Add Rubocop::Cops::Rails::ZeitwerkFriendlyConstant

config/default.yml

@@ -1218,7 +1218,11 @@ Rails/WhereNotWithMultipleConditions:
   Enabled: 'pending'
   Severity: warning
   VersionAdded: '2.17'
-  VersionChanged: '2.18'
+
+Rails/ZeitwerkFriendlyConstant:
+  Description: 'Ensure all constants defined in each file are independently loadable by Zeitwerk.'
+  Enabled: 'pending'
+  VersionAdded: '<<next>>'
 
 # Accept `redirect_to(...) and return` and similar cases.
 Style/AndOr:

lib/rubocop/cop/rails/zeitwerk_friendly_constant.rb

@@ -0,0 +1,161 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Ensures that every constant defined in a file matches the file name
+      # such a way that it is independently loadable by Zeitwerk.
+      #
+      # @example
+      #
+      #   Good
+      #
+      #     # /some/directory/foo.rb
+      #     module Foo
+      #     end
+      #
+      #     # /some/directory/foo.rb
+      #     module Foo
+      #       module Bar
+      #       end
+      #     end
+      #
+      #     # /some/directory/foo/bar.rb
+      #     module Foo
+      #       module Bar
+      #       end
+      #     end
+      #
+      #   Bad
+      #
+      #     # /some/directory/foo.rb
+      #     module Bar
+      #     end
+      #
+      #     # /some/directory/foo/bar.rb
+      #     module Foo
+      #       module Bar
+      #       end
+      #
+      #       module Baz
+      #       end
+      #     end
+      #
+      class ZeitwerkFriendlyConstant < Base
+        MSG = 'Constant name does not match filename.'
+        CLASS_MESSAGE = 'Class name does not match filename.'
+        MODULE_MESSAGE = 'Module name does not match filename.'
+        INCOMPATIBLE_FILE_PATH_MESSAGE = 'Constant names are mutually incompatible with file path.'
+
+        CONSTANT_NAME_MATCHER = /\A[[:upper:]_]*\Z/.freeze
+        CONSTANT_DEFINITION_TYPES = %i[module class casgn].freeze
+
+        def relevant_file?(file)
+          super && (File.extname(file) == '.rb')
+        end
+
+        def on_new_investigation
+          return if processed_source.blank?
+
+          common_anchors = nil
+
+          each_nested_constant(processed_source.ast) do |node, nesting|
+            anchors = nesting.anchors(path_segments)
+
+            if anchors.empty?
+              add_offense(node, message: offense_message(node))
+            else
+              common_anchors ||= anchors
+
+              if (common_anchors &= anchors).empty?
+                # Add an offense if there is no common anchor among constants.
+                add_offense(node, message: INCOMPATIBLE_FILE_PATH_MESSAGE)
+              end
+            end
+          end
+        end
+
+        private
+
+        Nesting = Struct.new(:namespace) do
+          def push(node)
+            self.namespace += [node]
+            @constants = nil
+          end
+
+          def constants
+            @constants ||= namespace.flat_map { |node| constant_name(node).split('::') }
+          end
+
+          # For a nesting like ["Foo", "Bar"] and path segments ["", "Some",
+          # "Dir", "Foo", "Bar"], return an array of all possible "anchors" of the
+          # nesting within the segments, if any (in this case, [3]).
+          def anchors(path_segments)
+            (1..constants.length).each_with_object([]) do |i, anchors|
+              anchors << i if path_segments[(path_segments.size - i)..] == constants[0, i]
+            end
+          end
+
+          def constant_name(node)
+            if (defined_module = node.defined_module)
+              defined_module.const_name
+            else
+              name = node.children[1].to_s
+              name = name.split('_').map(&:capitalize!).join if CONSTANT_NAME_MATCHER.match?(name)
+              name
+            end
+          end
+        end
+        # Traverse the AST from node and yield each constant, along with its
+        # nesting: an array of class/module names within which it is defined.
+        def each_nested_constant(node, nesting = Nesting.new([]), &block)
+          nesting.push(node) if constant_definition?(node)
+
+          any_yielded = node.child_nodes.map do |child_node|
+            each_nested_constant(child_node, nesting.dup, &block)
+          end.any?
+
+          # We only yield "leaves", i.e. constants that have no other nested
+          # constants within themselves. To do this we return true from this
+          # method if it itself has yielded, and only yield from parents if all
+          # recursive calls did not return true (i.e. they did not yield).
+          if !any_yielded && constant_definition?(node)
+            yield(node, nesting)
+            true
+          else
+            any_yielded
+          end
+        end
+
+        def path_segments
+          @path_segments ||= processed_source.file_path.delete_suffix('.rb').split('/').map! { |dir| camelize(dir) }
+        end
+
+        def constant_definition?(node)
+          CONSTANT_DEFINITION_TYPES.include?(node.type)
+        end
+
+        def offense_message(node)
+          case node.type
+          when :module
+            MODULE_MESSAGE
+          when :class
+            CLASS_MESSAGE
+          end
+        end
+
+        def camelize(path_segment)
+          path_segment.split('_').map! do |segment|
+            acronyms.key?(segment) ? acronyms[segment] : segment.capitalize
+          end.join
+        end
+
+        def acronyms
+          @acronyms ||= cop_config['Acronyms'].to_h do |acronym|
+            [acronym.downcase, acronym]
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -138,3 +138,4 @@
 require_relative 'rails/where_missing'
 require_relative 'rails/where_not'
 require_relative 'rails/where_not_with_multiple_conditions'
+require_relative 'rails/zeitwerk_friendly_constant'