hmModules/assertions: add "/share/zsh" assert from comment

nix/hmModules/assertions/default.nix

@@ -2,5 +2,6 @@
   imports = [
     # files
     ./gpg-agent.nix
+    ./zsh.nix
   ];
 }

nix/hmModules/assertions/zsh.nix

@@ -0,0 +1,24 @@
+{
+  config,
+  lib,
+  osConfig ? null,
+  ...
+}:
+let
+  cfg = config.programs.zsh;
+in
+{
+  config = lib.mkIf cfg.enable {
+
+    assertions = lib.mkIf (!builtins.isNull osConfig) [
+      # see https://github.com/nix-community/home-manager/blob/e1391fb22e18a36f57e6999c7a9f966dc80ac073/modules/programs/zsh.nix#L353
+      {
+        assertion = cfg.enableCompletion -> builtins.elem "/share/zsh" osConfig.environment.pathsToLink;
+        message = ''
+          for useful ZSH completion, add "/share/zsh" to NixOS environment.pathsToLink
+        '';
+      }
+    ];
+
+  };
+}

nix/nixos-modules/frontend/default.nix

@@ -38,7 +38,7 @@ in
     environment = {
 
       pathsToLink = [
-        "/share/zsh" # required for Home-Manager ZSH autocompletion, see https://github.com/nix-community/home-manager/blob/e1391fb22e18a36f57e6999c7a9f966dc80ac073/modules/programs/zsh.nix#L353
+        "/share/zsh" # for ZSH completion
       ];
 
       plasma6.excludePackages = with pkgs.kdePackages; [