If we can't read the old interface file for some reason (eg. it isn't
there, or it has the wrong version), then output the error if
-ddump-hi-diffs is on. It used to be part of -ddump-rn-trace, but
-ddump-hi-diffs seems more logical, since it covers all the other
information about why a module is being recompiled.
-> readIface iface_path `thenRn` \ read_result ->
case read_result of
Left err -> -- Old interface file not found, or garbled; give up
- traceRn (text "Bad old interface file" $$ nest 4 err) `thenRn_`
+ traceHiDiffsRn (
+ text "Cannot read old interface file:"
+ $$ nest 4 err) `thenRn_`
returnRn (outOfDate, Nothing)
Right parsed_iface