nixos is an example of a distro that is source based and can do the atomic rebuild here because it has a way to build packages against other packages that aren'

nixos is an example of a distro that is source based and can do the atomic rebuild here because it has a way to build packages against other packages that aren't "installed". In nixos, this is because there are very few things that get "installed" as the global, only thing to use. But one could imagine that Gentoo could build something that would allow them to at least build up one set of new packages without installing them and then install the files all at once.