Firstly, a huge thank you for bringing control+M to my attention. That's literally going to cumulatively save me days or even weeks of my life that I would have spent continuing to manually format.
Secondly, just so you know, there are some norms of the forum that would dictate that you make a new post in which you link to this very old post for context, instead of commenting directly on the very old post. See one relevant discussion on the topic here.
Thirdly, since I'm already posting on this bumped thread, I'll mention that I think I'm very much in favor of the idea. Especially now that we have variadic generics that allow comma-separated function arguments to play a role that used to require arrays, I think the occasions where one would wish one could end with a comma will be even more frequent. Yesterday while I was playing around with macros and editing the names parameter in @attached(member, names:) I probably bumped into the lack of this feature 10 times in the span of 20 minutes as I commented in/out arguments to the names: parameter.
@davide , you mind making a new post about this (in which you link to this one) so that we can continue the discussion without producing undesired notifications?