Update from_reflect to return a Result

[elbertronnie]

Objective

• Fixes Enable error returning from FromReflect trait #5967.

Solution

• Create a new Enum ReflectKind which contains all the "kinds" of types in Reflect
• Add a new method reflect_kind to Reflect which returns the appropriate kind
• Create a new Error type FromReflectError
• Update the function from_reflect to return a Result with the above Error
• Replaced some panic calls with Result inside from_reflect

---

Changelog

• Changed the return type of FromReflect::from_reflect to a Result with FromReflectError as the Error type
• Created a new Enum ReflectKind which contains all the "kinds" of types in Reflect
• Added a new method reflect_kind(&self) -> ReflectKind to Reflect which returns the appropriate kind

Migration Guide

• Replace from_reflect(...) with from_reflect(...).ok()
• If any of your types implement Reflect manually, add a new method reflect_kind(&self) -> ReflectKind to it

[MrGVSV]

Looking good so far. I think the main thing that should be addressed (imo anyways) is making the error struct an enum instead.

[elbertronnie]

I have chosen the name ReflectKind since a lot of documentation already mentions them as "kinds".

To differentiate between Struct and DynamicStruct on source type, I have added a from_kind field to each of the errors. from_kind will determine if it is a Struct, Enum, List, etc and TypeInfo will determine if it is Dynamic or not.

The same cannot be done for the target type since calling reflect_kind(&self) requires self which is not available. We cannot even remove &self from reflect_kind since it will become an associated function making the Trait not object-safe. So I made an assumption that target type will most likely not be Dynamic and therefore used TypeInfo to determine the kind of target type.

[MrGVSV]

Left a few more comments. Mostly documentation, organization, and other nitpicks.

I think we should also add some tests as well, to ensure that we get the proper errors from their corresponding cases. Could we add some to the from_reflect module? (We could continue to use lib.rs for this, but that file is already massive as is haha so it might be nicer to put them in their relevant module.)

[MrGVSV]

@elbertronnie are you interested in rebasing this PR? If not, we can mark this as S-Adopt-Me (and I'll probably pick this up in that case)

[elbertronnie]

@MrGVSV I am quite busy in other things currently. I can only work on this after about a month. If someone is willing to work on it now then I would suggest to mark it as S-Adopt-Me, otherwise I am willing to finish this incomplete PR after a month.

[elbertronnie]

I found a blog a year back that had a good syntax for error messages related to reflection of composed structures. I think this could be useful to when implementing this PR. This is much better than the error messages that are currently in this PR.

Abductive diagnostics for Müsli

[MrGVSV]

@MrGVSV I am quite busy in other things currently. I can only work on this after about a month. If someone is willing to work on it now then I would suggest to mark it as S-Adopt-Me, otherwise I am willing to finish this incomplete PR after a month.

I think we'll be okay to wait (unless you actually don't want to do it yourself, not trying to put any pressure on you haha). I’m planning to do some things with FromReflect soon, so if we decide we need it along with those changes then we can consider marking this for adoption.

[BenjaminBrienen]

Wouldn't TryFromReflect be a better name now?

[bas-ie]

Backlog cleanup: nice one, looks like this has a future so leaving it be!

[bas-ie]

Will check in again on this one in the new year 🙂

[BenjaminBrienen]

@elbertronnie I think now would be a good time to resolve merge conflicts if you are still interested

[elbertronnie]

After looking at the how bevy_reflect has changed over time, resolving merge conflicts felt harder than just writing code again. Since a lot parts like ReflectKind which are present in this PR are already integrated into the codebase. So I have created a new draft PR #17295 where further development will be tracked.