treewide: move NixOS-inherited defaults to their option definitions

modules/config/i18n.nix

@@ -16,9 +16,10 @@
 # https://github.com/NixOS/nixpkgs/blob/nixpkgs-unstable/pkgs/development/libraries/glibc/nix-locale-archive.patch
 
 {
-  lib,
-  pkgs,
   config,
+  lib,
+  nixosConfig,
+  pkgs,
   ...
 }:
 
@@ -55,9 +56,8 @@ in
 
         This option only applies to the Linux platform.
 
-        When Home Manager is configured with NixOS, the default value
-        will be set to {var}`i18n.glibcLocales` from the
-        system configuration.
+        If Home Manager is installed as a NixOS submodule, this
+        option defaults to NixOS' {var}`i18n.glibcLocales`.
       '';
       example = lib.literalExpression ''
         pkgs.glibcLocales.override {
@@ -65,9 +65,8 @@ in
           locales = [ "en_US.UTF-8/UTF-8" ];
         }
       '';
-      # NB. See nixos/default.nix for NixOS default.
-      default = pkgs.glibcLocales;
-      defaultText = lib.literalExpression "pkgs.glibcLocales";
+      default = nixosConfig.i18n.glibcLocales or pkgs.glibcLocales;
+      defaultText = lib.literalExpression "nixosConfig.i18n.glibcLocales or pkgs.glibcLocales";
     };
   };
 

modules/misc/fontconfig.nix

@@ -5,6 +5,7 @@
 {
   config,
   lib,
+  nixosConfig,
   pkgs,
   ...
 }:
@@ -70,7 +71,6 @@ in
     fonts.fontconfig = {
       enable = lib.mkOption {
         type = lib.types.bool;
-        default = false;
         description = ''
           Whether to enable fontconfig configuration. This will, for example,
           allow fontconfig to discover fonts and configurations installed through
@@ -80,6 +80,17 @@ in
           {var}`home-manager.useUserPackages` is enabled, this option defaults to the
           value of NixOS' {var}`fonts.fontconfig.enable`.
         '';
+        # On NixOS, the per-user directory inside /etc/profiles is not known by
+        # fontconfig by default.
+        default =
+          nixosConfig != null
+          && nixosConfig.home-manager.useUserPackages
+          && nixosConfig.fonts.fontconfig.enable;
+        defaultText = lib.literalExpression ''
+          nixosConfig != null
+          && nixosConfig.home-manager.useUserPackages
+          && nixosConfig.fonts.fontconfig.enable;
+        '';
       };
 
       defaultFonts = {

nixos/default.nix

@@ -68,29 +68,7 @@ in
   };
 
   config = lib.mkMerge [
-    {
-      home-manager = {
-        extraSpecialArgs.nixosConfig = config;
-
-        sharedModules = [
-          {
-            key = "home-manager#nixos-shared-module";
-
-            config = {
-              # The per-user directory inside /etc/profiles is not known by
-              # fontconfig by default.
-              #
-              # When changing the following line, remember to reflect these
-              # changes in the description of 'fonts.fontconfig.enable'!
-              fonts.fontconfig.enable = lib.mkDefault (cfg.useUserPackages && config.fonts.fontconfig.enable);
-
-              # Inherit glibcLocales setting from NixOS.
-              i18n.glibcLocales = lib.mkDefault config.i18n.glibcLocales;
-            };
-          }
-        ];
-      };
-    }
+    { home-manager.extraSpecialArgs.nixosConfig = config; }
 
     (mkIf (!cfg.startAsUserService && cfg.users != { }) {
       systemd.services = lib.mapAttrs' (