This is a tracking issue for the unstable mpsc_select feature in the standard library. Known points:

• The API is unsafe, and should probably never be stabilized.
• Servo apparently makes very heavy use of this API, so it can't be removed without replacement currently.
• The correctness of the implementation is somewhat dubious, it's incredibly complicated.
• The macro-based approach for selection is dubious.
• This is not a scalable implementation of selection (e.g. "poll everything each time wait() is called")
• This drags down some other channel-related code with a few mutexes in otherwise lock-free code.