AlecK/APPEALS-44512 - Zeitwerk Autoloader Transition

.rubocop.yml

@@ -1,10 +1,11 @@
+inherit_from: .rubocop_todo.yml
+
+require:
+  - .rubocop/custom_cop/top_level_constants_per_file.rb
+
 AllCops:
   TargetRubyVersion: 2.5
   TargetRailsVersion: 5.1
-  Include:
-    - '**/config.ru'
-    - '**/Rakefile'
-    - '**/*.rake'
   Exclude:
     - 'bin/**/*'
     - 'db/**/*'
@@ -62,3 +63,11 @@ Style/TrailingCommaInArrayLiteral:
 
 Style/TrailingCommaInHashLiteral:
   Enabled: false
+
+CustomCop/TopLevelConstantsPerFile:
+  Enabled: true
+  Include:
+    - 'app/**/*'
+    - 'lib/**/*'
+  Exclude:
+    - '**/*[^.rb]' # exclude non-.rb files (ex: .rake files)

.rubocop/custom_cop/top_level_constants_per_file.rb

@@ -0,0 +1,28 @@
+module RuboCop
+  module CustomCop
+    class TopLevelConstantsPerFile < RuboCop::Cop::Cop
+      MSG = "Multiple top-level constants detected in one file. The autoloader expects one top-level constant per file."
+
+      def investigate(processed_source)
+        return unless processed_source
+
+        # If more than one top-level constant in the file, add offense on the second one
+        if top_level_constant_nodes.size > 1
+          add_offense(top_level_constant_nodes[1], message: MSG)
+        end
+      end
+
+      private
+
+      def top_level_constant_nodes
+        @top_level_constant_nodes ||=
+          processed_source.ast.each_node(:class, :module).select { |node| top_level_constant?(node) }
+      end
+
+      def top_level_constant?(node)
+        # node is not nested within a class or module node?
+        node.ancestors.none? { |ancestor| ancestor.class_type? || ancestor.module_type? }
+      end
+    end
+  end
+end