Consider supporting conversion of matcher protocol

[yujinakayama]

rspec/rspec-expectations#270
rspec/rspec-expectations#373
rspec/rspec-expectations#375

[myronmarston]

If you're going to do anything here, I think transpec should focus only on updating uses of the custom matcher DSL, e.g. changing from this:

RSpec::Matchers.define :be_awesome do
  match_for_should { }
  match_for_should_not { }
  failure_message_for_should { }
  failure_message_for_should_not { }
end

...to this:

RSpec::Matchers.define :be_awesome do
  match { }
  match_when_negated { }
  failure_message { }
  failure_message_when_negated { }
end

For the case of a custom matcher being defined using a standard ruby class (as is the case in a gem like shoulda), I don't think transpec needs to do anything: they'll almost certainly want to support RSpec 2.x and 3.x (which is trivial: simply use aliases to support both forms of the protocol).

Note that the old protocol isn't removed from RSpec 3. It's simply deprecated (and the new protocol isn't even available in 2.99). We did that, rather than the typical "deprecate in 2.99, remove in 3.0" route, because one of the most common places where custom matchers show up is in extension gems, and we don't want people to not be able to upgrade because an extension gem they use (like shoulda) uses the old protocol. So we're having a longer period of deprecation with the plan to remove support for the old protocol in 4.0.

[yujinakayama]

Thanks. I'll try in accordance with your suggestion.