在我的例子中，我忘了在结构体中实现INetworkSerializable。一旦我这样做了，一切就都好了：

private struct ToolInfo : INetworkSerializable
{
    public Quaternion rot;
    public Vector3 pos;
    public Item.ItemInfo item;

    public void NetworkSerialize<T>(BufferSerializer<T> serializer) where T : IReaderWriter
    {
        if (serializer.IsReader)
        {
            var reader = serializer.GetFastBufferReader();
            reader.ReadValueSafe(out rot);
            reader.ReadValueSafe(out pos);
            reader.ReadValueSafe(out item);
        }
        else
        {
            var writer = serializer.GetFastBufferWriter();
            writer.WriteValueSafe(rot);
            writer.WriteValueSafe(pos);
            writer.WriteValueSafe(item);
        }
    }
}