What's the proper channel for asking about design rules? There's some rules I expect that I can't find any reference to

Try #efabless.

For the sky130 pdk?

Yep. I guess you already know about https://skywater-pdk.readthedocs.io/

yup

@User hope this helps

So, first of all, thanks. But I don't see it. The rules I'm looking for are "maximum distance of mosfet diffusion to welltap". I expect it to be on the order on 10 micro-meters (could be 5um, could be 50um), and it might vary by device type. Right now all my grepping and skimming has failed to find anything about it.

It’s <15um as I remember, I used 25um (tap-to-tap) in my designs

Maybe the problem is that sometimes it can be described as a antenna rule, so it can't be found among the periphery rules.

It's not an antenna rule though, because it doesn't have anything to do with electrostatic discharge or the device being destroyed. It's to warn you that the bulk connection is not solid and the voltage may drift - so the simulation/models will not represent the actual operation of the device. It also warns you about latchup.