presto-spi/src/main/java/io/prestosql/spi/block/VariableWidthBlockBuilder.java/VariableWidthBlockBuilder/toString
class VariableWidthBlockBuilder:    @Override
    public String toString()
    {
        StringBuilder sb = new StringBuilder("VariableWidthBlockBuilder{");
        sb.append("positionCount=").append(positions);
        sb.append(", size=").append(sliceOutput.size());
        sb.append('}');
        return sb.toString();
    }