An example of using an RSpec matcher as a duck-typed attribute matcher in a message expectation.

attribute_expectation_matcher.rb

RSpec::Matchers.define :user_with_email do |email|
  match do |user|
    user.email == email
  end
end

it "subscribes the user to contest" do
  # ... do something to cause the contests subscription to happen...
  Reverb::Contests::Subscription.should have_received(:create).with(user_with_email("foo@bar.com))
end