Add model support for POROs to Rails core? (like ActiveAttr or Virtus)

[shi...@kkvesper.jp]

I imagine this has been brought up before, but I'd really like Rails core to add a library which allows plain old Ruby objects (POROs) to be treated as models, such as ActiveAttr (https://github.com/cgriego/active_attr) or Virtus (https://github.com/solnic/virtus).

ActiveModel comes close to doing this, but there are quite a few gaps such as the ability to define attributes on models, e.g.:

class Person
  include ActiveAttr::Attributes

  attribute :first_name, default: "Bob"
  attribute :last_name, type: String     # type-casting
end

The main advantage I see in doing this is that developers no longer need to rely on a third-party library which may diverge from Rails standards. Moreover, there may be opportunity to consolidate functionality common to Mongoid and ActiveRecord in this library.

Thoughts?