Custom Rubocop rule to prevent unless with multiple conditions

.rubocop.yml

require:
  - ./.rubocop_custom_cops.rb

AllCops:
  # ...

.rubocop_custom_cops.rb

module RuboCop
  module Cop
    module Custom
      class UnlessMultipleConditions < Cop
        MSG = 'Avoid `unless` with multiple conditions. ' \
          'Rewrite the conditions with `if` instead.'.freeze

        def on_if(node)
          if node.unless? && node.condition.respond_to?(:operator)
            add_offense(node)
          end
        end
      end
    end
  end
end