“ORC is Nim’s all-new cycle collector based on ARC. It can be considered a full-blown GC since it includes a local tracing phase (contrary to most other tracing GCs which do global tracing). ORC is what you should use when working with Nim’s async because it contains cycles that need to be dealt with.”看起来ORC是个好东西，能解决ARC不回收循环引用的问题。