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

1 files changed, 3 insertions, 2 deletions

diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index 9ba606c045..c4d2950abd 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -726,9 +726,10 @@ module ActionMailer
     # 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: