code: func (t *Template) ExecuteFuncString(f TagFunc) string {
  	bb := t.byteBufferPool.Get()
  	if _, err := t.ExecuteFunc(bb, f); err != nil {
  		panic(fmt.Sprintf("unexpected error: %s", err))
  	}
  	s := string(bb.Bytes())
  	bb.Reset()
  	t.byteBufferPool.Put(bb)
  	return s
  }
docs: // ExecuteFuncString calls f on each template tag (placeholder) occurrence
  // and substitutes it with the data written to TagFunc's w.
  //
  // Returns the resulting string.
  //
  // This function is optimized for frozen templates.
  // Use ExecuteFuncString for constantly changing templates.