Interesting, looked at the Gentoo docs to understand USE flags. Nix has similar capabilities, where some packages expose configuration options that apply to the build, but it’s not a overtly named feature consistently applies across all packages. It seems that something like USE flags could be implemented rather easily by Nix but was either deemed not necessary or was an oversight. You can still change the build for any package but it might involve introspecting the package definition to figure out what to change so not meant as a first class mechanism like USE flags.