mount

# File actionpack/lib/action_dispatch/routing/mapper.rb, line 605
        def mount(app = nil, deprecated_options = nil, as: DEFAULT, via: nil, at: nil, defaults: nil, constraints: nil, anchor: false, format: false, path: nil, internal: nil, **mapping, &block)
          if deprecated_options.is_a?(Hash)
            as = assign_deprecated_option(deprecated_options, :as, :mount) if deprecated_options.key?(:as)
            via ||= assign_deprecated_option(deprecated_options, :via, :mount)
            at ||= assign_deprecated_option(deprecated_options, :at, :mount)
            defaults ||= assign_deprecated_option(deprecated_options, :defaults, :mount)
            constraints ||= assign_deprecated_option(deprecated_options, :constraints, :mount)
            anchor = assign_deprecated_option(deprecated_options, :anchor, :mount) if deprecated_options.key?(:anchor)
            format = assign_deprecated_option(deprecated_options, :format, :mount) if deprecated_options.key?(:format)
            path ||= assign_deprecated_option(deprecated_options, :path, :mount)
            internal ||= assign_deprecated_option(deprecated_options, :internal, :mount)
            assign_deprecated_options(deprecated_options, mapping, :mount)
          end

          path_or_action = at

          if app.nil?
            hash_app, hash_path = mapping.find { |key, _| key.respond_to?(:call) }
            mapping.delete(hash_app) if hash_app

            app ||= hash_app
            path_or_action ||= hash_path
          end

          raise ArgumentError, "A rack application must be specified" unless app.respond_to?(:call)
          raise ArgumentError, <<~MSG unless path_or_action
            Must be called with mount point

              mount SomeRackApp, at: "some_route"
              or
              mount(SomeRackApp => "some_route")
          MSG

          rails_app = rails_app? app
          as = app_name(app, rails_app) if as == DEFAULT

          target_as = name_for_action(as, path_or_action)
          via ||= :all

          match(path_or_action, to: app, as,, via,, defaults,, constraints,, anchor,, format,, path,, internal,, **mapping, &block)

          define_generate_prefix(app, target_as) if rails_app
          self
        end