Use shell packages to select the user's shell

The string type is still available for backward-compatiblity.

nixos/modules/programs/bash/bash.nix

@@ -200,7 +200,7 @@ in
     # Configuration for readline in bash.
     environment.etc."inputrc".source = ./inputrc;
 
-    users.defaultUserShell = mkDefault "/run/current-system/sw/bin/bash";
+    users.defaultUserShell = mkDefault pkgs.bashInteractive;
 
     environment.pathsToLink = optionals cfg.enableCompletion [
       "/etc/bash_completion.d"

nixos/modules/programs/shadow.nix

@@ -1,6 +1,6 @@
 # Configuration for the pwdutils suite of tools: passwd, useradd, etc.
 
-{ config, lib, pkgs, ... }:
+{ config, lib, utils, pkgs, ... }:
 
 with lib;
 
@@ -43,13 +43,13 @@ in
     users.defaultUserShell = lib.mkOption {
       description = ''
         This option defines the default shell assigned to user
-        accounts.  This must not be a store path, since the path is
+        accounts. This can be either a full system path or a shell package.
+
+        This must not be a store path, since the path is
         used outside the store (in particular in /etc/passwd).
-        Rather, it should be the path of a symlink that points to the
-        actual shell in the Nix store.
       '';
-      example = "/run/current-system/sw/bin/zsh";
-      type = types.path;
+      example = literalExample "pkgs.zsh";
+      type = types.either types.path types.shellPackage;
     };
 
   };
@@ -60,7 +60,9 @@ in
   config = {
 
     environment.systemPackages =
-      lib.optional config.users.mutableUsers pkgs.shadow;
+      lib.optional config.users.mutableUsers pkgs.shadow ++
+      lib.optional (types.shellPackage.check config.users.defaultUserShell)
+        config.users.defaultUserShell;
 
     environment.etc =
       [ { # /etc/login.defs: global configuration for pwdutils.  You
@@ -74,7 +76,7 @@ in
             ''
               GROUP=100
               HOME=/home
-              SHELL=${config.users.defaultUserShell}
+              SHELL=${utils.toShellPath config.users.defaultUserShell}
             '';
           target = "default/useradd";
         }