Add tags support behind feature flag

[vmx]

Tags are stored as Newtype Structs with the special name _TagStruct. The tags
are represented as a struct tuple consisting of the tag identifier and a value.

Example:

let tag = Value::Tag(123, Box::new(Value::Text("some value".to_string())));
let tag_encoded = to_vec(&tag).unwrap();

This implementation is heavily based on the msgpack-rust library, when support
for extensions were added 1. They have the same problem as CBOR has, that
such an extension/tagging type is not directly supported by Serde, hence a
workaround is needed. For me it makes sense to use a similarly working workaround
across different libraries.

This commit also contains an example on how to use custom types as tags. It can
be run via:

cargo run --example tags --features tags

This is an alternate proposal to #129.

[vmx]

I really thought this is a good idea. But after digging into all the discussions about tag support in Serde, I also come to the conclusion that this should be properly solved within Serde so that there's interoperability between different formats.

I would want to revive the approach take at serde-rs/serde#408. @pyfisch do you still have your tagged_values branch of cbor lying around somewhere (no worries if not, it shouldn't be too hard to implement)?

[pyfisch]

Hi @vmx, integrating with serde is a great idea. The tagged_values branch is still available on GitHub isn't it? https://github.com/pyfisch/serde/tree/tagged_values

I'd probably do some things differently than in this old branch. For example I'd use a TaggedVisitor as suggested in the discussion.

[pyfisch]

I will send a PR for the serialization part to the serde repo soon.

[vmx]

@pyfisch Wow, that's great to hear. I so far have ported the old version to master and I started playing around with it. I thought about creating a small example with a tag that is used in both, CBOR and msgpack, which is then (correctly) serialized/deserialized.

So please let me know if the new version of the patch for Serde is up. I really want to see this happen and I also have time to spend on this if there's a chance to get this upstream :)

[pyfisch]

See also #157

[pyfisch]

@vmx would you mind sharing the branch ported to master? I've just created a PR for serde to include tag serialization. (serde-rs/serde#1643) Having the example you suggest would be great to demonstrate why the feature is useful.

[vmx]

@pyfisch I haven't done that much, I pushed whatever I currently have though:

• Serde: https://github.com/vmx/serde/tree/wip-revive-tagged-values
• CBOR: https://github.com/vmx/cbor/tree/wip-revive-tagged-values

I kind of stopped when I saw that you're working on it again. But I'm happy to help, when you tell me what I should work on (my Serde knowledge is still pretty basic, though).

[rklaehn]

Tags are on master, so I guess we can close this.