toString mkString

mkStringを使いましょう。

final class StringBuilder(private val underlying: JavaStringBuilder)
...

  /** Returns a new String representing the data in this sequence.
   *
   *  @note    because toString is inherited from AnyRef and used for
   *           many purposes, it is better practice to call mkString
   *           to obtain a StringBuilder result.
   *  @return  the current contents of this sequence as a String
   */
  override def toString = underlying.toString

  /** Returns a new String representing the data in this sequence.
   *
   *  @return  the current contents of this sequence as a String
   */
  override def mkString = toString

Google翻訳さま「toStringはAnyRefから継承され、多くの目的に使用されるため、mkStringを呼び出してStringBuilderの結果を取得することをお勧めします。」