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

@@ -87,4 +87,14 @@ 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? -%>
+
+  config.generators.after_generate do |files|
+    # Apply autocorrection by RuboCop to files generated by `bin/rails generate`.
+    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 -%>
 end

railties/test/generators/app_generator_test.rb

@@ -628,6 +628,7 @@ 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
@@ -636,6 +637,9 @@ 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_usage_read_from_file