Add support for Format::Any

[JonathanBrouwer]

Summary

Hey there! I'm aware that the serde-reflection crate was developed in the context of serde-generate, but I'm using it for a different purpose, which is generating an XML schema for my serde-encoded types.

For this purpose I need to be able to detect types that require self-describing data structures, so I added a new variant to Format for this. I'm aware this is a breaking change and that you earlier rejected this proposal in #33, but I thought I'd try anyways, since otherwise I need to maintain my fork.

Feel free to propose changes to my implementation or reject it outright if you feel this doesn't fit the crate.

[ma2bd]

With a little bit of boilerplate in user code, we could have Format::Typename("Value") here.

Specifically, there could be a wrapper struct around serde_json::Value (say struct Value(serde_json::Value)) so that the (de)serialize trait implementations of the new type are treating it as a plain enum (hence compatible with the current framework of serde-{reflection,generate}) for non-human-readable (de)serializers.

[ma2bd]

Or alternatively use #[serde(with = "module")] instead of a wrapper type (doc).

Here is an example of conditional implementation of (de)serialize traits:
https://github.com/linera-io/linera-protocol/blob/152ebf5fb767ae967b381d3ed3b64dd9aedc8d9e/linera-base/src/data_types.rs#L60

[JonathanBrouwer]

If I understand correctly, this would indeed work perfectly. The problem is that the generated XML (which is the serde data format I need to use, sadly :p) would be less readable, explicitly encoding whether each field is a Value::Int, Value::String, etc. You'd get XML like this

<person>
  <object>
      <name><string>Jonathan</string></name>
      <age><number>23</number></age>
  </object>
</person>

Instead of

<person>
    <name>Jonathan</name>
    <age>23</age>
</person>

To clarify my goal, I'm building a new serde data format crate which can generate XML and outputs a corresponding XSD (xml schema) using the serde-reflection crate to find the structure of the serialized type. I want to handle types that require a self-describing data format gracefully during the generation of the XSD, emitting a wildcard for that part

[JonathanBrouwer]

Or alternatively use #[serde(with = "module")] instead of a wrapper type (doc).

Here is an example of conditional implementation of (de)serialize traits: https://github.com/linera-io/linera-protocol/blob/152ebf5fb767ae967b381d3ed3b64dd9aedc8d9e/linera-base/src/data_types.rs#L60

Oh I see, with conditional implementation of serialize/deserialize this could indeed work. Bit of a hacky solution, but I could try doing it that way

[ma2bd]

I understand that replacing serde_json::Value or annotating the fields could be annoying but for the rest what prevents you from obtaining the desired generated output once you have a format with a recognizable typename?

[JonathanBrouwer]

Currently my code was hitting the Err::NotSupported case in this crate's deserialize implementation, therefore I'm not getting a Registry at all. The example you sent above where a serialize/deserialize is switching its implementation based on the is_human_readable flag would be a workaround for this problem, so the deserialize_any call is not triggered and then I could manually filter out the types that use deserialize_any. This is a bit less nice to use than the implementation in this PR but an acceptable compromise

[ma2bd]

Yes. Obviously, you can pick the solution that works for you. Given that there is a workaround, though, I'm leaning towards keeping the current codebase as it is.

[JonathanBrouwer]

That's fair enough, then I'll close this PR, thanks a lot for the review!