Currently, if I import modules (I'm using the module system for something other than NixOS) that are not top-level files, errors will be reported similarly to:

 The option `foo' defined in `<unknown-file>' does not exist.

I think <unknown-file> could be improved by using unsafeGetAttrPos to try a bit harder to figure out where the module is defined. It obviously still won't work in all cases, but it will probably work in more cases.