annotations.rb

def nil.+@(*)
  if Module.last_method_defined
    method_name, klass = Module.last_method_defined
    klass.class_eval <<-RUBY
      alias_method "#{method_name}_without_logging", "#{method_name}"
      def #{method_name}(*args, &block)
        p "called #{method_name}"
        send("#{method_name}_without_logging", &block)
      end
    RUBY
    Module.last_method_defined = nil
  end
end

class Module
  class << self; attr_accessor :last_method_defined; end

  def method_added(method_name)
    Module.last_method_defined = [method_name, self]
  end
end

class Foo
  +def foo
    puts :foo
  end

  def bar
    puts :bar
  end
end

Foo.new.foo
Foo.new.bar