Is anyone using CircuitDag?

[mpharrigan]

I did some digging, since I've never personally used CircuitDag. I see it was added here #759 in response to this feature request: #547. Searching for usages turns up some utility in cirq.contrib.routing. Otherwise, my reading of these issues and some personal intuition is that this class was originally a "solution looking for a problem". Even in the case with of #3599, we found out that CircuitDag was close but not what we wanted.

One way of moving forward with #3599 is to extend CircuitDag to do what we need for factorizing circuits, but I think we'd need a better set of use cases.

Another proposal is to deprecate CircuitDag from main cirq prior to 1.0, move the functionality to cirq.contrib.routing and wait on a full design for the time at which we add routing to main-cirq

cc @bryano @cduck @Strilanc

[balopat]

Cirq Cynque verdict: deprecate nicely into contrib, where it will fall into the "archive" category eventually as we dismantle contrib.

[dstrain115]

@tanujkhattar What is the action item here? Should we deprecate CircuitDag?

[tanujkhattar]

+1, let's deprecate and move to contrib.