# Objective
Resolves#4597 (based on the work from #6056 and a refresh of #4147)
When using reflection, we may often end up in a scenario where we have a Dynamic representing a certain type. Unfortunately, we can't just call `MyType::from_reflect` as we do not have knowledge of the concrete type (`MyType`) at runtime.
Such scenarios happen when we call `Reflect::clone_value`, use the reflection deserializers, or create the Dynamic type ourselves.
## Solution
Add a `ReflectFromReflect` type data struct.
This struct allows us to easily convert Dynamic representations of our types into their respective concrete instances.
```rust
#[derive(Reflect, FromReflect)]
#[reflect(FromReflect)] // <- Register `ReflectFromReflect`
struct MyStruct(String);
let type_id = TypeId::of::<MyStruct>();
// Register our type
let mut registry = TypeRegistry::default();
registry.register::<MyStruct>();
// Create a concrete instance
let my_struct = MyStruct("Hello world".to_string());
// `Reflect::clone_value` will generate a `DynamicTupleStruct` for tuple struct types
let dynamic_value: Box<dyn Reflect> = my_struct.clone_value();
assert!(!dynamic_value.is::<MyStruct>());
// Get the `ReflectFromReflect` type data from the registry
let rfr: &ReflectFromReflect = registry
.get_type_data::<ReflectFromReflect>(type_id)
.unwrap();
// Call `FromReflect::from_reflect` on our Dynamic value
let concrete_value: Box<dyn Reflect> = rfr.from_reflect(&dynamic_value);
assert!(concrete_value.is::<MyStruct>());
```
### Why this PR?
###### Why now?
The three main reasons I closed#4147 were that:
1. Registering `ReflectFromReflect` is clunky (deriving `FromReflect` *and* registering `ReflectFromReflect`)
2. The ecosystem and Bevy itself didn't seem to pay much attention to deriving `FromReflect`
3. I didn't see a lot of desire from the community for such a feature
However, as time has passed it seems 2 and 3 are not really true anymore. Bevy is internally adding lots more `FromReflect` derives, which should make this feature all the more useful. Additionally, I have seen a growing number of people look for something like `ReflectFromReflect`.
I think 1 is still an issue, but not a horrible one. Plus it could be made much, much better using #6056. And I think splitting this feature out of #6056 could lead to #6056 being adopted sooner (or at least make the need more clear to users).
###### Why not just re-open #4147?
The main reason is so that this PR can garner more attention than simply re-opening the old one. This helps bring fresh eyes to the PR for potentially more perspectives/reviews.
---
## Changelog
* Added `ReflectFromReflect`
Co-authored-by: Gino Valente <49806985+MrGVSV@users.noreply.github.com>
