bevy_reflect_derive: Clean up attribute logic

[MrGVSV]

Objective

The code in bevy_reflect_derive could use some cleanup.

Solution

Took some of the changes in #11659 to create a dedicated PR for cleaning up the field and container attribute logic.

Updated Naming

I renamed ReflectTraits and ReflectFieldAttr to ContainerAttributes and FieldAttributes, respectively. I think these are clearer.

Updated Parsing

Readability

The parsing logic wasn't too bad before, but it was getting difficult to read. There was some duplicated logic between Meta::List and Meta::Path attributes. Additionally, all the logic was kept inside a large method.

To simply things, I replaced the nested meta parsing with ParseStream parsing. In my opinion, this is easier to follow since it breaks up the large match statement into a small set of single-line if statements, where each if-block contains a single call to the appropriate attribute parsing method.

Flexibility

On top of the added simplicity, this also makes our attribute parsing much more flexible. It allows us to more elegantly handle custom where clauses (i.e. #[reflect(where T: Foo)]) and it opens the door for more non-standard attribute syntax (e.g. #11659).

Errors

This also allows us to automatically provide certain errors when parsing. For example, since we can use stream.lookahead1(), we get errors like the following for free:

error: expected one of: `ignore`, `skip_serializing`, `default`
    --> crates/bevy_reflect/src/lib.rs:1988:23
     |
1988 |             #[reflect(foo)]
     |                       ^^^

---

Changelog

Note

All changes are internal to bevy_reflect_derive and should not affect the public API¹.

• Renamed ReflectTraits to ContainerAttributes
  • Renamed ReflectMeta::traits to ReflectMeta::attrs
• Renamed ReflectFieldAttr to FieldAttributes
• Updated parsing logic for field/container attribute parsing
  • Now uses a ParseStream directly instead of nested meta parsing
• General code cleanup of the field/container attribute modules for bevy_reflect_derive

Footnotes

1. Does not include errors, which may look slightly different. ↩

[soqb]

i love this!

[MrGVSV]

I updated the compile fail tests to account for the new attribution parsing. A side-effect of these changes is that it's now possible to include where attributes inside registration attributes like: #[reflect(Hash, where T: Debug)].

I decided not to remove it since we already allow this behavior for other non-registration attributes (i.e. from_reflect = false, no_field_bound, etc.) and because enforcing this with an error might be worth a separate discussion.