a rewriter to upgrade to Rails 5.1 style controller test actions

controller_actions_keyword_args.rb

=begin
USAGE: ruby-rewrite -l controller_actions_keyword_args.rb -m <FILE>

This rewriter moves your Rails controller tests from `get action, params, session, flash` to
`get action, params: params, session: session, flash: flash`.

Which is needed if you see the following message in your Rails 5.0 tests :

DEPRECATION WARNING: Using positional arguments in functional tests has been deprecated,
in favor of keyword arguments, and will be removed in Rails 5.1.

Deprecated style:
get :show, { id: 1 }, nil, { notice: "This is a flash message" }

New keyword style:
get :show, params: { id: 1 }, flash: { notice: "This is a flash message" },
  session: nil # Can safely be omitted.

=end

class ControllerActionsKeywordArgs < Parser::Rewriter
  CONTROLLER_TEST_ACTIONS = %i(get post put patch delete head)

  def on_send(node)
    rewrite_controller_test_action(node)
    write_out_xhr_test_calls(node)
    super
  end

  def rewrite_controller_test_action(node)
    receiver_node, method_name, *arg_nodes = *node

    if receiver_node.nil? && CONTROLLER_TEST_ACTIONS.include?(method_name)
      action, *further_args = arg_nodes
      unless kwarg_request?(further_args)
        parameters, session, flash = further_args
        transform_into_kwarg(parameters, "params") if parameters
        transform_into_kwarg(session, "session") if session
        transform_into_kwarg(flash, "flash") if flash
      end
    end
  end

  def write_out_xhr_test_calls(node)
    receiver_node, method_name, *arg_nodes = *node

    if receiver_node.nil? && %i(xhr xml_http_request).include?(method_name)
      http_method, action, *further_args = arg_nodes
      http_method_name = http_method.children[0]

      range = Parser::Source::Range.new(node.loc.expression.source_buffer, node.loc.expression.begin_pos, action.loc.expression.begin_pos)
      replace(range, http_method_name.to_s + " ")

      unless kwarg_request?(further_args)
        parameters, session, flash = further_args
        transform_into_kwarg(parameters, "params") if parameters
        transform_into_kwarg(session, "session") if session
        transform_into_kwarg(flash, "flash") if flash
      end
    end

    #TODO Add `xhr: true` at the end of the statement
  end

  protected
  def transform_into_kwarg(node, keyword)
    if node.type == :hash && !node.loc.expression.source.match(/\A{.*}\z/m)
      insert_before node.loc.expression, "#{keyword}: {"
      insert_after node.loc.expression, "}"
    else
      insert_before node.loc.expression, "#{keyword}: "
    end
  end

  REQUEST_KWARGS = %i(params session flash method body xhr)
  FORMAT_KWARGS = %i(format as)
  def kwarg_request?(args)
    return true if args.empty?
    return false unless args.size == 1 && args[0].type == :hash

    hash = args[0]
    hash_pairs = hash.children
    hash_keys = hash_pairs.map{|n| n.children.first}.flat_map(&:children)

    hash_keys.all? { |k| FORMAT_KWARGS.include?(k) } ||
      hash_keys.any? { |k| REQUEST_KWARGS.include?(k) }
  end
end

Needs attention to local variables. Do remember to transform them to something that the view can assess. The traditional way is @instance_var