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.

2 files changed, 31 insertions, 1 deletions

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!