output_index: usize) -> Box<dyn Reflect> {
  container.field_at(output_index).unwrap().clone_value()
}

// === NEW === //
/// Returns _exactly_ whatever was in the given struct
fn get_output(container: Box<dyn Struct>, output_index: usize) -> Box<dyn Reflect> {
  container.drain().remove(output_index).unwrap()
}
```

### Discussion

* Is `drain` the best method name? It makes sense that it "drains" all the fields and that it consumes the container in the process, but I'm open to alternatives.

---

## Changelog

* Added a `drain` method to the following traits:
  * `Struct`
  * `TupleStruct`
  * `Tuple`
  * `Array`
  * `List`
  * `Map`
  * `Enum`

crates/bevy_reflect/src/array.rs

@@ -30,6 +30,8 @@ pub trait Array: Reflect {
     }
     /// Returns an iterator over the collection.
     fn iter(&self) -> ArrayIter;
+    /// Drain the elements of this array to get a vector of owned values.
+    fn drain(self: Box<Self>) -> Vec<Box<dyn Reflect>>;
 
     fn clone_dynamic(&self) -> DynamicArray {
         DynamicArray {
@@ -246,6 +248,11 @@ impl Array for DynamicArray {
         }
     }
 
+    #[inline]
+    fn drain(self: Box<Self>) -> Vec<Box<dyn Reflect>> {
+        self.values.into_vec()
+    }
+
     #[inline]
     fn clone_dynamic(&self) -> DynamicArray {
         DynamicArray {

crates/bevy_reflect/src/impls/smallvec.rs

@@ -37,6 +37,12 @@ where
             index: 0,
         }
     }
+
+    fn drain(self: Box<Self>) -> Vec<Box<dyn Reflect>> {
+        self.into_iter()
+            .map(|value| Box::new(value) as Box<dyn Reflect>)
+            .collect()
+    }
 }
 
 impl<T: smallvec::Array + Send + Sync + 'static> List for SmallVec<T>

crates/bevy_reflect/src/impls/std.rs

@@ -123,6 +123,13 @@ impl<T: FromReflect> Array for Vec<T> {
             index: 0,
         }
     }
+
+    #[inline]
+    fn drain(self: Box<Self>) -> Vec<Box<dyn Reflect>> {
+        self.into_iter()
+            .map(|value| Box::new(value) as Box<dyn Reflect>)
+            .collect()
+    }
 }
 
 impl<T: FromReflect> List for Vec<T> {
@@ -261,6 +268,17 @@ impl<K: FromReflect + Eq + Hash, V: FromReflect> Map for HashMap<K, V> {
         }
     }
 
+    fn drain(self: Box<Self>) -> Vec<(Box<dyn Reflect>, Box<dyn Reflect>)> {
+        self.into_iter()
+            .map(|(key, value)| {
+                (
+                    Box::new(key) as Box<dyn Reflect>,
+                    Box::new(value) as Box<dyn Reflect>,
+                )
+            })
+            .collect()
+    }
+
     fn clone_dynamic(&self) -> DynamicMap {
         let mut dynamic_map = DynamicMap::default();
         dynamic_map.set_name(self.type_name().to_string());
@@ -409,6 +427,13 @@ impl<T: Reflect, const N: usize> Array for [T; N] {
             index: 0,
         }
     }
+
+    #[inline]
+    fn drain(self: Box<Self>) -> Vec<Box<dyn Reflect>> {
+        self.into_iter()
+            .map(|value| Box::new(value) as Box<dyn Reflect>)
+            .collect()
+    }
 }
 
 impl<T: Reflect, const N: usize> Reflect for [T; N] {

crates/bevy_reflect/src/lib.rs

@@ -519,6 +519,29 @@ mod tests {
         assert_eq!(foo, *foo2.downcast::<Foo>().unwrap());
     }
 
+    #[test]
+    fn should_drain_fields() {
+        let array_value: Box<dyn Array> = Box::new([123_i32, 321_i32]);
+        let fields = array_value.drain();
+        assert!(fields[0].reflect_partial_eq(&123_i32).unwrap_or_default());
+        assert!(fields[1].reflect_partial_eq(&321_i32).unwrap_or_default());
+
+        let list_value: Box<dyn List> = Box::new(vec![123_i32, 321_i32]);
+        let fields = list_value.drain();
+        assert!(fields[0].reflect_partial_eq(&123_i32).unwrap_or_default());
+        assert!(fields[1].reflect_partial_eq(&321_i32).unwrap_or_default());
+
+        let tuple_value: Box<dyn Tuple> = Box::new((123_i32, 321_i32));
+        let fields = tuple_value.drain();
+        assert!(fields[0].reflect_partial_eq(&123_i32).unwrap_or_default());
+        assert!(fields[1].reflect_partial_eq(&321_i32).unwrap_or_default());
+
+        let map_value: Box<dyn Map> = Box::new(HashMap::from([(123_i32, 321_i32)]));
+        let fields = map_value.drain();
+        assert!(fields[0].0.reflect_partial_eq(&123_i32).unwrap_or_default());
+        assert!(fields[0].1.reflect_partial_eq(&321_i32).unwrap_or_default());
+    }
+
     #[test]
     fn reflect_take() {
         #[derive(Reflect, Debug, PartialEq)]

crates/bevy_reflect/src/list.rs

@@ -137,6 +137,10 @@ impl Array for DynamicList {
         }
     }
 
+    fn drain(self: Box<Self>) -> Vec<Box<dyn Reflect>> {
+        self.values
+    }
+
     fn clone_dynamic(&self) -> DynamicArray {
         DynamicArray {
             name: self.name.clone(),

crates/bevy_reflect/src/map.rs

@@ -42,6 +42,9 @@ pub trait Map: Reflect {
     /// Returns an iterator over the key-value pairs of the map.
     fn iter(&self) -> MapIter;
 
+    /// Drain the key-value pairs of this map to get a vector of owned values.
+    fn drain(self: Box<Self>) -> Vec<(Box<dyn Reflect>, Box<dyn Reflect>)>;
+
     /// Clones the map, producing a [`DynamicMap`].
     fn clone_dynamic(&self) -> DynamicMap;
 
@@ -226,6 +229,10 @@ impl Map for DynamicMap {
             }
         }
     }
+
+    fn drain(self: Box<Self>) -> Vec<(Box<dyn Reflect>, Box<dyn Reflect>)> {
+        self.values
+    }
 }
 
 impl Reflect for DynamicMap {

crates/bevy_reflect/src/tuple.rs

@@ -40,6 +40,9 @@ pub trait Tuple: Reflect {
     /// Returns an iterator over the values of the tuple's fields.
     fn iter_fields(&self) -> TupleFieldIter;
 
+    /// Drain the fields of this tuple to get a vector of owned values.
+    fn drain(self: Box<Self>) -> Vec<Box<dyn Reflect>>;
+
     /// Clones the struct into a [`DynamicTuple`].
     fn clone_dynamic(&self) -> DynamicTuple;
 }
@@ -253,6 +256,11 @@ impl Tuple for DynamicTuple {
         }
     }
 
+    #[inline]
+    fn drain(self: Box<Self>) -> Vec<Box<dyn Reflect>> {
+        self.fields
+    }
+
     #[inline]
     fn clone_dynamic(&self) -> DynamicTuple {
         DynamicTuple {
@@ -451,6 +459,13 @@ macro_rules! impl_reflect_tuple {
                 }
             }
 
+            #[inline]
+            fn drain(self: Box<Self>) -> Vec<Box<dyn Reflect>> {
+                vec![
+                    $(Box::new(self.$index),)*
+                ]
+            }
+
             #[inline]
             fn clone_dynamic(&self) -> DynamicTuple {
                 let mut dyn_tuple = DynamicTuple {