Defer mailer include via ActiveSupport.on_load(:action_mailer)

lib/devise_invitable/rails.rb

@@ -8,10 +8,18 @@ class Engine < ::Rails::Engine
     # We use to_prepare instead of after_initialize here because Devise is a Rails engine; its
     # mailer is reloaded like the rest of the user's app.  Got to make sure that our mailer methods
     # are included each time Devise.mailer is (re)loaded.
+    #
+    # The work is wrapped in ActiveSupport.on_load(:action_mailer) so that resolving
+    # Devise.mailer (a String#constantize) doesn't force ActionMailer::Base to load
+    # before app initialization completes. Without this wrapper, Rails edge's
+    # guard_load_hooks support (rails/rails#56201) logs an early-load-hook warning
+    # for :action_mailer (and :active_job, via MailDeliveryJob) on every boot.
     config.to_prepare do
-      Devise.mailer.send :include, DeviseInvitable::Mailer
-      unless Devise.mailer.ancestors.include?(Devise::Mailers::Helpers)
-        Devise.mailer.send :include, Devise::Mailers::Helpers 
+      ActiveSupport.on_load(:action_mailer) do
+        Devise.mailer.send :include, DeviseInvitable::Mailer
+        unless Devise.mailer.ancestors.include?(Devise::Mailers::Helpers)
+          Devise.mailer.send :include, Devise::Mailers::Helpers
+        end
       end
     end
     # extend mapping with after_initialize because it's not reloaded