PseudoSingleton

pseudo_singleton.rb

require 'delegate'
require 'forwardable'

module PseudoSingleton
  def instance
    @instance ||= new
  end

  private

  attr_accessor :instance_delegator
  
  def self.extended(other)
    other.extend(SingleForwardable)
    other.send(:instance_delegator=, SimpleDelegator.new(other.instance))
    other.def_delegator :instance_delegator, :method_missing
  end
end

pseudo_singleton_spec.rb

require 'pseudo_singleton'

RSpec.describe PseudoSingleton do
  let(:described_class) do
    Class.new do
      extend PseudoSingleton

      def foo
        :bar
      end
    end
  end

  subject { described_class }

  describe '.instance' do

    it 'returns an instance of the class' do
      expect(subject.instance).to be_kind_of described_class
    end

    it 'always returns the *same* instance of the class' do
      expect(subject.instance).to be subject.instance
    end
  end

  describe 'forwarding of class methods to the instance' do
    it 'delegates any methods called on the class but not defined on the class to the instance' do
      expect(subject.foo).to eq :bar
    end
  end
end

I suppose the other option would be to forget about using a shared module and just change it so that classes that would extend PseudoSingleton instead become self-extending modules and then, for testing purposes (where you don't want to test the singleton version) you just define your subject as subject { Object.new.extend(described_class) }.