tree-wide: init fish completion module

nix/configurations/hydrogen.nix

@@ -58,15 +58,16 @@ in
       ./hydrogen/nixos/programs.nix
       ./hydrogen/nixos/connectivity.nix
 
-      # QUIRK:
-      # Had issue when building the installer as it fails to bootstrap itself
-      # Might be useful to disable for the first build.
-      ../nixosModules/extra/secure_dns.nix
+      ../nixosModules/common/fish.nix
       ../nixosModules/common/disable-command-not-found.nix
       ../nixosModules/common/network.nix
       ../nixosModules/common/sudo-conf.nix
       ../nixosModules/common/system-nixconf.nix
 
+      # QUIRK:
+      # Had issue when building the installer as it fails to bootstrap itself
+      # Might be useful to disable for the first build.
+      ../nixosModules/extra/secure_dns.nix
       ../nixosModules/extra/leana.nix
 
       #

nix/configurations/vanadium.nix

@@ -101,6 +101,7 @@ in
       ./vanadium/nixos/unfree-predicate.nix
       ./vanadium/nixos/programs.nix
 
+      ../nixosModules/common/fish.nix
       ../nixosModules/common/disable-command-not-found.nix
       ../nixosModules/common/network.nix
       ../nixosModules/common/sudo-conf.nix

nix/nixosModules/common/fish.nix

@@ -0,0 +1,22 @@
+#
+# This module links fish vendor completions, otherwise tab completion would be broken.
+#
+{
+  config,
+  lib,
+  ...
+}: let
+  fishNixOSEnabled = config.programs.fish.enable;
+  fishHMEnabled =
+    if config ? home-manager
+    then lib.any (userConfig: userConfig.programs.fish.enable) (lib.attrValues config.home-manager.users)
+    else false;
+
+  fishEnabled = fishNixOSEnabled || fishHMEnabled;
+in {
+  environment.pathsToLink = lib.mkIf fishEnabled [
+    "/share/fish/vendor_conf.d"
+    "/share/fish/vendor_completions.d"
+    "/share/fish/vendor_functions.d"
+  ];
+}

nix/nixosModules/extra/leana.nix

@@ -18,12 +18,6 @@
   };
   users.groups.leana = {};
 
-  environment.pathsToLink = [
-    "/share/fish/vendor_conf.d"
-    "/share/fish/vendor_completions.d"
-    "/share/fish/vendor_functions.d"
-  ];
-
   #
   # My nix binary cache
   #