code: public static function calculateTrailingWhitespaceIndent(Token $token)
      {
          if (!$token->isWhitespace()) {
              throw new \InvalidArgumentException(sprintf('The given token must be whitespace, got "%s".', $token->getName()));
          }
  
          $str = strrchr(
              str_replace(["\r\n", "\r"], "\n", $token->getContent()),
              "\n"
          );
  
          if (false === $str) {
              return '';
          }
  
          return ltrim($str, "\n");
      }
docs: Calculate the trailing whitespace.
  
  What we're doing here is grabbing everything after the final newline.
  
  @param Token $token
  
  @return string