Railsのautoload_pathsを設定しているソースの箇所

appの直下のサブディレクトリは自動でロードされるらしいと聞いたものの、ソースのどこでやっているのかを知らなかったので調べてみました。

      def paths
        @paths ||= begin
          paths = Rails::Paths::Root.new(@root)

          paths.add "app",                 eager_load: true, glob: "*"
          paths.add "app/assets",          glob: "*"
          paths.add "app/controllers",     eager_load: true
          paths.add "app/helpers",         eager_load: true
          paths.add "app/models",          eager_load: true
          paths.add "app/mailers",         eager_load: true
          paths.add "app/views"

          paths.add "app/controllers/concerns", eager_load: true
          paths.add "app/models/concerns",      eager_load: true

          paths.add "lib",                 load_path: true
          paths.add "lib/assets",          glob: "*"
          paths.add "lib/tasks",           glob: "**/*.rake"

          paths.add "config"
          paths.add "config/environments", glob: "#{Rails.env}.rb"
          paths.add "config/initializers", glob: "**/*.rb"
          paths.add "config/locales",      glob: "*.{rb,yml}"
          paths.add "config/routes.rb"

          paths.add "db"
          paths.add "db/migrate"
          paths.add "db/seeds.rb"

          paths.add "vendor",              load_path: true
          paths.add "vendor/assets",       glob: "*"

          paths
        end
      end

root, eager_load_paths, autoload_once_paths, autoload_pathsはその下で設定しています。
https://github.com/rails/rails/blob/v4.0.2/railties/lib/rails/engine/configuration.rb#L74-L88

      def root=(value)
        @root = paths.path = Pathname.new(value).expand_path
      end

      def eager_load_paths
        @eager_load_paths ||= paths.eager_load
      end

      def autoload_once_paths
        @autoload_once_paths ||= paths.autoload_once
      end

      def autoload_paths
        @autoload_paths ||= paths.autoload_paths
      end