nixos-configuration/nix-os/unstable-packages.nix

{
  config,
  lib,
  pkgs,
  self,
  ...
}:

let
  cfg = config.unstable;
  unstableOverlay = self.overlays.unstableWithMeta;
in
{
  options.unstable = {
    enable = lib.mkEnableOption (lib.mkDoc ''
      use of unstable packages in configuration. You can use `unstablePkgs` in configuration modules
    '') // { default = true; };
  };
  config = {
    _module.args.unstablePkgs = if config.unstable.enable then pkgs.unstable else pkgs;
    nixpkgs.overlays = lib.mkIf cfg.enable [ unstableOverlay ];
    system.extraSystemBuilderCmds = lib.mkIf config.unstable.enable ''
      echo ${pkgs.unstableVersion} > $out/nixos-unstable-version
    '';
  };
}