nixosMod/asserts: add mdns avahi & resolved clash check

nix/nixos-modules/assertions/default.nix

@@ -2,6 +2,7 @@
   imports = [
     # files
     ./fileSystems.nix
+    ./mdns.nix
     ./nixos.nix
   ];
 }

nix/nixos-modules/assertions/mdns.nix

@@ -0,0 +1,22 @@
+{ config, lib, ... }:
+let
+  cfgAvahi = config.services.avahi;
+  avahiMDNS = cfgAvahi.enable && (cfgAvahi.nssmdns4 || cfgAvahi.nssmdns6);
+  cfgResolved = config.services.resolved;
+  # TODO check settings when cfgResolved.settings exist
+  resolvedMDNS = cfgResolved.enable;
+in
+{
+  config = {
+
+    assertions = [
+      {
+        assertion = !(avahiMDNS && cfgResolved.enable);
+        message = ''
+          systemd-resolved is enabled while Avahi mDNS is enabled, disable one of both!
+        '';
+      }
+    ];
+
+  };
+}