[Pitch #2] Forward scan matching for trailing closures

[anandabits (Matthew Johnson)]

@Douglas_Gregor

My proposal's approach here might be wrong from an evolution standpoint: I'm saying we should commit ourselves to not having the heuristic in Swift 6, but leave space for the discussion of what should replace it. Alternatively, I could state the heuristic as part of the proposal, with future directions arguing that we should replace it with something else. Perhaps that better conveys that we have an acceptable "resting" place after this proposal, and leave it to later proposals to argue for more source breakage in this area.

I think we can agree that this pitch is a better "resting" place than the one we have right now. IIUC, your desire is to get this change in before Swift 5.3 ships, right? I would support that. We can discuss the longer term changes when there is more time.