Remove Reflect::type_descriptor (#90)

The idea with `Reflect::type_descriptor` was to get the type descriptor
for the reflected value however it had one big footgun: `<Value as
Reflect>::type_descriptor` returns an opaque type, _not_ the type the
`Value` was created from.

For example:

```rust
fn show_editor_ui(reflect: &mut dyn Reflect) {
    let type_descriptor = reflect.type_descriptor();
    // show ui...
}

struct Foo {}

let foo = Foo {};

// convert the `foo` into a `Value`, perhaps to serialize
// it and send across FFI
let mut foo_value = foo.to_value();

// because `Value` implements `Reflect` it works as a `&dyn mut Reflect`
// but our `show_editor_ui` wont really work because we've lost the type
// information when we called `foo.to_value()`
show_editor_ui(&mut foo_value);
```

The solution is to capture the type descriptor explicitly and store that
together with `foo_value`:

```rust
fn show_editor_ui(reflect: &mut dyn Reflect, type_descriptor: TypeDescriptor) {
    // show ui...
}

// store and serialize both the value and the original type descriptor
let mut foo_value = foo.to_value();
let foo_type_descriptor = <Foo as DescribeType>::type_descriptor();

show_editor_ui(&mut foo_value, foo_type_descriptor);
```

I think because of this it makes sense to remove
`Reflect::type_descriptor` as it should make it easier to users to do
the right thing.

I also considered making a `TypedValue` that is like `Value` but
preserves the type descriptor but I ran into a bunch of technical issues
with that.

CHANGELOG.md

@@ -7,9 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 # Unreleased
 
+- **breaking:** Remove `Reflect::type_descriptor` ([#90])
 - **fixed:** Fully qualify `FromReflect` in generated code ([#107])
 - **added:** Implement `Reflect`, and friends, for `Infallible` ([#111])
 
+[#90]: https://github.com/EmbarkStudios/mirror-mirror/pull/90
 [#107]: https://github.com/EmbarkStudios/mirror-mirror/pull/107
 [#111]: https://github.com/EmbarkStudios/mirror-mirror/pull/111
 
@@ -81,8 +83,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 # 0.1.2 (03. February, 2023)
 
 - **added:** Add `impl From<Key> for KeyPath` ([#88])
+- **breaking:** Remove `Reflect::type_descriptor`. Instead capture the type
+  descriptor explicitly with `<T as DescribeType::type_descriptor()` ([#90])
 
 [#88]: https://github.com/EmbarkStudios/mirror-mirror/pull/88
+[#90]: https://github.com/EmbarkStudios/mirror-mirror/pull/90
 
 # 0.1.1 (17. January, 2023)
 

crates/mirror-mirror-macros/src/derive_reflect/enum_.rs

@@ -294,12 +294,6 @@ fn expand_reflect(
         }
     };
 
-    let fn_type_info = quote! {
-        fn type_descriptor(&self) -> Cow<'static, TypeDescriptor> {
-            <Self as DescribeType>::type_descriptor()
-        }
-    };
-
     let fn_debug = attrs.fn_debug_tokens();
     let fn_clone_reflect = attrs.fn_clone_reflect_tokens();
 
@@ -327,7 +321,6 @@ fn expand_reflect(
                 self
             }
 
-            #fn_type_info
             #fn_patch
             #fn_to_value
             #fn_clone_reflect

crates/mirror-mirror-macros/src/derive_reflect/struct_named.rs

@@ -132,12 +132,6 @@ fn expand_reflect(
         }
     };
 
-    let fn_type_info = quote! {
-        fn type_descriptor(&self) -> Cow<'static, TypeDescriptor> {
-            <Self as DescribeType>::type_descriptor()
-        }
-    };
-
     let fn_debug = attrs.fn_debug_tokens();
     let fn_clone_reflect = attrs.fn_clone_reflect_tokens();
 
@@ -165,7 +159,6 @@ fn expand_reflect(
                 self
             }
 
-            #fn_type_info
             #fn_patch
             #fn_to_value
             #fn_clone_reflect

crates/mirror-mirror-macros/src/derive_reflect/tuple_struct.rs

@@ -134,12 +134,6 @@ fn expand_reflect(
         }
     };
 
-    let fn_type_info = quote! {
-        fn type_descriptor(&self) -> Cow<'static, TypeDescriptor> {
-            <Self as DescribeType>::type_descriptor()
-        }
-    };
-
     let fn_debug = attrs.fn_debug_tokens();
     let fn_clone_reflect = attrs.fn_clone_reflect_tokens();
     let Generics {
@@ -166,7 +160,6 @@ fn expand_reflect(
                 self
             }
 
-            #fn_type_info
             #fn_patch
             #fn_to_value
             #fn_clone_reflect

crates/mirror-mirror/src/foreign_impls/boxed.rs

@@ -1,4 +1,3 @@
-use alloc::borrow::Cow;
 use alloc::boxed::Box;
 use core::any::Any;
 use core::fmt;
@@ -12,7 +11,6 @@ use crate::Reflect;
 use crate::ReflectMut;
 use crate::ReflectOwned;
 use crate::ReflectRef;
-use crate::TypeDescriptor;
 use crate::Value;
 
 impl<T> DescribeType for Box<T>
@@ -28,10 +26,6 @@ impl<T> Reflect for Box<T>
 where
     T: Reflect + DescribeType,
 {
-    fn type_descriptor(&self) -> Cow<'static, TypeDescriptor> {
-        <T as DescribeType>::type_descriptor()
-    }
-
     fn as_any(&self) -> &dyn Any {
         <T as Reflect>::as_any(self)
     }

crates/mirror-mirror/src/lib.rs

@@ -258,7 +258,6 @@
 
 extern crate alloc;
 
-use alloc::borrow::Cow;
 use alloc::borrow::ToOwned;
 use alloc::boxed::Box;
 use alloc::string::String;
@@ -271,12 +270,6 @@ use crate::enum_::VariantKind;
 
 macro_rules! trivial_reflect_methods {
     () => {
-        fn type_descriptor(
-            &self,
-        ) -> alloc::borrow::Cow<'static, $crate::type_info::TypeDescriptor> {
-            <Self as $crate::type_info::DescribeType>::type_descriptor()
-        }
-
         fn as_any(&self) -> &dyn Any {
             self
         }
@@ -369,8 +362,6 @@ pub use self::value::Value;
 
 /// A reflected type.
 pub trait Reflect: Any + Send + 'static {
-    fn type_descriptor(&self) -> Cow<'static, TypeDescriptor>;
-
     fn as_any(&self) -> &dyn Any;
 
     fn as_any_mut(&mut self) -> &mut dyn Any;

crates/mirror-mirror/src/value.rs

@@ -27,7 +27,6 @@ use crate::ReflectRef;
 use crate::ScalarMut;
 use crate::ScalarOwned;
 use crate::ScalarRef;
-use crate::TypeDescriptor;
 
 /// A type erased value type.
 ///
@@ -190,10 +189,6 @@ impl DescribeType for Value {
 }
 
 impl Reflect for Value {
-    fn type_descriptor(&self) -> alloc::borrow::Cow<'static, TypeDescriptor> {
-        <Self as DescribeType>::type_descriptor()
-    }
-
     fn as_any(&self) -> &dyn Any {
         for_each_variant!(self, inner => inner)
     }