add rubocop cop to prevent the safe navigation operator

Close #596

.rubocop.yml

@@ -1,4 +1,5 @@
 # Project setup:
+require: ./extras/rubocop/cop/style/disallow_safe_navigation
 Rails:
   Enabled: true
 AllCops:

app/helpers/users_helper.rb

@@ -9,7 +9,7 @@ module UsersHelper
     capture do
       concat link_to(stories_displayed, "/newest/#{showing_user.username}")
 
-      concat(" (+#{stories_deleted} deleted)") if @user&.is_moderator? && stories_deleted > 0
+      concat(" (+#{stories_deleted} deleted)") if user_is_moderator? && stories_deleted > 0
 
       if tag
         concat ", most commonly tagged "
@@ -24,9 +24,15 @@ module UsersHelper
     capture do
       concat link_to(showing_user.comments_posted_count, "/threads/#{showing_user.username}")
 
-      if @user&.is_moderator? && comments_deleted > 0
+      if user_is_moderator? && comments_deleted > 0
         concat " (+#{comments_deleted} deleted)"
       end
     end
   end
+
+private
+
+  def user_is_moderator?
+    @user && @user.is_moderator?
+  end
 end

extras/rubocop/cop/style/disallow_safe_navigation.rb

@@ -0,0 +1,27 @@
+module RuboCop
+  module Cop
+    module Style
+      # The "safe navigation" operator &. makes it easier to work with and
+      # propagate nil values. This will disallow the use of the safe navigation
+      # operator
+      #
+      # @example
+      # 
+      #   # bad
+      #   foo&.bar
+      #   a.foo&.bar
+      #
+      class DisallowSafeNavigation < Cop
+        extend TargetRubyVersion
+
+        MSG = 'Do not use &.'.freeze
+
+        minimum_target_ruby_version 2.3
+
+        def on_csend(node)
+          add_offense(node)
+        end
+      end
+    end
+  end
+end