Environment

• Ruby 2.6.3
• Rails 6.0.0
• Devise 4.7.1

Current behavior

In a Rails 6 app without ActionMailer included (rails new testapp --skip-action-mailer), the default zeitwerk loader errors when eager-loading due to mailers/devise/mailer.rb not defining Devise::Mailer. Output from rails zeitwerk:check:
expected file /usr/local/bundle/gems/devise-4.7.1/app/mailers/devise/mailer.rb to define constant Devise::Mailer,

Expected behavior

Zeitwerk eager-loading should work without ActionMailer. Suggested options:

• (easy) Define a dummy Devise::Mailer when ActionMailer is not defined.
• (hard?) Customize the zeitwerk loader for when ActionMailer is not used.