Adding helpers for RSpec memoization

I thought initially it would be incredibly easy to add a wrapper method alongside existing let, let!, subject memoized helper methods in RSpec.

def current_user(&block)
    u = yield
    allow(User).to receive(:current_user).and_return(u)
  end

  describe 'stuff' do
    let(:artist) { Fabricate(:artist) }
    current_user { artist.user }
  end

But I kept getting. `artist` is not available on an example group when the current user block was yielded. I had to dig into RSpec::Core to find how let is defined and came up with and ensure that artist and current_user were in the same scope when calling one another.

module CurrentUserHelpers
  extend ActiveSupport::Concern

  module ClassMethods
    def current_user(&block)
      raise "#current_usercalled without a block" if block.nil?
      RSpec::Core::MemoizedHelpers.module_for(self).__send__(:define_method, :current_user, &block)

      # Apply the memoization. The method has been defined in an ancestor
      # module so we can use `super` here to get the value.
      define_method(:current_user) do
        u = __memoized.fetch_or_store(:current_user) do
          block.arity == 1 ? super(RSpec.current_example, &nil) : super(&nil)
        end
        allow(User).to receive(:current_user).and_return(u)
        u
      end
    end

    def current_user!(&block)
      current_user(&block)
      before { current_user }
    end
  end
end