Nudge engines to be compatible with include_all_helpers

When applications use this configuration setting, it may break an Engine
relying implicitly on all helpers being included.

By setting the option in the dummy app, we help engine authors be
compatible with this setting.

Co-authored-by: Adrianna Chang <adrianna.chang@shopify.com>

railties/lib/rails/generators/rails/plugin/plugin_generator.rb

@@ -141,6 +141,13 @@ module Rails
       if mountable?
         template "rails/routes.rb", "#{dummy_path}/config/routes.rb", force: true
       end
+      if engine? && !api?
+        insert_into_file "#{dummy_path}/config/application.rb", indent(<<~RUBY, 4), after: /^\s*config\.load_defaults.*\n/
+
+          # For compatibility with applications that use this config
+          config.action_controller.include_all_helpers = false
+        RUBY
+      end
     end
 
     def test_dummy_webpacker_assets

railties/test/generators/plugin_generator_test.rb

@@ -591,6 +591,34 @@ class PluginGeneratorTest < Rails::Generators::TestCase
     assert_file "test/dummy/config/application.rb", /^require "bukkits"/
   end
 
+  def test_dummy_application_sets_include_all_helpers_to_false_for_mountable
+    run_generator [destination_root, "--mountable"]
+
+    assert_file "test/dummy/config/application.rb", /^    config\.action_controller\.include_all_helpers = false$/
+  end
+
+  def test_dummy_application_sets_include_all_helpers_to_false_for_full
+    run_generator [destination_root, "--full"]
+
+    assert_file "test/dummy/config/application.rb", /include_all_helpers/
+  end
+
+  def test_dummy_application_does_not_set_include_all_helpers_for_regular
+    run_generator
+
+    assert_file "test/dummy/config/application.rb" do |content|
+      assert_no_match(/include_all_helpers/, content)
+    end
+  end
+
+  def test_dummy_application_does_not_set_include_all_helpers_for_api
+    run_generator [destination_root, "--mountable", "--api"]
+
+    assert_file "test/dummy/config/application.rb" do |content|
+      assert_no_match(/include_all_helpers/, content)
+    end
+  end
+
   def test_dummy_application_uses_dynamic_rails_version_number
     run_generator