github
diff --git a/‎lib/rubocop/cop/github/unreliable_subclasses.rb‎
Lines changed: 48 additions & 0 deletions b/‎lib/rubocop/cop/github/unreliable_subclasses.rb‎
Lines changed: 48 additions & 0 deletions
diff --git a/‎test/test_unreliable_subclasses.rb‎
Lines changed: 92 additions & 0 deletions b/‎test/test_unreliable_subclasses.rb‎
Lines changed: 92 additions & 0 deletions
@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+require "rubocop"
+
+module RuboCop
+  module Cop
+    module GitHub
+      # Inform people when they reach for Class#subclasses (Ruby) or
+      # Class#descendants (ActiveSupport).
+      #
+      # two reasons these can be unreliable:
+      #   1. autoload hasn't run yet, so half the tree is invisible
+      #   2. GC may have eaten dynamically-defined classes (i.e. in test suites)
+      #
+      # If you really need it, add a rubocop:disable on the line with a note
+      # explaining why future-you won't be sad.
+      class UnreliableSubclasses < Base
+        MSG = "Avoid `%<method>s` here. It may miss not-yet-autoloaded classes and depends on GC timing. " \
+              "Prefer an explicit registry or eager loading."
+
+        RESTRICT_ON_SEND = %i[descendants subclasses].freeze
+
+        # matches Foo.descendants, Foo::Bar.subclasses, self.descendants
+        # receiver has to be a constant or `self`
+        def_node_matcher :unreliable_call?, <<~PATTERN
+          (send {const self} {:descendants :subclasses})
+        PATTERN
+
+        # the same thing but with safe navigation operator
+        def_node_matcher :unreliable_csend?, <<~PATTERN
+          (csend {const self} {:descendants :subclasses})
+        PATTERN
+
+        def on_send(node)
+          return unless unreliable_call?(node)
+
+          add_offense(node, message: format(MSG, method: node.method_name))
+        end
+
+        def on_csend(node)
+          return unless unreliable_csend?(node)
+
+          add_offense(node, message: format(MSG, method: node.method_name))
+        end
+      end
+    end
+  end
+end
@@ -0,0 +1,92 @@
+# frozen_string_literal: true
+
+require_relative "cop_test"
+require "minitest/autorun"
+require "rubocop/cop/github/unreliable_subclasses"
+
+class TestUnreliableSubclasses < CopTest
+  def cop_class
+    RuboCop::Cop::GitHub::UnreliableSubclasses
+  end
+
+  def test_offended_by_descendants_call
+    offenses = investigate cop, <<~RUBY
+      ApplicationRecord.descendants
+    RUBY
+    assert_equal 1, offenses.size
+    assert_match(/Avoid `descendants`/, offenses.first.message)
+  end
+
+  def test_offended_by_subclasses_call
+    offenses = investigate cop, <<~RUBY
+      ApplicationRecord.subclasses
+    RUBY
+    assert_equal 1, offenses.size
+    assert_match(/Avoid `subclasses`/, offenses.first.message)
+  end
+
+  def test_offended_on_namespaced_constant
+    offenses = investigate cop, <<~RUBY
+      Foo::Bar::Baz.descendants
+    RUBY
+    assert_equal 1, offenses.size
+  end
+
+  def test_offended_on_self_receiver
+    # self.subclasses in a class method body is still Class#subclasses
+    offenses = investigate cop, <<~RUBY
+      class ApplicationRecord
+        def self.known_subclasses
+          self.subclasses
+        end
+      end
+    RUBY
+    assert_equal 1, offenses.size
+  end
+
+  def test_offended_when_chained
+    offenses = investigate cop, <<~RUBY
+      Tea.descendants.map(&:name)
+      Tea.subclasses.each { |k| k.foo }
+      Tea.descendants.size
+      Tea.subclasses.count
+    RUBY
+    assert_equal 4, offenses.size
+  end
+
+  def test_offended_by_safe_navigation_on_constant
+    offenses = investigate cop, <<~RUBY
+      Tea&.descendants
+      Tea&.subclasses
+    RUBY
+    assert_equal 2, offenses.size
+  end
+
+  def test_unoffended_by_non_constant_receiver
+    offenses = investigate cop, <<~RUBY
+      comment.descendants
+      category.subclasses
+      tree_node&.descendants
+      registry[:foo].subclasses
+    RUBY
+    assert_equal 0, offenses.size
+  end
+
+  def test_unoffended_when_called_with_arguments
+    # arity mismatch
+    offenses = investigate cop, <<~RUBY
+      Registry.subclasses(:include_abstract)
+      Tree.descendants(depth: 2)
+    RUBY
+    assert_equal 0, offenses.size
+  end
+
+  def test_unoffended_by_unrelated_methods
+    offenses = investigate cop, <<~RUBY
+      Tea.all
+      Tea.where(roasted: true)
+      ApplicationRecord.connection
+    RUBY
+    assert_equal 0, offenses.size
+  end
+end