`assert_emails` in block form use the given number as expected value

actionmailer/CHANGELOG.md

+*   `assert_emails` in block form use the given number as expected value.
+    This makes the error message much easier to understand.
+
+    *Yuji Yaginuma*
+
 *   Add support for inline images in mailer previews by using an interceptor
     class to convert cid: urls in image src attributes to data urls.
 

actionmailer/lib/action_mailer/test_helper.rb

@@ -34,7 +34,7 @@ def assert_emails(number)
         original_count = ActionMailer::Base.deliveries.size
         yield
         new_count = ActionMailer::Base.deliveries.size
-        assert_equal original_count + number, new_count, "#{number} emails expected, but #{new_count - original_count} were sent"
+        assert_equal number, new_count - original_count, "#{number} emails expected, but #{new_count - original_count} were sent"
       else
         assert_equal number, ActionMailer::Base.deliveries.size
       end

actionmailer/test/test_helper_test.rb

@@ -112,6 +112,17 @@ def test_assert_emails_too_many_sent
     assert_match(/1 .* but 2/, error.message)
   end
 
+  def test_assert_emails_message
+    TestHelperMailer.test.deliver_now
+    error = assert_raise ActiveSupport::TestCase::Assertion do
+      assert_emails 2 do
+        TestHelperMailer.test.deliver_now
+      end
+    end
+    assert_match "Expected: 2", error.message
+    assert_match "Actual: 1", error.message
+  end
+
   def test_assert_no_emails_failure
     error = assert_raise ActiveSupport::TestCase::Assertion do
       assert_no_emails do