restores the ability to manually eager load applications

The main interface to eager loading is config.eager_load. The logic that
implies happens during the boot process.

With the introduction of Zeitwerk, application code is loaded in the
finisher as everything else, but in previous versions of Rails users
could eager load the application code regardless of config.eager_load.

Use cases:

   * Some gems like indexers need to have everything in memory and would
   be a bad user experience to ask users to conditionally set the eager
   load flag.

   * Some tests may need to have everything in memory and would be a bad
   experience to have the flag enabled globally in the test environment.

I personally feel that the contract between this method and the entire
eager loading process is ill-defined. I believe this method is
essentially internal. The purpose of this patch is simply to restore this
functionality emulating what it did before because rethinking the design
of this interface may need time.

railties/lib/rails/application.rb

@@ -497,6 +497,15 @@ module Rails
       ordered_railties.flatten - [self]
     end
 
+    # Eager loads the application code.
+    def eager_load!
+      if Rails.autoloaders.zeitwerk_enabled?
+        Rails.autoloaders.each(&:eager_load)
+      else
+        super
+      end
+    end
+
   protected
     alias :build_middleware_stack :app
 

railties/lib/rails/engine.rb

@@ -471,7 +471,8 @@ module Rails
     end
 
     def eager_load!
-      # Already done by Zeitwerk::Loader.eager_load_all in the finisher.
+      # Already done by Zeitwerk::Loader.eager_load_all. We need this guard to
+      # easily provide a compatible API for both zeitwerk and classic modes.
       return if Rails.autoloaders.zeitwerk_enabled?
 
       config.eager_load_paths.each do |load_path|

railties/test/application/zeitwerk_integration_test.rb

@@ -192,6 +192,26 @@ class ZeitwerkIntegrationTest < ActiveSupport::TestCase
     assert $zeitwerk_integration_test_post
   end
 
+  test "eager loading loads the application code if invoked manually too (regression test)" do
+    $zeitwerk_integration_test_user = false
+    $zeitwerk_integration_test_post = false
+
+    app_file "app/models/user.rb", "class User; end; $zeitwerk_integration_test_user = true"
+    app_file "app/models/post.rb", "class Post; end; $zeitwerk_integration_test_post = true"
+
+    boot
+
+    # Preconditions.
+    assert !$zeitwerk_integration_test_user
+    assert !$zeitwerk_integration_test_post
+
+    Rails.application.eager_load!
+
+    # Postconditions.
+    assert $zeitwerk_integration_test_user
+    assert $zeitwerk_integration_test_post
+  end
+
   test "reloading is enabled if config.cache_classes is false" do
     boot