I don't think package-specific functions should be in lib. lib should be generic.

IMO, the kernel configuration is not a package-specific operation (rather platform/linux specific). There is already a library to configure the kernel that is used across nixpkgs:
system.requiredKernelConfig = with config.lib.kernelConfig; [
except it doesn't do anything sensible (the checks are just for show since we don't check the resultiung kenel config anyway).
My approach just moves the library from config.lib to lib with the goal of being more powerful:

• really check the kernel requirements
• allow packages to specify dependencies withotu the need to configure modules (some packages simply can't run without some kernel configuration, no need to add a module for that).

I deal with many types of kernels: out-of-tree kernels, computer-specific configurations, different versions as well as different testing setups (openvswitch/mininet/nixops-libvirtd with virtio reuquirements), builtin modules in order not to have to build an initramfs.

This is my single biggest complaint about nixos, it's all about reproducibility/dependency management, yet when it comes to the kernel we are not allowed to specify/check dependencies. It doesn't have to be like that. It's reasonibly easy to implement and can be opt-in if there are concerns.

for instance one can try to build a kernel with a configuration supposed to be checked by nixpkgs (module support is mandatory):

  kernel0 = (pkgs.linux_latest.override {
    structuredExtraConfig = { MODULES.tristate = "n"; };
  });

  kernel1 = (pkgs.linux_latest.override {
    extraConfig = ''
      MODULES n
    '';
  });

yet none of it is checked by nixpkgs, it just fails building. But for an option that doesn't break the build, it could easily go unnoticed.

There are multiple issues that show that kernel configuration checks could be useful

• Enable kernel configuration option FANOTIFY #6296
• Powertop no longer shows cpufreq_stats ( CPU_FREQ_STATS is turned off in kernel ) #56102
• criu package requires CONFIG_CHECKPOINT_RESTORE kernel option to work #3982
• and several others for containers/virtualization etc.

cc @dtzWill @Ericson2314 @worldofpeace

Looking good! Since this has been unnoticed for a while, is this still fresh or does it need adjustments before merging?

it's up to date ty !