Add serde behind feature flag for some builtins

[jrockett6]

Pretty straightforward here, though I hope I got all the types that made sense. I'm not familiar with the FFI stuff, so to be safe I left out anything with "Opaque" fields. Let me know if there is anything else here that I can help with.

[Bromeon]

Thanks for this!

In godot-rust/gdnative#743, we decided against serializing Rid because it's not a persistent value.

Also, implementing serde support entails more than the derive-macros, namely:

1. Finding a persisted representation for each type -- this is not always a 1:1 mapping of the struct/enum definition. For example for Projection, should we just store cols or something else?
2. This would need to be accompanied by tests, serde_json should be enough. Tests also serve as expressive code that visualizes how the persisted format looks.
3. While this wouldn't need to be implemented right now, it's probably good to think about how to support composite types like Array, PackedArray, Dictionary, Variant.

In general, it may be a bit early to decide all the things already. Some of the interfaces are still changing (including composite types). Also, we'd need to check that this doesn't make our CI run significantly longer; or just enable serde in the Linux unit-test.

[Bromeon]

@jrockett6 For an initial version, we would need concretely:

• Removal of support for Rid
• 1 test per type (based on serde_json). Each such test could reuse a helper function like this:

  fn roundtrip<T>(value: &T, expected_json: &str)
      where T: Deserialize + Serialize + PartialEq
  {
      // pseudo-code
      let json: String = serialize(value);
      let back: T = deserialize(json);
  
      assert_eq!(back, value, "serde round-trip changes value");
      assert_eq!(json, expected_value, "value does not conform to expected JSON");
  }

• enable the feature in the CI unit-test, but only for 1 job (the Linux one)

Do you think that's possible?

[jrockett6]

Yep, sounds good. I'll get a revision out this week.

[jrockett6]

@Bromeon I updated to include tests and remove RID support, cargo test -p godot-core --features serde runs successfully.

I think the only outstanding thing for the initial version (apart from potential review changes) is adding the feature flag to the linux CI. I'm a bit lost on where this should be added though, some suggestions on how to modify the yaml file would be very helpful 😁

[Bromeon]

Sure! 🙂
See e.g. this step in the CI workflow.

You can extend/rename:

          - name: linux-threads
            os: ubuntu-20.04
            artifact-name: linux
            godot-binary: godot.linuxbsd.editor.dev.x86_64
            rust-extra-args: --features threads

as follows:

          - name: linux-features
            os: ubuntu-20.04
            artifact-name: linux
            godot-binary: godot.linuxbsd.editor.dev.x86_64
            rust-extra-args: --features threads,serde

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-237

[Bromeon]

Thank you! 🙂

bors r+

[bors]

Build failed:

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[Bromeon]

bors r+

[bors]

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