Derive from_variants for FFI oneof fields

[ladvoc]

This PR derives from_variants on generated enums representing oneof fields from the FFI protocol, allowing a message to be converted into its corresponding oneof variant using into. This results in a significant boilerplate reduction in the FFI implementation.

[1egoman]

Makes sense to me!

Only one minor thought - how common are enum field that contain the same struct, resulting in conflicts in the from_variants implementations? Is it worth trying to think about some sort of more scalable solution than just piecemeal adding from_variants_skip_field to handle these or are they fairly uncommon?

[ladvoc]

Makes sense to me!

Only one minor thought - how common are enum field that contain the same struct, resulting in conflicts in the from_variants implementations? Is it worth trying to think about some sort of more scalable solution than just piecemeal adding from_variants_skip_field to handle these or are they fairly uncommon?

Good point! I did think about this, but I think this situation is pretty uncommon. A more scalable solution would be creating a custom proc_macro, but I think we should cross that bridge later if it becomes an issue.