lib/modules: Improve error when a configuration is imported

This is appears to be a fairly common mistake for beginners who want
to build larger things from the system configurations, such as NixOps
networks, etc. Further explanation seems appropriate.

1 files changed, 1 insertions, 0 deletions

diff --git a/lib/modules.nix b/lib/modules.nix
index 8233b4b9e84e..3bcd8d280b74 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -347,6 +347,7 @@ let
       };
 
       result = withWarnings {
+        _type = "configuration";
         options = checked options;
         config = checked (removeAttrs config [ "_module" ]);
         _module = checked (config._module);