Given the intricate relationship between fish and fish-foreign-env orchestrated by programs.fish, I wasn't able to test this patch. Any suggestions on how to do this are welcome.

Thank you. To test NixOS modules changes I generally do this:

1. write a configuration sample to test.nix, in this case something like:

 { config, pkgs, ...}:

{
  networking.hostName = "vmhost";
  users.extraUsers.vm = {
    isNormalUser = true;
    password     = "vm";
    shell        = "${pkgs.fish}/bin/fish";
    extraGroups  = [ "wheel" ];
  };
  programs.fish.enable = true;
}

2. Use nixos-rebuild -I nixpkgs=$HOME/src/nixpkgs -I nixos-config=$HOME/src/test.nix build-vm
   to build a virtual machine with the above configuration. (use the appropriate paths for nixpkgs and test.nix)

3. Run ./result/bin/run-vmhost-vm to start the VM and test the changes.

It seems ok, could you just fix the indentation in the patch file?

@rnhmjoj thank you very much, this is indeed a perfect way to test it. I did it and everything worked great.

I also amended this PR for indentation, thanks for noticing!