nixos/common: per-user useUserPackages option

to permit enabling this feature for users specified in
`home-manager.users` that have a corresponding entry in `users.users`
while still supporting users that do not.

modules/home-environment.nix

@@ -1,4 +1,4 @@
-{ config, lib, pkgs, ... }:
+{ config, lib, pkgs, ... }@args:
 
 let
   inherit (lib) literalExpression mkOption types;
@@ -500,6 +500,21 @@ in
         Whether to make programs use XDG directories whenever supported.
       '';
     };
+
+    home.useUserPackages = mkOption {
+      type = types.bool;
+      default = args.nixosConfig.home-manager.useUserPackages or false;
+      defaultText = lib.literalExpression "home-manager.useUserPackages";
+      description = ''
+        Enable installation of user packages through the
+        {option}`users.users.<name>.packages` option.
+
+        ::: {.warning}
+        This option is specific to NixOS configurations and has no effect
+        elsewhere.
+        :::
+      '';
+    };
   };
 
   config = {
@@ -521,6 +536,7 @@ in
         releaseMismatch =
           config.home.enableNixpkgsReleaseCheck
           && hmRelease != nixpkgsRelease;
+        wronglyUsingUserPackages = cfg.useUserPackages && (!(args ?  nixosConfig));
       in
         lib.optional releaseMismatch ''
           You are using
@@ -537,7 +553,11 @@ in
             home.enableNixpkgsReleaseCheck = false;
 
           to your configuration.
-        '';
+        ''
+        ++ lib.optional wronglyUsingUserPackages
+            "You have enabled `home.useUserPackages`, but this option has no effect outside of NixOS configurations.";
+
+
 
     home.username =
       lib.mkIf (lib.versionOlder config.home.stateVersion "20.09")

nixos/common.nix

@@ -10,6 +10,10 @@ let
 
   extendedLib = import ../modules/lib/stdlib-extended.nix lib;
 
+  usersUsingUserPackages =
+    lib.filterAttrs (_username: usercfg: usercfg.home.useUserPackages)
+    cfg.users;
+
   # `defaultOverridePriority` (and its legacy counterpart `defaultPriority`)
   # represents the priority of option definitions that do not explicity specify
   # a priority; that is, definitions of the form `{ foo = "bar"; }`.
@@ -36,7 +40,7 @@ let
       modulesPath = builtins.toString ../modules;
     } // cfg.extraSpecialArgs;
     modules = [
-      ({ name, ... }: {
+      ({ name, ... }@usercfg: {
         imports = import ../modules/modules.nix {
           inherit pkgs;
           lib = extendedLib;
@@ -45,7 +49,8 @@ let
 
         config = {
           submoduleSupport.enable = true;
-          submoduleSupport.externalPackageInstall = cfg.useUserPackages;
+          submoduleSupport.externalPackageInstall =
+            usercfg.config.home.useUserPackages;
 
           home.username = mkUserDefault config.users.users.${name}.name;
           home.homeDirectory = mkUserDefault config.users.users.${name}.home;
@@ -69,8 +74,8 @@ let
 in {
   options.home-manager = {
     useUserPackages = mkEnableOption ''
-      installation of user packages through the
-      {option}`users.users.<name>.packages` option'';
+      the per-user option {option}`home-manager.users.<name>.useUserPackages`
+      by default'';
 
     useGlobalPkgs = mkEnableOption ''
       using the system configuration's `pkgs`
@@ -121,10 +126,10 @@ in {
   };
 
   config = (lib.mkMerge [
-    # Fix potential recursion when configuring home-manager users based on values in users.users #594
-    (mkIf (cfg.useUserPackages && cfg.users != { }) {
+    (mkIf (usersUsingUserPackages != { }) {
       users.users = (lib.mapAttrs
-        (_username: usercfg: { packages = [ usercfg.home.path ]; }) cfg.users);
+        (_username: usercfg: { packages = [ usercfg.home.path ]; })
+        usersUsingUserPackages);
       environment.pathsToLink = [ "/etc/profile.d" ];
     })
     (mkIf (cfg.users != { }) {

nixos/default.nix

@@ -16,19 +16,22 @@ in {
       home-manager = {
         extraSpecialArgs.nixosConfig = config;
 
-        sharedModules = [{
-          key = "home-manager#nixos-shared-module";
+        sharedModules = [
+          ({ ... }@usercfg: {
+            key = "home-manager#nixos-shared-module";
 
-          config = {
-            # The per-user directory inside /etc/profiles is not known by
-            # fontconfig by default.
-            fonts.fontconfig.enable = lib.mkDefault
-              (cfg.useUserPackages && config.fonts.fontconfig.enable);
+            config = {
+              # The per-user directory inside /etc/profiles is not known by
+              # fontconfig by default.
+              fonts.fontconfig.enable = lib.mkDefault
+                (usercfg.config.home.useUserPackages != { }
+                  && config.fonts.fontconfig.enable);
 
-            # Inherit glibcLocales setting from NixOS.
-            i18n.glibcLocales = lib.mkDefault config.i18n.glibcLocales;
-          };
-        }];
+              # Inherit glibcLocales setting from NixOS.
+              i18n.glibcLocales = lib.mkDefault config.i18n.glibcLocales;
+            };
+          })
+        ];
       };
     }
     (lib.mkIf (cfg.users != { }) {