Provide unmatching enqueued email info in failure messages

The ActiveJob matcher provides output for each unmatching, enqueued job
in its failure messages.  Those unmatching jobs have certainly helped me
catch typos in the past, so it makes sense to provide similar feedback
in enqueued email failure messages.

Author: Joel Lubrano

lib/rspec/rails/matchers/active_job.rb

@@ -11,6 +11,8 @@ module ActiveJob
         # rubocop: disable Style/ClassLength
         # @private
         class Base < RSpec::Matchers::BuiltIn::BaseMatcher
+          attr_reader :matching_jobs, :unmatching_jobs
+
           def initialize
             @args = []
             @queue = nil

lib/rspec/rails/matchers/have_enqueued_mail.rb

@@ -54,6 +54,8 @@ def failure_message
           base_message.tap do |msg|
             msg << " #{expected_count_message}"
             msg << " with #{@args}" if @args.any?
+            msg << " but enqueued #{@job_matcher.matching_jobs.size}"
+            msg << "\n#{unmatching_mail_jobs_message}" if unmatching_mail_jobs.any?
           end
         end
 
@@ -103,6 +105,26 @@ def set_expected_count(relativity, count)
                             else Integer(count)
                             end
         end
+
+        def unmatching_mail_jobs
+          @job_matcher.unmatching_jobs.select do |job|
+            job[:job] == ActionMailer::DeliveryJob
+          end
+        end
+
+        def unmatching_mail_jobs_message
+          msg = "Queued deliveries:"
+
+          unmatching_mail_jobs.each do |job|
+            mailer_method = job[:args][0..1].join('.')
+            mailer_args = job[:args][3..-1]
+
+            msg << "\n  #{mailer_method}"
+            msg << " with #{mailer_args}" if mailer_args.any?
+          end
+
+          msg
+        end
       end
 
       # @api public

spec/rspec/rails/matchers/have_enqueued_mail_spec.rb

@@ -144,7 +144,21 @@ def email_with_optional_args(required_arg, optional_arg = nil); end
     it "generates a failure message with arguments" do
       expect {
         expect { }.to have_enqueued_email(TestMailer, :email_with_args).with(1, 2)
-      }.to raise_error(/expected to enqueue TestMailer.email_with_args exactly 1 time with \[1, 2\]/)
+      }.to raise_error(/expected to enqueue TestMailer.email_with_args exactly 1 time with \[1, 2\] but enqueued 0/)
+    end
+
+    it "generates a failure message with unmatching enqueued mail jobs" do
+      message = "expected to enqueue TestMailer.email_with_args exactly 1 time with [1, 2] but enqueued 0" + \
+                "\nQueued deliveries:" + \
+                "\n  TestMailer.test_email" + \
+                "\n  TestMailer.email_with_args with [3, 4]"
+
+      expect {
+        expect {
+          TestMailer.test_email.deliver_later
+          TestMailer.email_with_args(3, 4).deliver_later
+        }.to have_enqueued_email(TestMailer, :email_with_args).with(1, 2)
+      }.to raise_error(message)
     end
 
     it "throws descriptive error when no test adapter set" do