Buglet in reporting out of scope errors in rules

Most out of scope errors get reported by the type checker these
days, but not all.  Example, the function on the LHS of a RULE.

Trace #15659 pointed out that this less-heavily-used code path
produce a "wacky" error message.  Indeed so.  Easily fixed.

1 files changed, 6 insertions, 5 deletions

diff --git a/compiler/rename/RnUnbound.hs b/compiler/rename/RnUnbound.hs
index a77025fe7e..ce5d0dc315 100644
--- a/compiler/rename/RnUnbound.hs
+++ b/compiler/rename/RnUnbound.hs
@@ -12,7 +12,8 @@ module RnUnbound ( mkUnboundName
                  , WhereLooking(..)
                  , unboundName
                  , unboundNameX
-                 , perhapsForallMsg ) where
+                 , perhapsForallMsg
+                 , notInScopeErr ) where
 
 import GhcPrelude
 
@@ -60,8 +61,7 @@ unboundNameX :: WhereLooking -> RdrName -> SDoc -> RnM Name
 unboundNameX where_look rdr_name extra
   = do  { dflags <- getDynFlags
         ; let show_helpful_errors = gopt Opt_HelpfulErrors dflags
-              what = pprNonVarNameSpace (occNameSpace (rdrNameOcc rdr_name))
-              err = unknownNameErr what rdr_name $$ extra
+              err = notInScopeErr rdr_name $$ extra
         ; if not show_helpful_errors
           then addErr err
           else do { local_env  <- getLocalRdrEnv
@@ -72,12 +72,13 @@ unboundNameX where_look rdr_name extra
                   ; addErr (err $$ suggestions) }
         ; return (mkUnboundNameRdr rdr_name) }
 
-unknownNameErr :: SDoc -> RdrName -> SDoc
-unknownNameErr what rdr_name
+notInScopeErr :: RdrName -> SDoc
+notInScopeErr rdr_name
   = vcat [ hang (text "Not in scope:")
               2 (what <+> quotes (ppr rdr_name))
          , extra ]
   where
+    what = pprNonVarNameSpace (occNameSpace (rdrNameOcc rdr_name))
     extra | rdr_name == forall_tv_RDR = perhapsForallMsg
           | otherwise                 = Outputable.empty