Add support for :oneof fields on protobuf messages

[andrewn617]

Motivation

Google Protobuf messages support oneof field types:

add_message "MyCart" do
  oneof :contact_info do
    optional :phone_number, :int32, 1
    optional :email, :string, 2
  end
end

MyCart will have a #contact_info method that returns the name of the optional that has been used (or nil).

This PR adds a method definition to RBIs for messages with oneof fields.

Implementation

Descriptor provides an each_oneof method. I iterate over the oneof descriptors and create an RBI method for each.

Tests

👍

[asoules]

🏆

[slinkp]

I 🎩 this on Shopify/shopify and see the desired method sigs for oneOf fields. Thanks!!

[paracycle]

I wasn't sure how oneof fields worked, and it was not clear to me why the method would be returning a nilable Symbol. So I looked up the documentation for it and understood that the oneof field holds the name of the field that ends up being populated.

Here is a code snippet that describes the situation for everyone's sake:

cart = MyCart.new

# Fields have their defaults.
raise unless cart.email == ""
raise unless cart.phone_number == 0
raise unless cart.contact_info == nil

cart.email = "[email protected]"
raise unless cart.email == "[email protected]"
raise unless cart.phone_number == 0
raise unless cart.contact_info == :email

# Setting phone_number clears email.
cart.phone_number = 5552456789
raise unless cart.email == ""
raise unless cart.contact_info == :phone_number

# Setting phone_number to nil clears the oneof.
cart.phone_number = nil
raise unless cart.contact_info == nil