This new version of Mesa had been up about 45 minutes today. It broke all X, no new apps could be opened, logout screen crashed, bootup fail in SDDM.
It was removed as soon as this became clear, and probably no-one else tried to reboot with this version, but just in case anyone on the build repo did reboot and find themselves with login failing at SDDM, just go to tty2 and run
sudo pacman -Syyuu and restart SDDM,
sudo systemctl restart sddm.