Revert "Revert "Merge pull request #211603 from farcaller/fish""

This reverts commit 876008b845.

nixos/modules/config/users-groups.nix

@@ -699,7 +699,20 @@ in {
               users.groups.${user.name} = {};
             '';
           }
-        ]
+        ] ++ (map (shell: {
+            assertion = (user.shell == pkgs.${shell}) -> (config.programs.${shell}.enable == true);
+            message = ''
+              users.users.${user.name}.shell is set to ${shell}, but
+              programs.${shell}.enable is not true. This will cause the ${shell}
+              shell to lack the basic nix directories in its PATH and might make
+              logging in as that user impossible. You can fix it with:
+              programs.${shell}.enable = true;
+            '';
+          }) [
+          "fish"
+          "xonsh"
+          "zsh"
+        ])
     ));
 
     warnings =