Add new Rails/ControllerTesting cop

Add controller testing cop to discourage use of
ActionController::TestCase. ActionDispatch::IntegrationTest should be
used instead to test controllers.

changelog/new_rails_controller_testing_cop.md

@@ -0,0 +1 @@
+* [#638](https://github.com/rubocop/rubocop-rails/pull/614): Add new `Rails/ControllerTesting` cop. ([@gmcgibbon][])

config/default.yml

@@ -199,6 +199,16 @@ Rails/ContentTag:
   Exclude:
     - app/models/**/*.rb
 
+Rails/ControllerTesting:
+  Description: 'Use `ActionDispatch::IntegrationTest` instead of `ActionController::TestCase`.'
+  Reference:
+    - 'https://api.rubyonrails.org/classes/ActionController/TestCase.html'
+  Enabled: true
+  VersionAdded: '<<next>>'
+  Include:
+    - test/**/*.rb
+
+
 Rails/CreateTableWithTimestamps:
   Description: >-
                   Checks the migration for which timestamps are not included

lib/rubocop/cop/rails/controller_testing.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Using ActionController::TestCase is discouraged and should be replaced by
+      # ActionDispatch::IntegrationTest. Controller tests are too close to the
+      # internals of a controller whereas integration tests mimick the browser/user.
+      #
+      # @safety
+      #   This cop's autocorrection is unsafe because the API of each test case class is different.
+      #   Make sure to update each test of your controller test cases after changing the superclass.
+      #
+      # @example
+      #   # bad
+      #   class MyControllerTest < ActionController::TestCase
+      #   end
+      #
+      #   # good
+      #   class MyControllerTest < ActionDispatch::IntegrationTest
+      #   end
+      #
+      class ControllerTesting < Base
+        extend AutoCorrector
+        extend TargetRailsVersion
+
+        MSG = 'Use ActionDispatch::IntegrationTest instead. See https://api.rubyonrails.org/classes/ActionController/TestCase.html'
+
+        minimum_target_rails_version 5.0
+
+        def_node_matcher :controller_test_case?, <<~PATTERN
+          (class
+            (const nil? _)
+            (const (const nil? :ActionController) :TestCase) nil?)
+        PATTERN
+
+        def on_class(node)
+          return unless controller_test_case?(node)
+
+          add_offense(node.parent_class) do |corrector|
+            corrector.replace(node.parent_class, 'ActionDispatch::IntegrationTest')
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -26,6 +26,7 @@
 require_relative 'rails/bulk_change_table'
 require_relative 'rails/compact_blank'
 require_relative 'rails/content_tag'
+require_relative 'rails/controller_testing'
 require_relative 'rails/create_table_with_timestamps'
 require_relative 'rails/date'
 require_relative 'rails/default_scope'

spec/rubocop/cop/rails/controller_testing_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::ControllerTesting, :config do
+  context 'Rails 4.2', :rails42, :config do
+    it 'does not add offense when extending ActionController::TestCase' do
+      expect_no_offenses(<<~RUBY)
+        class MyControllerTest < ActionController::TestCase
+        end
+      RUBY
+    end
+  end
+
+  it 'adds offense when extending ActionController::TestCase' do
+    expect_offense(<<~RUBY)
+      class MyControllerTest < ActionController::TestCase
+                               ^^^^^^^^^^^^^^^^^^^^^^^^^^ Use ActionDispatch::IntegrationTest instead. See https://api.rubyonrails.org/classes/ActionController/TestCase.html
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      class MyControllerTest < ActionDispatch::IntegrationTest
+      end
+    RUBY
+  end
+
+  it 'does not add offense when extending ActionDispatch::IntegrationTest' do
+    expect_no_offenses(<<~RUBY)
+      class MyControllerTest < ActionDispatch::IntegrationTest; end
+    RUBY
+  end
+end