Introduce zeitwerk:check:boot task

This task boots your application and reports if any autoloaded constants were loaded during the boot process.

railties/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Introduce `zeitwerk:check:boot` task
+
+    This task boots your application and reports if any autoloaded constants were loaded during the boot
+    process.
+
+    *Andrew Novoselac*
+
 *   Do not include redis by default in generated Dev Containers.
 
     Now that applications use the Solid Queue and Solid Cache gems by default, we do not need to include redis

railties/lib/rails/tasks/zeitwerk.rake

@@ -20,6 +20,22 @@ report_unchecked = ->(unchecked) do
   puts
 end
 
+report_autoloads_on_boot = ->(autoloads) do
+  puts
+  puts <<~EOS
+    WARNING: The following autoload constants were loaded during
+    the boot process:
+  EOS
+  puts
+
+  autoloads.each { |constant| puts "  #{constant}" }
+  puts
+
+  puts <<~EOS
+    SUMMARY: #{autoloads.count} autoload constants eagerly loaded during boot.
+  EOS
+end
+
 namespace :zeitwerk do
   desc "Check project structure for Zeitwerk compatibility"
   task check: :environment do
@@ -38,4 +54,20 @@ namespace :zeitwerk do
       puts "Otherwise, all is good!"
     end
   end
+
+  namespace :check do
+    task boot: :environment do
+      begin
+        autoloads_on_boot = Rails::ZeitwerkChecker.check_boot
+      rescue Zeitwerk::NameError => e
+        abort e.message.sub(/#{Regexp.escape(Rails.root.to_s)}./, "")
+      end
+
+      if autoloads_on_boot.empty?
+        puts "All is good!"
+      else
+        report_autoloads_on_boot[autoloads_on_boot]
+      end
+    end
+  end
 end

railties/lib/rails/zeitwerk_checker.rb

@@ -12,4 +12,8 @@ def self.check
     unchecked.select! { |dir| Dir.exist?(dir) && !Dir.empty?(dir)  }
     unchecked
   end
+
+  def self.check_boot
+    Zeitwerk::Registry.loaders.flat_map { |loader| loader.__to_unload.keys }
+  end
 end

railties/test/application/zeitwerk_checker_test.rb

@@ -80,4 +80,23 @@ def boot(env = "development")
 
     assert_equal ["#{app_path}/dir1", "#{app_path}/dir2"], Rails::ZeitwerkChecker.check.sort
   end
+
+  test "booting the app does not load any autoload constants by default" do
+    boot
+
+    assert_empty Rails::ZeitwerkChecker.check_boot
+  end
+
+  test "loading an autoloaded constant during the boot process is reported" do
+    app_file "app/models/user.rb", "class User < ActiveRecord::Base; end"
+    app_file "config/initializers/extra_config.rb", <<~INITIALIZER
+      Rails.application.config.to_prepare do
+        Rails.application.config.my_favourite_model = User
+      end
+    INITIALIZER
+
+    boot
+
+    assert_equal ["User"], Rails::ZeitwerkChecker.check_boot
+  end
 end