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

actionmailer/CHANGELOG.md

@@ -1,6 +1,23 @@
 ## Rails 4.0.0 (unreleased) ##
 
-*   Eager loading made to use relation's in_clause_length instead of host's one.
+*   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
+
+    *Olek Janiszewski*
+
+*   Eager loading made to use relation's `in_clause_length` instead of host's one.
     Fix #8474
 
     *Boris Staal*

actionmailer/lib/action_mailer/base.rb

@@ -726,9 +726,10 @@ def set_content_type(m, user_content_type, class_default)
     # Translates the +subject+ using Rails I18n class under <tt>[mailer_scope, action_name]</tt> scope.
     # If it does not find a translation for the +subject+ under the specified scope it will default to a
     # humanized version of the <tt>action_name</tt>.
-    def default_i18n_subject #:nodoc:
+    # If the subject has interpolations, you can pass them through the +interpolations+ parameter.
+    def default_i18n_subject(interpolations = {})
       mailer_scope = self.class.mailer_name.tr('/', '.')
-      I18n.t(:subject, scope: [mailer_scope, action_name], default: action_name.humanize)
+      I18n.t(:subject, interpolations.merge(scope: [mailer_scope, action_name], default: action_name.humanize))
     end
 
     def collect_responses(headers) #:nodoc:

actionmailer/test/base_test.rb

@@ -209,6 +209,12 @@ def teardown
     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

actionmailer/test/mailers/base_mailer.rb

@@ -123,4 +123,8 @@ def with_nil_as_return_value(hash = {})
     mail(:template_name => "welcome")
     nil
   end
+
+  def with_subject_interpolations
+    mail(subject: default_i18n_subject(rapper_or_impersonator: 'Slim Shady'), body: '')
+  end
 end