This is different though. The attackers of the AUR don't have access to do anything to upstream and any malicious dependency they add would have to be either 1) already built as an official package or 2) also taken from the AUR... in which case the person building it would need to audit the dependency as well.

So you have two "AUR hygiene" principals at play: One, know what software you're even installing, and Two, know that the PKGBUILD does what it says on the tin. If you neglect either of these and YOLO then it's kind of on you.

Arch users should really know that the AUR is something to approach with a massive amount of caution. It's better than "curl bash" from some rando web site, but that's only due to the fact that you can easily audit and diff the payload of the install recipe yourself.

When you `makepkg -s`, makepkg will get the dependencies it can from the vetted and maintained pacman repos. Only the dependencies that are not present there would have to be obtained from the AUR the same way as the package you're currently reviewing: git clone, manually review, makepkg, etc.

Having dependencies in the AUR is not that common in my experience. I think I've had rarely 1 or 2 deps in the AUR; maybe once or twice I had like 6 deps. It can happen, and it's a bit of a chore, but it can be done.

Having code reviewed the PKGBUILD doesn't mean the upstream software is safe to use, having reviewed the upstream software and it's dependency tree doesn't mean the PKGBUILD is safe to use.