Allow SceneMapSerializer to accept Box<dyn Reflect>s as well

[aecsocket]

Objective

NOTE: This is NOT a working PR! See below.

Currently, SceneMapSerializer looks like this:

pub struct SceneMapSerializer<'a> {
    /// List of boxed values of unique type to serialize.
    pub entries: &'a [Box<dyn PartialReflect>],
// ...

Which works fine for the use-case of SceneSerializer and EntitySerializer, since components there are PartialReflects. However, I want to serialize a Vec<Box<dyn Reflect>> using this serializer, which is currently impossible without cloning the Box<dyn Reflect>s first.

Solution

This PR attempts to solve this issue by changing the signature to:

pub struct SceneMapSerializer<'a, Entry: AsRef<dyn PartialReflect>> {
    /// List of values of unique type to serialize.
    pub entries: &'a [Entry],
// ...

Since the serializer logic doesn't actually need a Box<dyn PartialReflect>, it just needs some way to get a &dyn PartialReflect from the entry. This means that Vec<Box<dyn PartialReflect>>, Vec<&dyn PartialReflect>, and Vec<Arc<dyn PartialReflect>> can all be serialized now.

However, this doesn't solve the original issue of serializing Box<dyn Reflect>s since that doesn't impl AsRef<dyn PartialReflect>. I'm not sure how to solve this, so I'd like to open this PR and see if anyone else has a good idea on how to achieve this.

Current ideas:

• Use CastPartialReflect from bevy_reflect: Add casting traits to support Box and other wrapper types #15532
  • Would be clean, since a Vec<&dyn PartialReflect> and Vec<&dyn Reflect> could be accepted into the same interface
• Make entries a impl Iterator<Item = impl AsRef<dyn PartialReflect>>, and manually entries.iter().map(Reflect::as_partial_reflect) when constructing the SceneMapSerializer
  • Most flexible solution, and it also allows users to apply extra logic like filtering, mapping beforehand
  • Does not work with serde::Serialize since fn serialize(&self) takes a shared ref to self, but we would need to mutate the iterator to use it (unless we mutex, but like, really?)
    • Is there some alternate trait to serde::Serialize we can use?
  • You could pass in a Vec<&dyn Reflect> but you need to do that iterator mapping trick, so it's not quite as clean as the first solution
• Some combination of the 2? impl Iterator<Item = impl CastPartialReflect>?
• Are we ok with the extra generic in SceneMapSerializer?

Testing

Added a unit test to serde.rs which should be used to validate the outcome of this PR. It checks that passing in different combinations of Box<..>, Arc<..>, PartialReflect and Reflect all compile.

Migration Guide

This section is optional. If there are no breaking changes, you can delete this section.

• If this PR is a breaking change (relative to the last release of Bevy), describe how a user might need to migrate their code to support these changes
• Simply adding new functionality is not a breaking change.
• Fixing behavior that was definitely a bug, rather than a questionable design choice is not a breaking change.