bevy_reflect: Better proxies (#6971)

# Objective

> This PR is based on discussion from #6601

The Dynamic types (e.g. `DynamicStruct`, `DynamicList`, etc.) act as
both:
1. Dynamic containers which may hold any arbitrary data
2. Proxy types which may represent any other type

Currently, the only way we can represent the proxy-ness of a Dynamic is
by giving it a name.

```rust
// This is just a dynamic container
let mut data = DynamicStruct::default();

// This is a "proxy"
data.set_name(std::any::type_name::<Foo>());
```

This type name is the only way we check that the given Dynamic is a
proxy of some other type. When we need to "assert the type" of a `dyn
Reflect`, we call `Reflect::type_name` on it. However, because we're
only using a string to denote the type, we run into a few gotchas and
limitations.

For example, hashing a Dynamic proxy may work differently than the type
it proxies:

```rust
#[derive(Reflect, Hash)]
#[reflect(Hash)]
struct Foo(i32);

let concrete = Foo(123);
let dynamic = concrete.clone_dynamic();

let concrete_hash = concrete.reflect_hash();
let dynamic_hash = dynamic.reflect_hash();

// The hashes are not equal because `concrete` uses its own `Hash` impl
// while `dynamic` uses a reflection-based hashing algorithm
assert_ne!(concrete_hash, dynamic_hash);
```

Because the Dynamic proxy only knows about the name of the type, it's
unaware of any other information about it. This means it also differs on
`Reflect::reflect_partial_eq`, and may include ignored or skipped fields
in places the concrete type wouldn't.

## Solution

Rather than having Dynamics pass along just the type name of proxied
types, we can instead have them pass around the `TypeInfo`.

Now all Dynamic types contain an `Option<&'static TypeInfo>` rather than
a `String`:

```diff
pub struct DynamicTupleStruct {
-    type_name: String,
+    represented_type: Option<&'static TypeInfo>,
    fields: Vec<Box<dyn Reflect>>,
}
```

By changing `Reflect::get_type_info` to
`Reflect::represented_type_info`, hopefully we make this behavior a
little clearer. And to account for `None` values on these dynamic types,
`Reflect::represented_type_info` now returns `Option<&'static
TypeInfo>`.

```rust
let mut data = DynamicTupleStruct::default();

// Not proxying any specific type
assert!(dyn_tuple_struct.represented_type_info().is_none());

let type_info = <Foo as Typed>::type_info();
dyn_tuple_struct.set_represented_type(Some(type_info));
// Alternatively:
// let dyn_tuple_struct = foo.clone_dynamic();

// Now we're proxying `Foo`
assert!(dyn_tuple_struct.represented_type_info().is_some());
```

This means that we can have full access to all the static type
information for the proxied type. Future work would include
transitioning more static type information (trait impls, attributes,
etc.) over to the `TypeInfo` so it can actually be utilized by Dynamic
proxies.

### Alternatives & Rationale

> **Note** 
> These alternatives were written when this PR was first made using a
`Proxy` trait. This trait has since been removed.

<details>
<summary>View</summary>

#### Alternative: The `Proxy<T>` Approach

I had considered adding something like a `Proxy<T>` type where `T` would
be the Dynamic and would contain the proxied type information.

This was nice in that it allows us to explicitly determine whether
something is a proxy or not at a type level. `Proxy<DynamicStruct>`
proxies a struct. Makes sense.

The reason I didn't go with this approach is because (1) tuples, (2)
complexity, and (3) `PartialReflect`.

The `DynamicTuple` struct allows us to represent tuples at runtime. It
also allows us to do something you normally can't with tuples: add new
fields. Because of this, adding a field immediately invalidates the
proxy (e.g. our info for `(i32, i32)` doesn't apply to `(i32, i32,
NewField)`). By going with this PR's approach, we can just remove the
type info on `DynamicTuple` when that happens. However, with the
`Proxy<T>` approach, it becomes difficult to represent this behavior—
we'd have to completely control how we access data for `T` for each `T`.

Secondly, it introduces some added complexities (aside from the manual
impls for each `T`). Does `Proxy<T>` impl `Reflect`? Likely yes, if we
want to represent it as `dyn Reflect`. What `TypeInfo` do we give it?
How would we forward reflection methods to the inner type (remember, we
don't have specialization)? How do we separate this from Dynamic types?
And finally, how do all this in a way that's both logical and intuitive
for users?

Lastly, introducing a `Proxy` trait rather than a `Proxy<T>` struct is
actually more inline with the [Unique Reflect
RFC](bevyengine/rfcs#56). In a way, the `Proxy`
trait is really one part of the `PartialReflect` trait introduced in
that RFC (it's technically not in that RFC but it fits well with it),
where the `PartialReflect` serves as a way for proxies to work _like_
concrete types without having full access to everything a concrete
`Reflect` type can do. This would help bridge the gap between the
current state of the crate and the implementation of that RFC.

All that said, this is still a viable solution. If the community
believes this is the better path forward, then we can do that instead.
These were just my reasons for not initially going with it in this PR.

#### Alternative: The Type Registry Approach

The `Proxy` trait is great and all, but how does it solve the original
problem? Well, it doesn't— yet!

The goal would be to start moving information from the derive macro and
its attributes to the generated `TypeInfo` since these are known
statically and shouldn't change. For example, adding `ignored: bool` to
`[Un]NamedField` or a list of impls.

However, there is another way of storing this information. This is, of
course, one of the uses of the `TypeRegistry`. If we're worried about
Dynamic proxies not aligning with their concrete counterparts, we could
move more type information to the registry and require its usage.

For example, we could replace `Reflect::reflect_hash(&self)` with
`Reflect::reflect_hash(&self, registry: &TypeRegistry)`.

That's not the _worst_ thing in the world, but it is an ergonomics loss.

Additionally, other attributes may have their own requirements, further
restricting what's possible without the registry. The `Reflect::apply`
method will require the registry as well now. Why? Well because the
`map_apply` function used for the `Reflect::apply` impls on `Map` types
depends on `Map::insert_boxed`, which (at least for `DynamicMap`)
requires `Reflect::reflect_hash`. The same would apply when adding
support for reflection-based diffing, which will require
`Reflect::reflect_partial_eq`.

Again, this is a totally viable alternative. I just chose not to go with
it for the reasons above. If we want to go with it, then we can close
this PR and we can pursue this alternative instead.

#### Downsides

Just to highlight a quick potential downside (likely needs more
investigation): retrieving the `TypeInfo` requires acquiring a lock on
the `GenericTypeInfoCell` used by the `Typed` impls for generic types
(non-generic types use a `OnceBox which should be faster). I am not sure
how much of a performance hit that is and will need to run some
benchmarks to compare against.

</details>

### Open Questions

1. Should we use `Cow<'static, TypeInfo>` instead? I think that might be
easier for modding? Perhaps, in that case, we need to update
`Typed::type_info` and friends as well?
2. Are the alternatives better than the approach this PR takes? Are
there other alternatives?

---

## Changelog

### Changed

- `Reflect::get_type_info` has been renamed to
`Reflect::represented_type_info`
- This method now returns `Option<&'static TypeInfo>` rather than just
`&'static TypeInfo`

### Added

- Added `Reflect::is_dynamic` method to indicate when a type is dynamic
- Added a `set_represented_type` method on all dynamic types

### Removed

- Removed `TypeInfo::Dynamic` (use `Reflect::is_dynamic` instead)
- Removed `Typed` impls for all dynamic types

## Migration Guide

- The Dynamic types no longer take a string type name. Instead, they
require a static reference to `TypeInfo`:

    ```rust
    #[derive(Reflect)]
    struct MyTupleStruct(f32, f32);
    
    let mut dyn_tuple_struct = DynamicTupleStruct::default();
    dyn_tuple_struct.insert(1.23_f32);
    dyn_tuple_struct.insert(3.21_f32);
    
    // BEFORE:
    let type_name = std::any::type_name::<MyTupleStruct>();
    dyn_tuple_struct.set_name(type_name);
    
    // AFTER:
    let type_info = <MyTupleStruct as Typed>::type_info();
    dyn_tuple_struct.set_represented_type(Some(type_info));
    ```

- `Reflect::get_type_info` has been renamed to
`Reflect::represented_type_info` and now also returns an
`Option<&'static TypeInfo>` (instead of just `&'static TypeInfo`):

    ```rust
    // BEFORE:
    let info: &'static TypeInfo = value.get_type_info();
    // AFTER:
let info: &'static TypeInfo = value.represented_type_info().unwrap();
    ```

- `TypeInfo::Dynamic` and `DynamicInfo` has been removed. Use
`Reflect::is_dynamic` instead:
   
    ```rust
    // BEFORE:
    if matches!(value.get_type_info(), TypeInfo::Dynamic) {
      // ...
    }
    // AFTER:
    if value.is_dynamic() {
      // ...
    }
    ```

---------

Co-authored-by: radiish <[email protected]>

Author: MrGVSV

crates/bevy_reflect/bevy_reflect_derive/src/impls/enums.rs

@@ -192,8 +192,8 @@ pub(crate) fn impl_enum(reflect_enum: &ReflectEnum) -> TokenStream {
             }
 
             #[inline]
-            fn get_type_info(&self) -> &'static #bevy_reflect_path::TypeInfo {
-                <Self as #bevy_reflect_path::Typed>::type_info()
+            fn get_represented_type_info(&self) -> #FQOption<&'static #bevy_reflect_path::TypeInfo> {
+                #FQOption::Some(<Self as #bevy_reflect_path::Typed>::type_info())
             }
 
             #[inline]

crates/bevy_reflect/bevy_reflect_derive/src/impls/structs.rs

@@ -151,7 +151,7 @@ pub(crate) fn impl_struct(reflect_struct: &ReflectStruct) -> TokenStream {
 
             fn clone_dynamic(&self) -> #bevy_reflect_path::DynamicStruct {
                 let mut dynamic: #bevy_reflect_path::DynamicStruct = #FQDefault::default();
-                dynamic.set_name(::std::string::ToString::to_string(#bevy_reflect_path::Reflect::type_name(self)));
+                dynamic.set_represented_type(#bevy_reflect_path::Reflect::get_represented_type_info(self));
                 #(dynamic.insert_boxed(#field_names, #bevy_reflect_path::Reflect::clone_value(&self.#field_idents));)*
                 dynamic
             }
@@ -164,8 +164,8 @@ pub(crate) fn impl_struct(reflect_struct: &ReflectStruct) -> TokenStream {
             }
 
             #[inline]
-            fn get_type_info(&self) -> &'static #bevy_reflect_path::TypeInfo {
-                <Self as #bevy_reflect_path::Typed>::type_info()
+            fn get_represented_type_info(&self) -> #FQOption<&'static #bevy_reflect_path::TypeInfo> {
+                #FQOption::Some(<Self as #bevy_reflect_path::Typed>::type_info())
             }
 
             #[inline]

crates/bevy_reflect/bevy_reflect_derive/src/impls/tuple_structs.rs

@@ -122,7 +122,7 @@ pub(crate) fn impl_tuple_struct(reflect_struct: &ReflectStruct) -> TokenStream {
 
             fn clone_dynamic(&self) -> #bevy_reflect_path::DynamicTupleStruct {
                 let mut dynamic: #bevy_reflect_path::DynamicTupleStruct = #FQDefault::default();
-                dynamic.set_name(::std::string::ToString::to_string(#bevy_reflect_path::Reflect::type_name(self)));
+                dynamic.set_represented_type(#bevy_reflect_path::Reflect::get_represented_type_info(self));
                 #(dynamic.insert_boxed(#bevy_reflect_path::Reflect::clone_value(&self.#field_idents));)*
                 dynamic
             }
@@ -135,8 +135,8 @@ pub(crate) fn impl_tuple_struct(reflect_struct: &ReflectStruct) -> TokenStream {
             }
 
             #[inline]
-            fn get_type_info(&self) -> &'static #bevy_reflect_path::TypeInfo {
-                <Self as #bevy_reflect_path::Typed>::type_info()
+            fn get_represented_type_info(&self) -> #FQOption<&'static #bevy_reflect_path::TypeInfo> {
+                #FQOption::Some(<Self as #bevy_reflect_path::Typed>::type_info())
             }
 
             #[inline]

crates/bevy_reflect/bevy_reflect_derive/src/impls/values.rs

@@ -49,8 +49,8 @@ pub(crate) fn impl_value(meta: &ReflectMeta) -> TokenStream {
             }
 
             #[inline]
-            fn get_type_info(&self) -> &'static #bevy_reflect_path::TypeInfo {
-                <Self as #bevy_reflect_path::Typed>::type_info()
+            fn get_represented_type_info(&self) -> #FQOption<&'static #bevy_reflect_path::TypeInfo> {
+                #FQOption::Some(<Self as #bevy_reflect_path::Typed>::type_info())
             }
 
             #[inline]

crates/bevy_reflect/src/array.rs

@@ -1,7 +1,4 @@
-use crate::{
-    utility::{reflect_hasher, NonGenericTypeInfoCell},
-    DynamicInfo, Reflect, ReflectMut, ReflectOwned, ReflectRef, TypeInfo, Typed,
-};
+use crate::{utility::reflect_hasher, Reflect, ReflectMut, ReflectOwned, ReflectRef, TypeInfo};
 use std::{
     any::{Any, TypeId},
     fmt::Debug,
@@ -67,7 +64,7 @@ pub trait Array: Reflect {
     /// Clones the list, producing a [`DynamicArray`].
     fn clone_dynamic(&self) -> DynamicArray {
         DynamicArray {
-            name: self.type_name().to_string(),
+            represented_type: self.get_represented_type_info(),
             values: self.iter().map(|value| value.clone_value()).collect(),
         }
     }
@@ -167,22 +164,22 @@ impl ArrayInfo {
 /// [`DynamicList`]: crate::DynamicList
 #[derive(Debug)]
 pub struct DynamicArray {
-    pub(crate) name: String,
+    pub(crate) represented_type: Option<&'static TypeInfo>,
     pub(crate) values: Box<[Box<dyn Reflect>]>,
 }
 
 impl DynamicArray {
     #[inline]
     pub fn new(values: Box<[Box<dyn Reflect>]>) -> Self {
         Self {
-            name: String::default(),
+            represented_type: None,
             values,
         }
     }
 
     pub fn from_vec<T: Reflect>(values: Vec<T>) -> Self {
         Self {
-            name: String::default(),
+            represented_type: None,
             values: values
                 .into_iter()
                 .map(|field| Box::new(field) as Box<dyn Reflect>)
@@ -191,26 +188,37 @@ impl DynamicArray {
         }
     }
 
-    #[inline]
-    pub fn name(&self) -> &str {
-        &self.name
-    }
+    /// Sets the [type] to be represented by this `DynamicArray`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the given [type] is not a [`TypeInfo::Array`].
+    ///
+    /// [type]: TypeInfo
+    pub fn set_represented_type(&mut self, represented_type: Option<&'static TypeInfo>) {
+        if let Some(represented_type) = represented_type {
+            assert!(
+                matches!(represented_type, TypeInfo::Array(_)),
+                "expected TypeInfo::Array but received: {:?}",
+                represented_type
+            );
+        }
 
-    #[inline]
-    pub fn set_name(&mut self, name: String) {
-        self.name = name;
+        self.represented_type = represented_type;
     }
 }
 
 impl Reflect for DynamicArray {
     #[inline]
     fn type_name(&self) -> &str {
-        self.name.as_str()
+        self.represented_type
+            .map(|info| info.type_name())
+            .unwrap_or_else(|| std::any::type_name::<Self>())
     }
 
     #[inline]
-    fn get_type_info(&self) -> &'static TypeInfo {
-        <Self as Typed>::type_info()
+    fn get_represented_type_info(&self) -> Option<&'static TypeInfo> {
+        self.represented_type
     }
 
     #[inline]
@@ -281,6 +289,11 @@ impl Reflect for DynamicArray {
     fn reflect_partial_eq(&self, value: &dyn Reflect) -> Option<bool> {
         array_partial_eq(self, value)
     }
+
+    #[inline]
+    fn is_dynamic(&self) -> bool {
+        true
+    }
 }
 
 impl Array for DynamicArray {
@@ -312,7 +325,7 @@ impl Array for DynamicArray {
     #[inline]
     fn clone_dynamic(&self) -> DynamicArray {
         DynamicArray {
-            name: self.name.clone(),
+            represented_type: self.represented_type,
             values: self
                 .values
                 .iter()
@@ -322,13 +335,6 @@ impl Array for DynamicArray {
     }
 }
 
-impl Typed for DynamicArray {
-    fn type_info() -> &'static TypeInfo {
-        static CELL: NonGenericTypeInfoCell = NonGenericTypeInfoCell::new();
-        CELL.get_or_set(|| TypeInfo::Dynamic(DynamicInfo::new::<Self>()))
-    }
-}
-
 /// An iterator over an [`Array`].
 pub struct ArrayIter<'a> {
     array: &'a dyn Array,

crates/bevy_reflect/src/enums/dynamic_enum.rs

@@ -1,8 +1,6 @@
-use crate::utility::NonGenericTypeInfoCell;
 use crate::{
-    enum_debug, enum_hash, enum_partial_eq, DynamicInfo, DynamicStruct, DynamicTuple, Enum,
-    Reflect, ReflectMut, ReflectOwned, ReflectRef, Struct, Tuple, TypeInfo, Typed,
-    VariantFieldIter, VariantType,
+    enum_debug, enum_hash, enum_partial_eq, DynamicStruct, DynamicTuple, Enum, Reflect, ReflectMut,
+    ReflectOwned, ReflectRef, Struct, Tuple, TypeInfo, VariantFieldIter, VariantType,
 };
 use std::any::Any;
 use std::fmt::Formatter;
@@ -58,7 +56,6 @@ impl From<()> for DynamicVariant {
 ///
 /// // Create a DynamicEnum to represent the new value
 /// let mut dyn_enum = DynamicEnum::new(
-///   Reflect::type_name(&value),
 ///   "None",
 ///   DynamicVariant::Unit
 /// );
@@ -71,7 +68,7 @@ impl From<()> for DynamicVariant {
 /// ```
 #[derive(Default, Debug)]
 pub struct DynamicEnum {
-    name: String,
+    represented_type: Option<&'static TypeInfo>,
     variant_name: String,
     variant_index: usize,
     variant: DynamicVariant,
@@ -82,17 +79,12 @@ impl DynamicEnum {
     ///
     /// # Arguments
     ///
-    /// * `name`: The type name of the enum
     /// * `variant_name`: The name of the variant to set
     /// * `variant`: The variant data
     ///
-    pub fn new<I: Into<String>, V: Into<DynamicVariant>>(
-        name: I,
-        variant_name: I,
-        variant: V,
-    ) -> Self {
+    pub fn new<I: Into<String>, V: Into<DynamicVariant>>(variant_name: I, variant: V) -> Self {
         Self {
-            name: name.into(),
+            represented_type: None,
             variant_index: 0,
             variant_name: variant_name.into(),
             variant: variant.into(),
@@ -103,33 +95,40 @@ impl DynamicEnum {
     ///
     /// # Arguments
     ///
-    /// * `name`: The type name of the enum
     /// * `variant_index`: The index of the variant to set
     /// * `variant_name`: The name of the variant to set
     /// * `variant`: The variant data
     ///
     pub fn new_with_index<I: Into<String>, V: Into<DynamicVariant>>(
-        name: I,
         variant_index: usize,
         variant_name: I,
         variant: V,
     ) -> Self {
         Self {
-            name: name.into(),
+            represented_type: None,
             variant_index,
             variant_name: variant_name.into(),
             variant: variant.into(),
         }
     }
 
-    /// Returns the type name of the enum.
-    pub fn name(&self) -> &str {
-        &self.name
-    }
+    /// Sets the [type] to be represented by this `DynamicEnum`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the given [type] is not a [`TypeInfo::Enum`].
+    ///
+    /// [type]: TypeInfo
+    pub fn set_represented_type(&mut self, represented_type: Option<&'static TypeInfo>) {
+        if let Some(represented_type) = represented_type {
+            assert!(
+                matches!(represented_type, TypeInfo::Enum(_)),
+                "expected TypeInfo::Enum but received: {:?}",
+                represented_type
+            );
+        }
 
-    /// Sets the type name of the enum.
-    pub fn set_name(&mut self, name: String) {
-        self.name = name;
+        self.represented_type = represented_type;
     }
 
     /// Set the current enum variant represented by this struct.
@@ -142,11 +141,11 @@ impl DynamicEnum {
     pub fn set_variant_with_index<I: Into<String>, V: Into<DynamicVariant>>(
         &mut self,
         variant_index: usize,
-        name: I,
+        variant_name: I,
         variant: V,
     ) {
         self.variant_index = variant_index;
-        self.variant_name = name.into();
+        self.variant_name = variant_name.into();
         self.variant = variant.into();
     }
 
@@ -161,9 +160,9 @@ impl DynamicEnum {
     ///
     /// This is functionally the same as [`DynamicEnum::from`] except it takes a reference.
     pub fn from_ref<TEnum: Enum>(value: &TEnum) -> Self {
-        match value.variant_type() {
+        let type_info = value.get_represented_type_info();
+        let mut dyn_enum = match value.variant_type() {
             VariantType::Unit => DynamicEnum::new_with_index(
-                value.type_name(),
                 value.variant_index(),
                 value.variant_name(),
                 DynamicVariant::Unit,
@@ -174,7 +173,6 @@ impl DynamicEnum {
                     data.insert_boxed(field.value().clone_value());
                 }
                 DynamicEnum::new_with_index(
-                    value.type_name(),
                     value.variant_index(),
                     value.variant_name(),
                     DynamicVariant::Tuple(data),
@@ -187,13 +185,15 @@ impl DynamicEnum {
                     data.insert_boxed(name, field.value().clone_value());
                 }
                 DynamicEnum::new_with_index(
-                    value.type_name(),
                     value.variant_index(),
                     value.variant_name(),
                     DynamicVariant::Struct(data),
                 )
             }
-        }
+        };
+
+        dyn_enum.set_represented_type(type_info);
+        dyn_enum
     }
 }
 
@@ -276,7 +276,7 @@ impl Enum for DynamicEnum {
 
     fn clone_dynamic(&self) -> DynamicEnum {
         Self {
-            name: self.name.clone(),
+            represented_type: self.represented_type,
             variant_index: self.variant_index,
             variant_name: self.variant_name.clone(),
             variant: self.variant.clone(),
@@ -287,12 +287,14 @@ impl Enum for DynamicEnum {
 impl Reflect for DynamicEnum {
     #[inline]
     fn type_name(&self) -> &str {
-        &self.name
+        self.represented_type
+            .map(|info| info.type_name())
+            .unwrap_or_default()
     }
 
     #[inline]
-    fn get_type_info(&self) -> &'static TypeInfo {
-        <Self as Typed>::type_info()
+    fn get_represented_type_info(&self) -> Option<&'static TypeInfo> {
+        self.represented_type
     }
 
     #[inline]
@@ -418,10 +420,3 @@ impl Reflect for DynamicEnum {
         write!(f, ")")
     }
 }
-
-impl Typed for DynamicEnum {
-    fn type_info() -> &'static TypeInfo {
-        static CELL: NonGenericTypeInfoCell = NonGenericTypeInfoCell::new();
-        CELL.get_or_set(|| TypeInfo::Dynamic(DynamicInfo::new::<Self>()))
-    }
-}