Allow dynamic lists with validates_inclusion_of :in

Suppose you have a validation rule such as this:
validates_inclusion_of :foo, :in => [1,2,3]

Currently, you can only pass a constant value to :in.

Suppose that the list of valid values changes. In that case, you have to resort to workarounds like this:

validate :validate_foo

def validate_foo
  unless my_foo_list.include?(foo)
    self.errors.add(:foo, I18n.t("activerecord.errors.models.bar.attributes.foo.inclusion"))
  end
end

And you have to write this for every attribute in every model. I suppose you could do some trickery to refactor this, but it's still a lot of extra code that's not very DRY.

It would be nice if you could pass a symbol to :in, and have that symbol evaluated.

validates_inclusion_of :foo, :in => :valid_foo_values_list

def valid_foo_values_list
 if things?
   [1,2,3]
 else
   [4,5,6]
 end
end

In lib/active_record/validations.rb, in validates_inclusion_of, we have this call
enum = configuration[:in] || configuration[:within]

A simple would be to add this line:
enum = self.send(enum) if enum.is_a?(Symbol)

However, I suspect there are other implications.