Add Rails/ClassDescendants cop

I trust Ben Sheldon regarding that 😉

github/rubocop-github#110

Author: alexanderadam

changelog/new_class_descendants_cop.md

@@ -0,0 +1 @@
+* [#1544](https://github.com/rubocop/rubocop-rails/pull/1544): Add new `Rails/ClassDescendants` cop to discourage use of `Class#descendants` due to autoloading issues and non-deterministic behavior with Garbage Collection. ([@alexanderadam][])

config/default.yml

@@ -298,6 +298,14 @@ Rails/BulkChangeTable:
   Include:
     - db/**/*.rb
 
+Rails/ClassDescendants:
+  Description: >-
+                  Avoid using `Class#descendants` as it may not include classes that have yet
+                  to be autoloaded and is non-deterministic with regards to Garbage Collection.
+  Enabled: pending
+  Severity: warning
+  VersionAdded: '<<next>>'
+
 Rails/CompactBlank:
   Description: 'Checks if collection can be blank-compacted with `compact_blank`.'
   Enabled: pending

lib/rubocop/cop/rails/class_descendants.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Checks for the use of `Class#descendants` which has several issues:
+      #
+      # 1. It doesn't know about classes that have yet to be autoloaded.
+      # 2. It's non-deterministic with regards to Garbage Collection of classes.
+      #    If you use `Class.descendants` in tests where there is a pattern to
+      #    dynamically define classes, GC is unpredictable for when those classes
+      #    are cleaned up and removed.
+      #
+      # Consider using alternatives like maintaining an explicit registry of subclasses
+      # or using Rails' `ActiveSupport::DescendantsTracker` with proper autoloading
+      # considerations.
+      #
+      # @example
+      #   # bad
+      #   User.descendants
+      #   ApplicationRecord.descendants.map(&:name)
+      #
+      #   # bad
+      #   MyClass.descendants.each do |klass|
+      #     klass.do_something
+      #   end
+      #
+      #   # good - use subclasses if you don't need recursive descendants
+      #   MyClass.subclasses
+      #
+      class ClassDescendants < Base
+        MSG = 'Avoid using `%<method>s` as it may not include classes that have yet to be autoloaded ' \
+              'and is non-deterministic with regards to Garbage Collection.'
+
+        RESTRICT_ON_SEND = %i[descendants].freeze
+
+        # @!method descendants_call?(node)
+        def_node_matcher :descendants_call?, <<~PATTERN
+          (send _ :descendants ...)
+        PATTERN
+
+        def on_send(node)
+          return unless descendants_call?(node)
+
+          add_offense(node, message: format(MSG, method: node.method_name))
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -32,6 +32,7 @@
 require_relative 'rails/belongs_to'
 require_relative 'rails/blank'
 require_relative 'rails/bulk_change_table'
+require_relative 'rails/class_descendants'
 require_relative 'rails/compact_blank'
 require_relative 'rails/content_tag'
 require_relative 'rails/create_table_with_timestamps'

spec/rubocop/cop/rails/class_descendants_spec.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::ClassDescendants, :config do
+  it 'registers an offense when using `descendants`' do
+    expect_offense(<<~RUBY)
+      User.descendants
+      ^^^^^^^^^^^^^^^^ Avoid using `descendants` as it may not include classes that have yet to be autoloaded and is non-deterministic with regards to Garbage Collection.
+    RUBY
+  end
+end