Add $primitive for serializing unit variants as string

[fwcd]

Fixes #283

This PR updates the serializer for unit variants to serialize them as string primitives instead of self-closing tags if prefixed with $primitive. For example, if we define

enum X {
    #[serde(rename = "$primitive=A")]
    A
}

struct Y {
    x: X
}

X::A will now serialize to A instead of <A/> and Y { x: X::A } will now serialize to <Y x="A"/> instead of <Y><A/></Y>.

[bachp]

@fwcd Could this be somehow controlled via an #[] attribute on an enum or enum variant level?

[fwcd]

Perhaps, I am not that familiar with serde's macro system though. We might be able to use a special dollar-named prefix, like already used for unflattening structs (see here).

I have a feeling that most use cases where unit variants are involved are concerned with serializing 'simple' enums (i.e. without associated values) that are stored as a field, and making unit variants-as-strings (i.e. this PR) the default behavior would match the intuitive serialization there. Any thoughts?

[tafia]

I think the default behavior isn't necessarily unintuitive. Sadly this discussion is probably to late because I am afraid to break quite a lot of code.

I agree with a new $ attribute to make it work. This is far from perfect but xml really isn't that great with serde and this is the best workaround I can see.

Eventually we'll have to properly document all these $ attributes.

[fwcd]

I have restored the previous behavior and added a $primitive= prefix that can be placed on unit variants to optionally serialize them as strings. A new paragraph in the README documents this behavior.

[djc]

@tafia I would really like to be able to use this. Is there some chance of getting this merged?

[tafia]

Thanks, I'll publish a new version this weekend

[djc]

@tafia awesome, thanks!

[djc]

@tafia happy new year! Gentle ping on getting this out to crates.io?