Fix unused-import warningswip/T13064

This patch fixes a fairly long-standing bug (dating back to 2015) in
RdrName.bestImport, namely

   commit 9376249b6b78610db055a10d05f6592d6bbbea2f
   Author: Simon Peyton Jones <simonpj@microsoft.com>
   Date:   Wed Oct 28 17:16:55 2015 +0000

   Fix unused-import stuff in a better way

In that patch got the sense of the comparison back to front, and
thereby failed to implement the unused-import rules described in
  Note [Choosing the best import declaration] in RdrName

This led to Trac #13064 and #15393

Fixing this bug revealed a bunch of unused imports in libraries;
the ones in the GHC repo are part of this commit.

The two important changes are

* Fix the bug in bestImport

* Modified the rules by adding (a) in
     Note [Choosing the best import declaration] in RdrName
  Reason: the previosu rules made Trac #5211 go bad again.  And
  the new rule (a) makes sense to me.

In unravalling this I also ended up doing a few other things

* Refactor RnNames.ImportDeclUsage to use a [GlobalRdrElt] for the
  things that are used, rather than [AvailInfo]. This is simpler
  and more direct.

* Rename greParentName to greParent_maybe, to follow GHC
  naming conventions

* Delete dead code RdrName.greUsedRdrName

* Slightly change the spec of the unused function
  HsImpExpr.ieLWrappedName, and use it in RnNames.findImportUsage

1 files changed, 9 insertions, 10 deletions

diff --git a/compiler/rename/RnEnv.hs b/compiler/rename/RnEnv.hs
index 16897c2681..76634b7c0d 100644
--- a/compiler/rename/RnEnv.hs
+++ b/compiler/rename/RnEnv.hs
@@ -77,7 +77,6 @@ import ListSetOps       ( minusList )
 import qualified GHC.LanguageExtensions as LangExt
 import RnUnbound
 import RnUtils
-import Data.Maybe (isJust)
 import qualified Data.Semigroup as Semi
 import Data.Either      ( partitionEithers )
 import Data.List        (find)
@@ -638,21 +637,21 @@ lookupSubBndrOcc_helper must_have_parent warn_if_deprec parent rdr_name
             NoParent -> Nothing
 
         picked_gres :: [GlobalRdrElt] -> DisambigInfo
+        -- For Unqual, find GREs that are in scope qualified or unqualified
+        -- For Qual,   find GREs that are in scope with that qualification
         picked_gres gres
           | isUnqual rdr_name
-              = mconcat (map right_parent gres)
+          = mconcat (map right_parent gres)
           | otherwise
-              = mconcat (map right_parent (pickGREs rdr_name gres))
-
+          = mconcat (map right_parent (pickGREs rdr_name gres))
 
         right_parent :: GlobalRdrElt -> DisambigInfo
         right_parent p
-          | Just cur_parent <- getParent p
-            = if parent == cur_parent
-                then DisambiguatedOccurrence p
-                else NoOccurrence
-          | otherwise
-            = UniqueOccurrence p
+          = case getParent p of
+               Just cur_parent
+                  | parent == cur_parent -> DisambiguatedOccurrence p
+                  | otherwise            -> NoOccurrence
+               Nothing                   -> UniqueOccurrence p
 
 
 -- This domain specific datatype is used to record why we decided it was