Using Module prepend with an argument

authenticated.rb

# spec/support/authenticated.rb
class Authenticated < Module
  def initialize(current_user:)
    super() do
      define_method :current_user do
        current_user
      end
    end
  end
end

RSpec.shared_context "authenticated", shared_context: :metadata do
  before do
    described_class.prepend Authenticated.new(current_user: current_user)
  end
end

create_spec.rb

# spec/admin/controllers/study/create_spec.rb
RSpec.describe Admin::Controllers::Study::Create, type: :action do
  let(:action) { described_class.new }
  let(:params) { Hash[] }
  let(:admin) { Fabricate.build(:admin) }

  include_context "authenticated" do
    let(:current_user) { admin }
  end

  it 'is successful' do
    response = action.call(params)
    expect(response[0]).to eq 302
  end
end

Thanks to @epidemian and his suggestion to mock the described class:

# spec/support/authenticated.rb
RSpec.shared_context "authenticated", shared_context: :metadata do
  before do
    allow_any_instance_of(described_class).
      to receive(:current_user).
      and_return(current_user)
  end
end

I find this to be much more straightforward and clear. I think I tried something akin to this but for some reason it didn't work. Not sure why. Anyway, this is very nice indeed!