class_eval_test.rb

class Module
  def append_features(base)
    base.class_eval(&@_included_block) if instance_variable_defined?("@_included_block")
  end

  def included(klass = nil, &block)
    if block_given?
      @_included_block = block
    else
      super if defined?(super)
    end
  end
end

module Foo
  FOO = 1
  
  included do
    p Module.nesting
    puts FOO
  end
end

class Bar
  include Foo
  # Ruby 1.8: 1
  # Ruby 1.9.current: error
  # Ruby 1.9.shugo: 1
end

class Baz
  FOO = 2
  
  include Foo
  # Ruby 1.8: 1
  # Ruby 1.9.current: 2
  # Ruby 1.9.shugo: 2
end