Allow passing interpolations to `#default_i18n_subject`, e.g.:

    # config/locales/en.yml
    en:
      user_mailer:
        welcome:
          subject: 'Hello, %{username}'

    # app/mailers/user_mailer.rb
    class UserMailer < ActionMailer::Base
      def welcome(user)
        mail(subject: default_i18n_subject(username: user.name))
      end
    end

2 files changed, 10 insertions, 0 deletions

diff --git a/actionmailer/test/base_test.rb b/actionmailer/test/base_test.rb
index b06c465380..b9c56c540d 100644
--- a/actionmailer/test/base_test.rb
+++ b/actionmailer/test/base_test.rb
@@ -209,6 +209,12 @@ class BaseTest < ActiveSupport::TestCase
     assert_equal "New Subject!", email.subject
   end
 
+  test 'default subject can have interpolations' do
+    I18n.backend.store_translations('en', base_mailer: {with_subject_interpolations: {subject: 'Will the real %{rapper_or_impersonator} please stand up?'}})
+    email = BaseMailer.with_subject_interpolations
+    assert_equal 'Will the real Slim Shady please stand up?', email.subject
+  end
+
   test "translations are scoped properly" do
     I18n.backend.store_translations('en', base_mailer: {email_with_translations: {greet_user: "Hello %{name}!"}})
     email = BaseMailer.email_with_translations
diff --git a/actionmailer/test/mailers/base_mailer.rb b/actionmailer/test/mailers/base_mailer.rb
index 8fca6177bd..504ca36483 100644
--- a/actionmailer/test/mailers/base_mailer.rb
+++ b/actionmailer/test/mailers/base_mailer.rb
@@ -123,4 +123,8 @@ class BaseMailer < ActionMailer::Base
     mail(:template_name => "welcome")
     nil
   end
+
+  def with_subject_interpolations
+    mail(subject: default_i18n_subject(rapper_or_impersonator: 'Slim Shady'), body: '')
+  end
 end