Add rubocop-rails-omakase to new Rails applications (#50486)

* Add rubocop to new rails app generator

This setups a basic rubocop config for new rails apps using the `rubocop-rails-omakase` gem:
https://github.com/rails/rubocop-rails-omakase

It can be skipped with the `--skip-rubocop` flag.

railties/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Add RuboCop with rules from rubocop-rails-omakase by default. Skip with --skip-rubocop.
+
+    *DHH* and *zzak*
+
 *   Use `bin/rails runner --skip-executor` option to not wrap the runner script
     with an Executor.
 

railties/lib/rails/generators/app_base.rb

@@ -100,6 +100,9 @@ def self.add_shared_options_for(name)
         class_option :skip_dev_gems,       type: :boolean, default: nil,
                                            desc: "Skip development gems (e.g., web-console)"
 
+        class_option :skip_rubocop,        type: :boolean, default: nil,
+                                           desc: "Skip RuboCop setup"
+
         class_option :dev,                 type: :boolean, default: nil,
                                            desc: "Set up the #{name} with Gemfile pointing to your Rails checkout"
 
@@ -379,6 +382,10 @@ def skip_propshaft?
         skip_asset_pipeline? || options[:asset_pipeline] != "propshaft"
       end
 
+      def skip_rubocop?
+        options[:skip_rubocop]
+      end
+
 
       class GemfileEntry < Struct.new(:name, :version, :comment, :options, :commented_out)
         def initialize(name, version, comment, options = {}, commented_out = false)

railties/lib/rails/generators/rails/app/app_generator.rb

@@ -82,6 +82,10 @@ def dockerfiles
       chmod "bin/docker-entrypoint", 0755 & ~File.umask, verbose: false
     end
 
+    def rubocop
+      template "rubocop.yml", ".rubocop.yml"
+    end
+
     def version_control
       if !options[:skip_git] && !options[:pretend]
         run git_init_command, capture: options[:quiet], abort_on_failure: false
@@ -98,7 +102,8 @@ def app
     end
 
     def bin
-      directory "bin" do |content|
+      options = skip_rubocop? ? { exclude_pattern: /rubocop/ } : {}
+      directory "bin", **options do |content|
         "#{shebang}\n" + content
       end
       chmod "bin", 0755 & ~File.umask, verbose: false
@@ -367,6 +372,11 @@ def create_dockerfiles
         build(:dockerfiles)
       end
 
+      def create_rubocop_file
+        return if skip_rubocop?
+        build(:rubocop)
+      end
+
       def create_config_files
         build(:config)
       end

railties/lib/rails/generators/rails/app/templates/Gemfile.tt

@@ -40,6 +40,11 @@ end
 <% end -%>
 
 group :development do
+<%- unless options.skip_rubocop? -%>
+  # Omakase Ruby styling [https://github.com/rails/rubocop-rails-omakase/]
+  gem "rubocop-rails-omakase", require: false
+
+<%- end -%>
 <%- unless options.api? || options.skip_dev_gems? -%>
   # Use console on exceptions pages [https://github.com/rails/web-console]
   gem "web-console"

railties/lib/rails/generators/rails/app/templates/bin/rubocop.tt

@@ -0,0 +1,7 @@
+require "rubygems"
+require "bundler/setup"
+
+# explicit rubocop config increases performance slightly while avoiding config confusion.
+ARGV.unshift("--config", File.expand_path("../.rubocop.yml", __dir__))
+
+load Gem.bin_path("rubocop", "rubocop")

railties/lib/rails/generators/rails/app/templates/config/application.rb.tt

@@ -18,7 +18,7 @@ module <%= app_const_base %>
     # Please, add to the `ignore` list any other `lib` subdirectories that do
     # not contain `.rb` files, or that should not be reloaded or eager loaded.
     # Common ones are `templates`, `generators`, or `middleware`, for example.
-    config.autoload_lib(ignore: %w(assets tasks))
+    config.autoload_lib(ignore: %w[assets tasks])
 
     # Configuration for the application, engines, and railties goes here.
     #

railties/lib/rails/generators/rails/app/templates/rubocop.yml.tt

@@ -0,0 +1,8 @@
+# Omakase Ruby styling for Rails
+inherit_gem: { rubocop-rails-omakase: rubocop.yml }
+
+# Overwrite or add rules to create your own house style
+#
+# # Use `[a, [b, c]]` not `[ a, [ b, c ] ]`
+# Layout/SpaceInsideArrayLiteralBrackets:
+#   Enabled: false

railties/lib/rails/generators/rails/app/templates/test/application_system_test_case.rb.tt

@@ -1,5 +1,5 @@
 require "test_helper"
 
 class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
-  driven_by :selenium, using: :chrome, screen_size: [1400, 1400]
+  driven_by :selenium, using: :chrome, screen_size: [ 1400, 1400 ]
 end

railties/test/generators/app_generator_test.rb

@@ -8,6 +8,7 @@
   .gitattributes
   .gitignore
   .dockerignore
+  .rubocop.yml
   .ruby-version
   README.md
   Gemfile
@@ -39,6 +40,7 @@
   bin/docker-entrypoint
   bin/rails
   bin/rake
+  bin/rubocop
   bin/setup
   config/application.rb
   config/boot.rb
@@ -623,6 +625,19 @@ def test_inclusion_of_a_debugger
     end
   end
 
+  def test_inclusion_of_rubocop
+    run_generator
+    assert_gem "rubocop-rails-omakase"
+  end
+
+  def test_rubocop_is_skipped_if_required
+    run_generator [destination_root, "--skip-rubocop"]
+
+    assert_no_gem "rubocop"
+    assert_no_file "bin/rubocop"
+    assert_no_file ".rubocop.yml"
+  end
+
   def test_usage_read_from_file
     assert_called(File, :read, returns: "USAGE FROM FILE") do
       assert_equal "USAGE FROM FILE", Rails::Generators::AppGenerator.desc