It would be great to somehow store the state of the configuration used in a package build inside the package. Think, a hash over the config files used by savedconfig.eclass or similar mechanisms. Then, upon binary package installation, portage could check if the hash still matches and reject the binary package otherwise. At the moment I see this as a problem that * partially needs to be solved in "eclass- or ebuild-space" (figure out what things all critically influence the build, e.g. list of config files, hash that and pass the hash somehow to portage), and * partially needs to be solved in portage itself (add the hash to binary packages, check the hash on binary package usage). Most important use case I can see is savedconfig + kernel builds in releng (we have boot media with more than one kernel / kernel config).
Much simpler idea, which would solve the same problem: * Introduce an emerge command line option that can "tag" a binary package upon creation emerge --buildpkg --set-tag="minimal" busybox * Introduce an emerge command line option that requires a tag from a package emerge -k --need-tag="minimal" busybox
