optimizes eager loading in :zeitwerk mode

During initialization, the eager load paths of engines are unshifted
into AS::Dependencies.autoload_paths. After that, the collection is
frozen. (See the initializers in railties/lib/rails/engine.rb.)

Hence, there is no eager load path that is not an autoload path too, and
so the array difference in the deleted code is always an empty array.

Just do nothing.

railties/lib/rails/engine.rb

@@ -469,13 +469,16 @@ module Rails
       self
     end
 
-    # Eager load the application by loading all ruby
-    # files inside eager_load paths.
     def eager_load!
-      if Rails.autoloaders.zeitwerk_enabled?
+      # Already done by Zeitwerk::Loader.eager_load_all in the finisher.
-        eager_load_with_zeitwerk!
+      return if Rails.autoloaders.zeitwerk_enabled?
-      else
+
-        eager_load_with_dependencies!
+      config.eager_load_paths.each do |load_path|
+        # Starts after load_path plus a slash, ends before ".rb".
+        relname_range = (load_path.to_s.length + 1)...-3
+        Dir.glob("#{load_path}/**/*.rb").sort.each do |file|
+          require_dependency file[relname_range]
+        end
       end
     end
 
@@ -654,22 +657,6 @@ module Rails
 
     private
 
-      def eager_load_with_zeitwerk!
-        (config.eager_load_paths - Zeitwerk::Loader.all_dirs).each do |path|
-          Dir.glob("#{path}/**/*.rb").sort.each { |file| require file }
-        end
-      end
-
-      def eager_load_with_dependencies!
-        config.eager_load_paths.each do |load_path|
-          # Starts after load_path plus a slash, ends before ".rb".
-          relname_range = (load_path.to_s.length + 1)...-3
-          Dir.glob("#{load_path}/**/*.rb").sort.each do |file|
-            require_dependency file[relname_range]
-          end
-        end
-      end
-
       def load_config_initializer(initializer) # :doc:
         ActiveSupport::Notifications.instrument("load_config_initializer.railties", initializer: initializer) do
           load(initializer)

railties/test/application/zeitwerk_integration_test.rb

@@ -124,12 +124,26 @@ class ZeitwerkIntegrationTest < ActiveSupport::TestCase
 
     app_file "app/models/user.rb", "class User; end; $zeitwerk_integration_test_user = true"
     app_file "app/models/post.rb", "class Post; end; $zeitwerk_integration_test_post = true"
+
     boot("production")
 
     assert $zeitwerk_integration_test_user
     assert $zeitwerk_integration_test_post
   end
 
+  test "eager loading loads code in engines" do
+    $test_blog_engine_eager_loaded = false
+
+    engine("blog") do |bukkit|
+      bukkit.write("lib/blog.rb", "class BlogEngine < Rails::Engine; end")
+      bukkit.write("app/models/post.rb", "Post = $test_blog_engine_eager_loaded = true")
+    end
+
+    boot("production")
+
+    assert $test_blog_engine_eager_loaded
+  end
+
   test "eager loading loads anything managed by Zeitwerk" do
     $zeitwerk_integration_test_user = false
     app_file "app/models/user.rb", "class User; end; $zeitwerk_integration_test_user = true"