Apply autocorrection by RuboCop to files generated by `bin/rails gene…

…rate`

## Motivation / Background

RuboCop has now been included by default (rails#50456).
By adding the following tip to the default configuration, user can apply RuboCop's autocorrection to
code generated by `bin/rails generate` (e.g., migration file):

https://github.com/rubocop/rubocop-rails#rails-configuration-tip

This means that the generated files will be formatted according to user's .rubocop.yml custom configuration.

## Detail

Since `bin/rails generate` and `bin/rubocop` are used only in the development environment,
the target files are limited to only `config/environments/development.rb`.

## Additional information

This feature was introduced in Rails 6.1 by rails#38870.

railties/lib/rails/configuration.rb

@@ -131,6 +131,15 @@ def after_generate(&block)
         @after_generate_callbacks << block
       end
 
+      def apply_autocorrect_after_generate!
+        after_generate do |files|
+          parsable_files = files.filter { |file| file.end_with?(".rb") }
+          unless parsable_files.empty?
+            system("bin/rubocop -A --fail-level=E #{parsable_files.shelljoin}", exception: true)
+          end
+        end
+      end
+
       def method_missing(method, *args)
         method = method.to_s.delete_suffix("=").to_sym
 

railties/lib/rails/generators/rails/app/templates/config/environments/development.rb.tt

@@ -87,4 +87,9 @@ Rails.application.configure do
   <%- end -%>
   # Raise error when a before_action's only/except options reference missing actions
   config.action_controller.raise_on_missing_callback_actions = true
+  <%- unless skip_rubocop? -%>
+
+  # Apply autocorrection by RuboCop to files generated by `bin/rails generate`.
+  config.generators.apply_autocorrect_after_generate!
+  <%- end -%>
 end

railties/test/application/generators_test.rb

@@ -255,5 +255,14 @@ def check_expected
 
       assert_match(/# Add comment to model/, File.read(model_file))
     end
+
+    test "generators with apply_autocorrect_after_generate!" do
+      with_config do |c|
+        c.generators.apply_autocorrect_after_generate!
+      end
+
+      output = rails("generate", "model", "post", "title:string", "body:string")
+      assert_match(/3 files inspected, no offenses detected/, output)
+    end
   end
 end

railties/test/generators/app_generator_test.rb

@@ -629,6 +629,7 @@ def test_inclusion_of_a_debugger
   def test_inclusion_of_rubocop
     run_generator
     assert_gem "rubocop-rails-omakase"
+    assert_file "config/environments/development.rb", %r|# Apply autocorrection by RuboCop to files generated by `bin/rails generate`\.|
   end
 
   def test_rubocop_is_skipped_if_required
@@ -637,6 +638,9 @@ def test_rubocop_is_skipped_if_required
     assert_no_gem "rubocop"
     assert_no_file "bin/rubocop"
     assert_no_file ".rubocop.yml"
+    assert_file "config/environments/development.rb" do |content|
+      assert_no_match(%r|# Apply autocorrection by RuboCop to files generated by `bin/rails generate`\.|, content)
+    end
   end
 
   def test_inclusion_of_brakeman