I hadn’t done any system maintenance in quite a while:

$ sudo pacman -Syu

And when I did, I got this message:

error: failed to commit transaction (conflicting files)
rpi4-post-install: /etc/udev/rules.d/99-vcio-rewrite.rules exists in filesystem
Errors occurred, no packages were upgraded.

Which means (I’m told)

Previously we added this file to fix a few issues, but it was not owned by any package. So with this update, we switched it to be owned by rpi4-post-install, which results in this file conflict. Workaround: Run

sudo pacman -Syu --overwrite /etc/udev/rules.d/99-vcio-rewrite.rules.