fake_di.rb

class Injectee
  def hello
    puts :hello
  end
end

module DI
  class Injector
    def initialize(module_object)
      @module = module_object
    end

    def to(var_name)
      mod = if @module.instance_method(:initialize).arity == 0
        Module.new do
          module_eval %Q|
          def initialize
            #{var_name.to_s} = #{"Injectee"}.new
            super
          end
          |
        end
      else
        Module.new do
          module_eval %Q|
          def initialize(*args, **params, &block)
            #{var_name.to_s} = #{"Injectee"}.new
            super
          end
          |
        end
      end
      @module.instance_eval do
        prepend mod
      end
    end
  end

  def self.included(mod)
    mod.instance_variable_set(:@inject, Injector.new(mod))
  end
end

class Foo
  include DI
  @inject.to( :@hoge )

  def foo
    @hoge.hello
  end
end

Foo.new.foo #=> "hello"