Currently there is an undocumented contract inside nextpnr that site PIPs should only be used for nets that originate/terminate inside that site: https://github.com/YosysHQ/nextpnr/blob/c322cda3f875a5e5dd2575d3a390cbe1cee073e0/fpga_interchange/arch.cc#L1769-L1791

In particular, site PIPs shouldn't be used as general route-throughs, for example through sites that aren't otherwise occupied at all. Tile pseudo-PIPs should be used for these cases.

However, there are two problems:

• this contract currently isn't documented anywhere (I suspect it might originate from an implicit contract of Vivado) and would be a surprise to anyone implementing another PnR implementation without looking at nextpnr
• there are edge cases not covered by this as discussed later on in Wrong LUT equation when used as LUT-thru python-fpga-interchange#76. In particular for the case where a net re-enters a site via a site pin being used for another purpose. Use of site route-through PIPs should probably be disallowed in this edge case too but we need to formally document the exact allowed/illegal behaviour here and then implement accordingly in nextpnr.