[project @ 2001-05-31 11:25:51 by simonmar]

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.

ghc/compiler/rename/Rename.lhs

@@ -413,7 +413,9 @@ checkOldIface ghci_mode dflags hit hst pcs iface_path source_unchanged maybe_ifa
           -> 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