Fix Zeitwerk autoloading when ActionMailer is not present.

lib/devise/rails.rb

@@ -47,5 +47,11 @@ class Engine < ::Rails::Engine
           )
         end
     end
+
+    initializer "devise.configure_zeitwerk" do
+      if Rails.autoloaders.zeitwerk_enabled? && !defined?(ActionMailer)
+        Rails.autoloaders.main.ignore("#{root}/app/mailers/devise/mailer.rb")
+      end
+    end
   end
 end

test/zeitwerk_loader_test.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+ENV["RAILS_ENV"] = 'test'
+
+require 'rails'
+require 'rails/test_help'
+
+require 'devise'
+
+class RailsTest < ActiveSupport::TestCase
+  test 'boot without ActionMailer' do
+    class MinimalApp < Rails::Application
+      config.root = __dir__
+      config.eager_load = false
+      config.active_support.to_time_preserves_timezone = :zone
+    end
+
+    Devise.setup do |config|
+      config.parent_controller = 'ActionController::Base'
+    end
+
+    MinimalApp.initialize!
+    MinimalApp.eager_load!
+  end
+end