Automatically match NixOS version and repository branch

[arunoruto]

When updating nixos, I need to be more active in updating all occurrences of the nixos version throughout my config.
Since nixvim also needs the version for the branch, I came up with a minor "fix" for this "problem":

{ config, pkgs, lib, ... }:
let
  sysconfig = (import <nixpkgs/nixos> {}).config;
  nixvim = import (builtins.fetchGit {
    url = "https://github.com/nix-community/nixvim";
    ref = "nixos-${sysconfig.system.stateVersion}";
  });
in
{
...
}

This will always fetch the correct version of nixvim after a system update.

My next problem was that I was also deploying my configuration to systems that only provide the Nix package manager (and home manager) but not NixOS itself. On such systems, the line sysconfig = (import <nixpkgs/nixos> {}).config; will give an error since there is no nixos config to be fetched! I have a non-elegant way of solving this, but it works. I am checking if the configuration.nix file is on the system and choosing the appropriate branch to be fetched:

{ config, pkgs, lib, ... }:
let
  sysconfig = if builtins.pathExists "/etc/nixos/configuration.nix" then
    (import <nixpkgs/nixos> {}).config
  else
    "other";
  nixvim = import (builtins.fetchGit {
    url = "https://github.com/nix-community/nixvim";
    ref = if (sysconfig == "other") then "main" else "nixos-${sysconfig.system.stateVersion}";
  });
in
{
...
}

If someone has a more elegant solution or has a more elegant way of solving this, please leave some feedback!