nixosMod/extends: add assertions about nix settings & dependencies

nix/nixos-modules/extends/default.nix

@@ -1,6 +1,7 @@
 {
   imports = [
     # files
+    ./nixos.nix
     ./podman.nix
   ];
 }

nix/nixos-modules/extends/nixos.nix

@@ -0,0 +1,41 @@
+{ config, lib, ... }:
+let
+  channelsEn = config.nix.channel.enable;
+  nixFeature = lib.trivial.flip builtins.elem config.nix.settings.experimental-features;
+  packageNames = map lib.strings.getName config.environment.systemPackages;
+  gitInst = builtins.elem "git" packageNames;
+  gitEn = config.programs.git.enable;
+in
+{
+  config = {
+
+    assertions = [
+      {
+        assertion = !channelsEn -> nixFeature "flakes";
+        message = ''
+          You disabled Nix channels, then you should enable flakes, otherwise you cannot build a new config.
+        '';
+      }
+      {
+        assertion = (!channelsEn && nixFeature "flakes") -> (gitInst || gitEn);
+        message = ''
+          Missing git, which is required to interact with most flakes.
+        '';
+      }
+      {
+        assertion = nixFeature "flakes" -> nixFeature "nix-command";
+        message = ''
+          Nix experimental-feature "flakes" requires feature "nix-command"
+        '';
+      }
+    ];
+
+    warnings = [
+      # TODO add link to this file
+      (lib.mkIf (gitEn && !gitInst) ''
+        (not relevant for you, please report to the module author) git package was not detected properly, fallback to programs.git module
+      '')
+    ];
+
+  };
+}