https://github.com/rails/rails/compare/fa736e69a197...2d9dbf416b14

例子： Code before:

resources :messages do
  resources :comments
end

resources :posts do
  resources :comments
  resources :images, only: :index
end

Code after:

concern :commentable do
  resources :comments
end

concern :image_attachable do
  resources :images, only: :index
end

resources :messages, concerns: :commentable

resources :posts, concerns: [:commentable, :image_attachable]