Suspicious treatment of renaming of field labels

While working on my rework of interface file names (Phab:D2467) I noticed that there is some funniness in the treatment of renaming field selectors. Namely, ifaceConDeclFields are IfaceTopBndrs (and should therefore be renamed), renaming them (in RnModIface) causes bkpreex06 to fail with the panic find_lbl missing foo. Looking at the environment reveals that there are selectors named foo in scope, but they were not renamed.

I believe this is due to the implementation of tcIfaceDataCons, which typechecks the fields with mapM (traverse lookupIfaceTop) (ifaceConDeclFields if_cons). In essence, this projects the field labels back to OccNames (in ifaceConDeclFields) and then looks each of these OccNames up in the name cache, thereby retrieving the un-renamed Name. This seems wrong.