allow partial args when using have_enqueued_mail (#2118)

this provide parity with have_enqueued_job with partial arguments
which would be useful especially in negated case where
we want to make sure that absolutely no emails are enqueued

lib/rspec/rails/matchers/have_enqueued_mail.rb

@@ -22,7 +22,7 @@ def initialize(mailer_class, method_name)
         end
 
         def description
-          "enqueues #{@mailer_class.name}.#{@method_name}"
+          "enqueues #{mailer_class_name}.#{@method_name}"
         end
 
         def with(*args, &block)
@@ -50,7 +50,7 @@ def failure_message_when_negated
         private
 
         def base_message
-          "#{@mailer_class.name}.#{@method_name}".tap do |msg|
+          [mailer_class_name, @method_name].compact.join('.').tap do |msg|
             msg << " #{expected_count_message}"
             msg << " with #{@mail_args}," if @mail_args.any?
             msg << " on queue #{@queue}," if @queue
@@ -63,10 +63,14 @@ def expected_count_message
           "#{message_expectation_modifier} #{@expected_number} #{@expected_number == 1 ? 'time' : 'times'}"
         end
 
+        def mailer_class_name
+          @mailer_class ? @mailer_class.name : 'ActionMailer::Base'
+        end
+
         def mailer_args
           if @mail_args.any?
             base_mailer_args + @mail_args
-          else
+          elsif @mailer_class && @method_name
             mailer_method_arity = @mailer_class.instance_method(@method_name).arity
 
             number_of_args = if mailer_method_arity < 0
@@ -76,11 +80,15 @@ def mailer_args
                              end
 
             base_mailer_args + Array.new(number_of_args) { anything }
+          elsif @mailer_class
+            [mailer_class_name, any_args]
+          else
+            []
           end
         end
 
         def base_mailer_args
-          [@mailer_class.name, @method_name.to_s, MAILER_JOB_METHOD]
+          [mailer_class_name, @method_name.to_s, MAILER_JOB_METHOD]
         end
 
         def yield_mail_args(block)
@@ -135,6 +143,14 @@ def mailer_job
       # @example
       #     expect {
       #       MyMailer.welcome(user).deliver_later
+      #     }.to have_enqueued_mail
+      #
+      #     expect {
+      #       MyMailer.welcome(user).deliver_later
+      #     }.to have_enqueued_mail(MyMailer)
+      #
+      #     expect {
+      #       MyMailer.welcome(user).deliver_later
       #     }.to have_enqueued_mail(MyMailer, :welcome)
       #
       #     # Using alias
@@ -162,7 +178,7 @@ def mailer_job
       #     expect {
       #       MyMailer.welcome(user).deliver_later(queue: :urgent_mail)
       #     }.to have_enqueued_mail(MyMailer, :welcome).on_queue(:urgent_mail)
-      def have_enqueued_mail(mailer_class, mail_method_name)
+      def have_enqueued_mail(mailer_class = nil, mail_method_name = nil)
         HaveEnqueuedMail.new(mailer_class, mail_method_name)
       end
       alias_method :have_enqueued_email, :have_enqueued_mail

spec/rspec/rails/matchers/have_enqueued_mail_spec.rb

@@ -10,6 +10,10 @@ def test_email; end
     def email_with_args(arg1, arg2); end
     def email_with_optional_args(required_arg, optional_arg = nil); end
   end
+
+  class AnotherTestMailer < ActionMailer::Base
+    def test_email; end
+  end
 end
 
 RSpec.describe "HaveEnqueuedMail matchers", :skip => !RSpec::Rails::FeatureCheck.has_active_job? do
@@ -53,6 +57,34 @@ def email_with_optional_args(required_arg, optional_arg = nil); end
       expect { }.not_to have_enqueued_mail(TestMailer, :test_email)
     end
 
+    it "passes when given 0 arguments" do
+      expect {
+        TestMailer.test_email.deliver_later
+      }.to have_enqueued_email
+    end
+
+    it "passes when negated with 0 arguments" do
+      expect { }.not_to have_enqueued_email
+    end
+
+    it "passes when only given mailer argument" do
+      expect {
+        TestMailer.test_email.deliver_later
+      }.to have_enqueued_email(TestMailer)
+    end
+
+    it "passes when negated with only mailer arguments" do
+      expect { }.not_to have_enqueued_email(TestMailer)
+    end
+
+    it "ensure that the right mailer is enqueued" do
+      expect {
+        expect {
+          AnotherTestMailer.test_email.deliver_later
+        }.to have_enqueued_mail(TestMailer)
+      }.to raise_error(/expected to enqueue TestMailer exactly 1 time but enqueued 0/)
+    end
+
     it "counts only emails enqueued in the block" do
       TestMailer.test_email.deliver_later
 
@@ -126,6 +158,18 @@ def email_with_optional_args(required_arg, optional_arg = nil); end
       }.to have_enqueued_mail(TestMailer, :test_email).at_most(:twice)
     end
 
+    it "generates a failure message when given 0 argument" do
+      expect {
+        expect { }.to have_enqueued_mail.at_least(:once)
+      }.to raise_error(/expected to enqueue ActionMailer::Base at least 1 time but enqueued 0/)
+    end
+
+    it "generates a failure message when given only mailer argument" do
+      expect {
+        expect { }.to have_enqueued_mail(TestMailer).at_least(:once)
+      }.to raise_error(/expected to enqueue TestMailer at least 1 time but enqueued 0/)
+    end
+
     it "generates a failure message with at least hint" do
       expect {
         expect { }.to have_enqueued_mail(TestMailer, :test_email).at_least(:once)