struct_for_instance_variable.rb

class Struct
  unless self.respond_to? :org_new_for_access_instance_variable
    class << self
      alias_method :org_new_for_access_instance_variable, :new
      def new(*ary, &block)
        st = org_new_for_access_instance_variable(*ary, &block)
        st.module_eval {
          alias_method :org_init, :initialize
          def initialize(*ary)
            org_init(*ary)
            obj_singleton = class << self; self end
            self.class.members.each {|m|
              self.instance_variable_set '@' + m, self.__send__(m)
              obj_singleton.__send__(:attr_accessor, m)
            }
          end
        }
        st
      end
    end
  end
end

A = Struct.new(:a)
class A
  def b
    @a
  end
end

A.new('hoge').b #=> "hoge"