class RuboCop::Cop::Performance::RedundantStringChars

def on_send(node)
  return unless (receiver, method, args = redundant_chars_call?(node))
  return if method == :last && !args.empty?
  range = offense_range(receiver, node)
  message = build_message(method, args)
  add_offense(range, message: message) do |corrector|
    range = correction_range(receiver, node)
    replacement = build_good_method(method, args)
    corrector.replace(range, replacement)
  end
end