Discourage direct use of ignored_columns in models

This discourages the use of `self.ignored_columns += ...` in models and
instead suggests to use `IgnorableColumns` concern and `ignore_column`
methods provided.

app/models/concerns/ignorable_columns.rb

@@ -21,7 +21,7 @@ module IgnorableColumns
       raise ArgumentError, 'Please indicate when we can stop ignoring columns with remove_after (date string), example: ignore_columns(:name, remove_after: \'2019-12-01\', remove_with: \'12.6\')' unless remove_after
       raise ArgumentError, 'Please indicate in which release we can stop ignoring columns with remove_with, example: ignore_columns(:name, remove_after: \'2019-12-01\', remove_with: \'12.6\')' unless remove_with
 
-      self.ignored_columns += columns.flatten
+      self.ignored_columns += columns.flatten # rubocop:disable Cop/IgnoredColumns
 
       unless defined?(@ignored_columns_details)
         @ignored_columns_details = superclass.try(:ignored_columns_details)&.dup || {}

rubocop/cop/ignored_columns.rb

+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+
+    # Cop that blacklists the usage of Group.public_or_visible_to_user
+    class IgnoredColumns < RuboCop::Cop::Cop
+      MSG = 'Use `IgnoredColumns` concern instead of adding to `self.ignored_columns`.'
+
+      def_node_matcher :ignored_columns?, <<~PATTERN
+        (send (self) :ignored_columns)
+      PATTERN
+
+      def on_send(node)
+        return unless ignored_columns?(node)
+
+        add_offense(node, location: :expression)
+      end
+    end
+  end
+end

rubocop/rubocop.rb

@@ -54,3 +54,4 @@ require_relative 'cop/group_public_or_visible_to_user'
 require_relative 'cop/inject_enterprise_edition_module'
 require_relative 'cop/graphql/authorize_types'
 require_relative 'cop/graphql/descriptions'
+require_relative 'cop/ignored_columns'

spec/rubocop/cop/ignored_columns_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'rubocop'
+require 'rubocop/rspec/support'
+require_relative '../../../rubocop/cop/ignored_columns'
+
+describe RuboCop::Cop::IgnoredColumns do
+  include CopHelper
+
+  subject(:cop) { described_class.new }
+
+  it 'flags the use of destroy_all with a local variable receiver' do
+    inspect_source(<<~RUBY)
+      class Foo < ApplicationRecord
+        self.ignored_columns += %i[id]
+      end
+    RUBY
+
+    expect(cop.offenses.size).to eq(1)
+  end
+end