[Fix rubocop#368] Add DelegatePrivate cop

changelog/add_delegate_private_cop.md

@@ -0,0 +1 @@
+* [#368](https://github.com/rubocop/rubocop-rails/issues/368): Add DelegatePrivate cop. ([@povilasjurcys][])

config/default.yml

@@ -336,6 +336,11 @@ Rails/DelegateAllowBlank:
   Enabled: true
   VersionAdded: '0.44'
 
+Rails/DelegatePrivate:
+  Description: 'Use delegate with `private: true` option in private scope.'
+  Enabled: true
+  VersionAdded: '2.20.2'
+
 Rails/DeprecatedActiveModelErrorsMethods:
   Description: 'Avoid manipulating ActiveModel errors hash directly.'
   Enabled: pending

lib/rubocop/cop/rails/delegate_private.rb

@@ -0,0 +1,106 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # @example DelegatePrivate
+      #   # bad
+      #   class Foo
+      #     def bar=Bar.new
+      #
+      #     private
+      #
+      #     delegate :baz, to: :bar
+      #   end
+      #
+      #   # bad
+      #   class Foo
+      #     delegate :baz, to: :bar, private: true
+      #
+      #     def bar=Bar.new
+      #   end
+      #
+      #   # good
+      #   class Foo
+      #     def bar=Bar.new
+      #
+      #     private
+      #
+      #     delegate :baz, to: :bar, private: true
+      #   end
+      class DelegatePrivate < Base
+        extend TargetRailsVersion
+
+        MSG_MISSING_PRIVATE = '`delegate` in private section should have `private: true` option'
+        MSG_WRONG_PRIVATE = 'private `delegate` should be put in private section'
+
+        minimum_target_rails_version 6.0
+
+        def on_send(node)
+          mark_scope(node)
+          return unless delegate_node?(node)
+
+          if private_scope?(node) && !private_delegate?(node)
+            add_offense(node, message: MSG_MISSING_PRIVATE)
+          elsif public_scope?(node) && private_delegate?(node)
+            add_offense(node, message: MSG_WRONG_PRIVATE)
+          end
+        end
+
+        private
+
+        def private_delegate?(node)
+          node.arguments.select(&:hash_type?).each do |hash_node|
+            hash_node.each_pair do |key_node, value_node|
+              return true if key_node.value == :private && value_node.true_type?
+            end
+          end
+
+          false
+        end
+
+        def mark_scope(node)
+          receiver_node, method_name, arg_node = *node
+          return if receiver_node || arg_node
+
+          @private_ranges ||= []
+
+          if method_name == :private
+            add_private_range(node)
+          elsif method_name == :public
+            cut_private_range_from(node.location.first_line)
+          end
+        end
+
+        def delegate_node?(node)
+          return false if node.receiver
+
+          node.method_name == :delegate
+        end
+
+        def private_scope?(node)
+          @private_ranges&.any? { |range| range.include?(node.location.first_line) }
+        end
+
+        def public_scope?(node)
+          !private_scope?(node)
+        end
+
+        def add_private_range(node)
+          @private_ranges ||= []
+          @private_ranges += [node.location.first_line..node.parent.last_line]
+        end
+
+        def cut_private_range_from(from_line)
+          @private_ranges ||= []
+          @private_ranges = @private_ranges.each.with_object([]) do |range, new_ranges|
+            next if range.begin > from_line
+
+            new_range = range.include?(from_line) ? (range.begin...from_line) : range
+            new_ranges << new_range
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -36,6 +36,7 @@
 require_relative 'rails/default_scope'
 require_relative 'rails/delegate'
 require_relative 'rails/delegate_allow_blank'
+require_relative 'rails/delegate_private'
 require_relative 'rails/deprecated_active_model_errors_methods'
 require_relative 'rails/dot_separated_keys'
 require_relative 'rails/duplicate_association'

spec/rubocop/cop/rails/delegate_private_spec.rb

@@ -0,0 +1,115 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::DelegatePrivate, :config, :rails60 do
+  context 'when no delegate is provided' do
+    it 'registers no offense' do
+      expect_no_offenses(<<~RUBY)
+        class User
+        end
+      RUBY
+    end
+  end
+
+  context 'when delegate is provided in public scope without "private: true"' do
+    it 'registers no offense' do
+      expect_no_offenses(<<~RUBY)
+        class User
+          delegate :name, to: :user
+        end
+      RUBY
+    end
+  end
+
+  context 'when delegate is provided in public scope with "private: true"' do
+    it 'registers an offense' do
+      expect_offense(<<~RUBY)
+        class User
+          delegate :name, to: :user, private: true
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ private `delegate` should be put in private section
+        end
+      RUBY
+    end
+  end
+
+  context 'when delegate is provided in private scope without "private: true"' do
+    it 'registers an offense' do
+      expect_offense(<<~RUBY)
+        class User
+          private
+
+          delegate :name, to: :user
+          ^^^^^^^^^^^^^^^^^^^^^^^^^ `delegate` in private section should have `private: true` option
+        end
+      RUBY
+    end
+  end
+
+  context 'when delegate is provided in private scope with "private: true"' do
+    it 'registers no offense' do
+      expect_no_offenses(<<~RUBY)
+        class User
+          private
+
+          delegate :name, to: :user, private: true
+        end
+      RUBY
+    end
+  end
+
+  context 'when delegate is provided in public scope without "private: true" in outer class' do
+    it 'registers no offense' do
+      expect_no_offenses(<<~RUBY)
+        class User
+          class InnerUser
+            private
+            def foo; end
+          end
+
+          delegate :name, to: :user
+        end
+      RUBY
+    end
+  end
+
+  context 'when `private: true` is in explicit public scope' do
+    it 'registers no offense' do
+      expect_no_offenses(<<~RUBY)
+        class User
+          private
+
+          def foo
+          end
+
+          public
+
+          delegate :name, to: :user
+        end
+      RUBY
+    end
+  end
+
+  context 'when private scope is set on method, but `private: true` is used in public scope' do
+    it 'registers no offense' do
+      expect_no_offenses(<<~RUBY)
+        class User
+          private def foo
+          end
+
+          delegate :name, to: :user
+        end
+      RUBY
+    end
+  end
+
+  context 'with rails < 6.0', :rails52 do
+    it 'registers no offense' do
+      expect_no_offenses(<<~RUBY)
+        class User
+          private
+
+          delegate :name, to: :user
+        end
+      RUBY
+    end
+  end
+end