Move plumbing to enable yjit to inside the framework

There is no reason to expose all those details to users and this
has the benefit that now are can ensure that the YJIT is enabled
after all initialization is done.

Author: rafaelfranca

guides/source/configuring.md

@@ -65,6 +65,7 @@ Below are the default values associated with each target version. In cases of co
 - [`config.active_record.postgresql_adapter_decode_dates`](#config-active-record-postgresql-adapter-decode-dates): `true`
 - [`config.active_record.validate_migration_timestamps`](#config-active-record-validate-migration-timestamps): `true`
 - [`config.active_storage.web_image_content_types`](#config-active-storage-web-image-content-types): `%w[image/png image/jpeg image/gif image/webp]`
+- [`config.yjit`](#config-yjit): `true`
 
 #### Default Values for Target Version 7.1
 
@@ -613,6 +614,16 @@ Used to easily add nested custom configuration to the application config object
 
 See [Custom Configuration](#custom-configuration)
 
+#### `config.yjit`
+
+Enables YJIT as of Ruby 3.3, to bring sizeable performance improvements. If you are
+deploying to a memory constrained environment you may want to set this to `false`.
+
+| Starting with version | The default value is |
+| --------------------- | -------------------- |
+| (original)            | `false`              |
+| 7.2                   | `true`               |
+
 ### Configuring Assets
 
 #### `config.assets.css_compressor`

railties/lib/rails/application/configuration.rb

@@ -23,7 +23,7 @@ class Configuration < ::Rails::Engine::Configuration
                     :content_security_policy_nonce_generator, :content_security_policy_nonce_directives,
                     :require_master_key, :credentials, :disable_sandbox, :sandbox_by_default,
                     :add_autoload_paths_to_load_path, :rake_eager_load, :server_timing, :log_file_size,
-                    :dom_testing_default_html_version
+                    :dom_testing_default_html_version, :yjit
 
       attr_reader :encoding, :api_only, :loaded_config_version, :log_level
 
@@ -81,6 +81,7 @@ def initialize(*)
         @rake_eager_load                         = false
         @server_timing                           = false
         @dom_testing_default_html_version        = :html4
+        @yjit                                    = false
       end
 
       # Loads default configuration values for a target version. This includes
@@ -318,6 +319,8 @@ def load_defaults(target_version)
         when "7.2"
           load_defaults "7.1"
 
+          self.yjit = true
+
           if respond_to?(:active_job)
             active_job.enqueue_after_transaction_commit = :default
           end

railties/lib/rails/application/finisher.rb

@@ -231,6 +231,12 @@ def self.complete(_state)
           ActiveSupport::DescendantsTracker.disable_clear!
         end
       end
+
+      initializer :enable_yjit do
+        if config.yjit && defined?(RubyVM::YJIT.enable)
+          RubyVM::YJIT.enable
+        end
+      end
     end
   end
 end

railties/lib/rails/generators/rails/app/templates/config/initializers/enable_yjit.rb.tt

@@ -55,7 +55,6 @@
   config/environments/test.rb
   config/initializers/assets.rb
   config/initializers/content_security_policy.rb
-  config/initializers/enable_yjit.rb
   config/initializers/filter_parameter_logging.rb
   config/initializers/inflections.rb
   config/initializers/permissions_policy.rb

railties/test/generators/app_generator_test.rb

@@ -54,7 +54,6 @@
   test/dummy/config/environments/production.rb
   test/dummy/config/environments/test.rb
   test/dummy/config/initializers/content_security_policy.rb
-  test/dummy/config/initializers/enable_yjit.rb
   test/dummy/config/initializers/filter_parameter_logging.rb
   test/dummy/config/initializers/inflections.rb
   test/dummy/config/initializers/permissions_policy.rb