Replace autoloader accessors with Rails.autoloaders.{main,once}

Rails.autoloader and Rails.once_autoloader was just tentative API good
enough for a first patch. Rails.autoloader is singular and does not
convey in its name that there is another autoloader. That might be
confusing, for example if you set a logger and miss traces. On the other
hand, the name `once_autoloader` is very close to being horrible.

Rails.autoloaders.main and Rails.autoloaders.once read better for my
taste, and have a nice symmetry. Also, both "main" and "once" are four
letters long, short and same length.

They are tagged as "rails.main" and "rails.once", respectively.

References #35235.

3 files changed, 33 insertions, 18 deletions

diff --git a/railties/lib/rails.rb b/railties/lib/rails.rb
index bca2cf34e1..1f533a8c04 100644
--- a/railties/lib/rails.rb
+++ b/railties/lib/rails.rb
@@ -13,6 +13,7 @@ require "active_support/core_ext/object/blank"
 
 require "rails/application"
 require "rails/version"
+require "rails/autoloaders"
 
 require "active_support/railtie"
 require "action_dispatch/railtie"
@@ -111,24 +112,8 @@ module Rails
       application && Pathname.new(application.paths["public"].first)
     end
 
-    def autoloader
-      if configuration.autoloader == :zeitwerk
-        @autoloader ||= Zeitwerk::Loader.new
-      end
-    end
-
-    def once_autoloader
-      if configuration.autoloader == :zeitwerk
-        @once_autoloader ||= Zeitwerk::Loader.new
-      end
-    end
-
     def autoloaders
-      if configuration.autoloader == :zeitwerk
-        [autoloader, once_autoloader]
-      else
-        []
-      end
+      Autoloaders
     end
   end
 end
diff --git a/railties/lib/rails/autoloaders.rb b/railties/lib/rails/autoloaders.rb
new file mode 100644
index 0000000000..4b71f90c6b
--- /dev/null
+++ b/railties/lib/rails/autoloaders.rb
@@ -0,0 +1,30 @@
+module Rails
+  module Autoloaders # :nodoc:
+    class << self
+      include Enumerable
+
+      def main
+        if zeitwerk_enabled?
+          @main ||= Zeitwerk::Loader.new.tap { |loader| loader.tag = "rails.main" }
+        end
+      end
+
+      def once
+        if zeitwerk_enabled?
+          @once ||= Zeitwerk::Loader.new.tap { |loader| loader.tag = "rails.once" }
+        end
+      end
+
+      def each
+        if zeitwerk_enabled?
+          yield main
+          yield once
+        end
+      end
+
+      def zeitwerk_enabled?
+        Rails.configuration.autoloader == :zeitwerk
+      end
+    end
+  end
+end
diff --git a/railties/lib/rails/engine.rb b/railties/lib/rails/engine.rb
index 2485158a7b..76be6a8ac7 100644
--- a/railties/lib/rails/engine.rb
+++ b/railties/lib/rails/engine.rb
@@ -472,7 +472,7 @@ module Rails
     # Eager load the application by loading all ruby
     # files inside eager_load paths.
     def eager_load!
-      if Rails.autoloader
+      if Rails.autoloaders.zeitwerk_enabled?
         eager_load_with_zeitwerk!
       else
         eager_load_with_dependencies!