serde support

[hicqu]

I think serde_json is a useful feature. If it's opened, the generated code should be like:

#[derive(Serialize, Deserialize)]
struct S {
    a: i64,
}

So that we can parse {"a": 100} to the struct directly.

Current status:

• Basic implementation
• Repeated message fields
• Bytes

[stepancheg]

I think two thoughts:

• I'm opposed to adding #[derive] unconditionally: currently rust-protobuf has no mandatory dependencies, and I'd like to keep it that way. However, I think it would probably be useful to have codegen option to add those derives (FYI, currently rust-protobuf has an option to use bytes crate).
• I think rust-protobuf should suport JSON encoding and decoding natively. That encoding should follow official specification

[bachp]

I think deriving Serialize would be useful to use the generated types in Template generators like or Tera. They can automatically render types that implement Serialize.

Hiding this behind a feature would be fine tough.

[repi]

This would indeed be nice as an optional feature!

[stepancheg]

Actually now I think that probably it's possible to have it always on.

E. g. generated files could be like this:

#[cfg_attr(protobuf_serde, derive(Serialize, Deserialize))]
struct GeneratedMessage {
   ...
}

So users who need serde could simply enable flag, and codegen flag won't be needed.

However! I believe JSON serialization and parsing should be done without serde (because it can be done better/more protobuf standard conforming than serde).

I've almost finished reflection in protobuf (now protobuf reflection can mutate messages, not just read), and I plan to implement JSON parsing/serialization in protobuf soon.

[stepancheg]

Three additions.

• protobuf now natively supports JSON serialization, serde is probably not that important
• other formats can be implemented with protobuf runtime reflection
• I've created an issue for code generation callbacks, so serde annotations could be inserted in these callbacks

[stepancheg]

The current state of serde support:

• protobuf crate has with-serde feature
• no serde annotations are generated by default
• annotations are generated if serde_derive codegen option is specified
• annotations are guarded with #[cfg_attr(smth, ...)] if serde_derive_cfg=smth option is specified (this part is done mostly for implementation of with-serde feature, not sure it is needed for users)

This API design is not final. Feedback is welcome!

[stepancheg]

Published version 2.1.0.