I added the non-nix docs, but it only covers hm, because I don’t do anything custom with nixos 
However I still think the basic pattern would be the same. Assuming your custom modules don’t live in glorious isolation from nixos, then what you want to is to eval nixos using your root configuration. You can figure this out in two ways.
- Know that
nixos-rebuild replsomehow is doing what you want - Know that building
<nixpkgs/nixos>does something useful.
Lets assume mystically you’ve decided to look at nixpkgs/nixos/default.nix you’ll notice it takes an argument which is the path to a specific configuration.
❯ nix repl --file '<nixpkgs>'
Nix 2.24.14
Type :? for help.
Loading installable ''...
Added 23037 variables.
nix-repl> options = (import <nixpkgs/nixos> { configuration = /etc/nixos/configuration.nix; }).options
nix-repl> options.
options._module options.ec2 options.jobs options.nixpkgs options.security options.time
options.appstream options.environment options.krb5 options.oci options.services options.users
options.assertions options.fileSystems options.lib options.openstack options.snapraid options.virtualisation
options.boot options.fonts options.location options.passthru options.sound options.warnings
options.console options.gtk options.meta options.power options.specialisation options.xdg
options.containers options.hardware options.nesting options.powerManagement options.stubby options.zramSwap
options.docker-containers options.i18n options.networking options.programs options.swapDevices
options.documentation options.ids options.nix options.qt options.system
options.dysnomia options.isSpecialisation options.nixops options.qt5 options.systemd
These options will contain any custom options you imported under your configuration.
You could test this out then a contribution to the nixd docs