Create Cop to enforce use of namespaced classes

A class must be nested inside a module representing
the domain where it operates.

rubocop/cop/gitlab/namespaced_class.rb

+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Gitlab
+      # Cop that enforces use of namespaced classes in order to better identify
+      # high level domains within the codebase.
+
+      # @example
+      #   # bad
+      #   class MyClass
+      #   end
+      #
+      #   # good
+      #   module MyDomain
+      #     class MyClass
+      #     end
+      #   end
+
+      class NamespacedClass < RuboCop::Cop::Cop
+        MSG = 'Classes must be declared inside a module indicating a product domain namespace. For more info: https://gitlab.com/gitlab-org/gitlab/-/issues/212156'
+
+        def_node_matcher :compact_namespaced_class?, <<~PATTERN
+          (class (const (const ...) ...) ...)
+        PATTERN
+
+        def on_module(node)
+          @namespaced = true
+        end
+
+        def on_class(node)
+          return if @namespaced
+
+          add_offense(node) unless compact_namespaced_class?(node)
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/gitlab/namespaced_class_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+require 'rubocop'
+require 'rubocop/rspec/support'
+require_relative '../../../../rubocop/cop/gitlab/namespaced_class'
+
+RSpec.describe RuboCop::Cop::Gitlab::NamespacedClass do
+  subject(:cop) { described_class.new }
+
+  it 'flags a class definition without namespace' do
+    expect_offense(<<~SOURCE)
+      class MyClass
+      ^^^^^^^^^^^^^ #{described_class::MSG}
+      end
+    SOURCE
+  end
+
+  it 'flags a class definition with inheritance without namespace' do
+    expect_offense(<<~SOURCE)
+      class MyClass < ApplicationRecord
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ #{described_class::MSG}
+        def some_method
+          true
+        end
+      end
+    SOURCE
+  end
+
+  it 'does not flag the class definition with namespace in separate lines' do
+    expect_no_offenses(<<~SOURCE)
+      module MyModule
+        class MyClass < ApplicationRecord
+        end
+
+        class MyOtherClass
+          def other_method
+            1 + 1
+          end
+        end
+      end
+    SOURCE
+  end
+
+  it 'does not flag the class definition with nested namespace in separate lines' do
+    expect_no_offenses(<<~SOURCE)
+      module TopLevelModule
+        module NestedModule
+          class MyClass
+          end
+        end
+      end
+    SOURCE
+  end
+
+  it 'does not flag the class definition nested inside namespaced class' do
+    expect_no_offenses(<<~SOURCE)
+      module TopLevelModule
+        class TopLevelClass
+          class MyClass
+          end
+        end
+      end
+    SOURCE
+  end
+
+  it 'does not flag a compact namespaced class definition' do
+    expect_no_offenses(<<~SOURCE)
+      class MyModule::MyClass < ApplicationRecord
+      end
+    SOURCE
+  end
+end