move directory existence check for code statistics task inside of the…

railties/lib/rails/tasks/statistics.rake

@@ -26,12 +26,13 @@ STATS_DIRECTORIES ||= [
   %w(Channel\ tests     test/channels),
   %w(Integration\ tests test/integration),
   %w(System\ tests      test/system),
-].collect do |name, dir|
-  [ name, "#{File.dirname(Rake.application.rakefile_location)}/#{dir}" ]
-end.select { |name, dir| File.directory?(dir) }
+]
 
 desc "Report code statistics (KLOCs, etc) from the application or engine"
 task :stats do
   require "rails/code_statistics"
-  CodeStatistics.new(*STATS_DIRECTORIES).to_s
+  stat_directories = STATS_DIRECTORIES.collect do |name, dir|
+    [ name, "#{File.dirname(Rake.application.rakefile_location)}/#{dir}" ]
+  end.select { |name, dir| File.directory?(dir) }
+  CodeStatistics.new(*stat_directories).to_s
 end

railties/test/commands/statistics_test.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+require "isolation/abstract_unit"
+require "rails/command"
+
+class Rails::Command::DevTest < ActiveSupport::TestCase
+  setup :build_app
+  teardown :teardown_app
+
+  test "`bin/rails stats` handles non-existing directories added by third parties" do
+    Dir.chdir(app_path) do
+      app_file("lib/tasks/custom.rake", <<~CODE
+        task stats: "custom:statsetup"
+        namespace :custom do
+          task statsetup: :environment do
+            require "rails/code_statistics"
+            ::STATS_DIRECTORIES << ["app/non_existing"]
+          end
+        end
+      CODE
+      )
+      assert rails "stats"
+    end
+  end
+end