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

10 files changed, 6 insertions, 12 deletions

diff --git a/compiler/typecheck/ClsInst.hs b/compiler/typecheck/ClsInst.hs
index de957b71fd..c0a54e48cc 100644
--- a/compiler/typecheck/ClsInst.hs
+++ b/compiler/typecheck/ClsInst.hs
@@ -29,9 +29,7 @@ import Id
 import Type
 import MkCore ( mkStringExprFS, mkNaturalExpr )
 
-import Unique ( hasKey )
 import Name   ( Name )
-import Var    ( DFunId )
 import DataCon
 import TyCon
 import Class
diff --git a/compiler/typecheck/TcDeriv.hs b/compiler/typecheck/TcDeriv.hs
index 6f749fc60f..409cccbc72 100644
--- a/compiler/typecheck/TcDeriv.hs
+++ b/compiler/typecheck/TcDeriv.hs
@@ -26,7 +26,7 @@ import TcValidity( allDistinctTyVars )
 import TcClassDcl( instDeclCtxt3, tcATDefault, tcMkDeclCtxt )
 import TcEnv
 import TcGenDeriv                       -- Deriv stuff
-import TcValidity
+import TcValidity( checkValidInstHead )
 import InstEnv
 import Inst
 import FamInstEnv
diff --git a/compiler/typecheck/TcErrors.hs b/compiler/typecheck/TcErrors.hs
index bc5b061a9e..ce75259203 100644
--- a/compiler/typecheck/TcErrors.hs
+++ b/compiler/typecheck/TcErrors.hs
@@ -64,7 +64,7 @@ import qualified Data.Set as Set
 
 import {-# SOURCE #-} TcHoleErrors ( findValidHoleFits )
 
-import Data.Semigroup   ( Semigroup )
+-- import Data.Semigroup   ( Semigroup )
 import qualified Data.Semigroup as Semigroup
 
 
diff --git a/compiler/typecheck/TcHsSyn.hs b/compiler/typecheck/TcHsSyn.hs
index 77e2a246cb..ee00acaad3 100644
--- a/compiler/typecheck/TcHsSyn.hs
+++ b/compiler/typecheck/TcHsSyn.hs
@@ -56,7 +56,6 @@ import TcEvidence
 import TysPrim
 import TyCon
 import TysWiredIn
-import TyCoRep( CoercionHole(..) )
 import Type
 import Coercion
 import ConLike
diff --git a/compiler/typecheck/TcInstDcls.hs-boot b/compiler/typecheck/TcInstDcls.hs-boot
index e7240903e4..ea0f50fd36 100644
--- a/compiler/typecheck/TcInstDcls.hs-boot
+++ b/compiler/typecheck/TcInstDcls.hs-boot
@@ -9,7 +9,6 @@ import HsSyn
 import TcRnTypes
 import TcEnv( InstInfo )
 import TcDeriv
-import HsExtension ( GhcRn )
 
 -- We need this because of the mutual recursion
 -- between TcTyClsDecls and TcInstDcls
diff --git a/compiler/typecheck/TcInteract.hs b/compiler/typecheck/TcInteract.hs
index 028b755187..cb1747cdb0 100644
--- a/compiler/typecheck/TcInteract.hs
+++ b/compiler/typecheck/TcInteract.hs
@@ -27,7 +27,7 @@ import Class
 import TyCon
 import FunDeps
 import FamInst
-import ClsInst( ClsInstResult(..), InstanceWhat(..), safeOverlap )
+import ClsInst( InstanceWhat(..), safeOverlap )
 import FamInstEnv
 import Unify ( tcUnifyTyWithTFs, ruleMatchTyKiX )
 
diff --git a/compiler/typecheck/TcPatSyn.hs b/compiler/typecheck/TcPatSyn.hs
index fabe16a1e0..c9dbc0f02f 100644
--- a/compiler/typecheck/TcPatSyn.hs
+++ b/compiler/typecheck/TcPatSyn.hs
@@ -17,10 +17,9 @@ import GhcPrelude
 
 import HsSyn
 import TcPat
-import Type( mkEmptyTCvSubst, tidyTyVarBinders, tidyTypes, tidyType )
+import Type( tidyTyVarBinders, tidyTypes, tidyType )
 import TcRnMonad
 import TcSigs( emptyPragEnv, completeSigFromId )
-import TcType( mkMinimalBySCs )
 import TcEnv
 import TcMType
 import TcHsSyn( zonkTyVarBindersX, zonkTcTypeToTypes
diff --git a/compiler/typecheck/TcRnTypes.hs b/compiler/typecheck/TcRnTypes.hs
index ca5639b297..b240585f65 100644
--- a/compiler/typecheck/TcRnTypes.hs
+++ b/compiler/typecheck/TcRnTypes.hs
@@ -155,7 +155,7 @@ import TcEvidence
 import Type
 import Class    ( Class )
 import TyCon    ( TyCon, TyConFlavour, tyConKind )
-import TyCoRep  ( CoercionHole(..), coHoleCoVar )
+import TyCoRep  ( coHoleCoVar )
 import Coercion ( Coercion, mkHoleCo )
 import ConLike  ( ConLike(..) )
 import DataCon  ( DataCon, dataConUserType, dataConOrigArgTys )
@@ -556,6 +556,7 @@ data TcGblEnv
 
         tcg_dus       :: DefUses,   -- ^ What is defined in this module and what is used.
         tcg_used_gres :: TcRef [GlobalRdrElt],  -- ^ Records occurrences of imported entities
+          -- One entry for each occurrence; but may have different GREs for the same Name
           -- See Note [Tracking unused binding and imports]
 
         tcg_keep :: TcRef NameSet,
diff --git a/compiler/typecheck/TcSMonad.hs b/compiler/typecheck/TcSMonad.hs
index 5bf5cefe01..ad7d332781 100644
--- a/compiler/typecheck/TcSMonad.hs
+++ b/compiler/typecheck/TcSMonad.hs
@@ -143,7 +143,6 @@ import Kind
 import TcType
 import DynFlags
 import Type
-import TyCoRep( coHoleCoVar )
 import Coercion
 import Unify
 
diff --git a/compiler/typecheck/TcTypeable.hs b/compiler/typecheck/TcTypeable.hs
index ad266f658f..146c8515b6 100644
--- a/compiler/typecheck/TcTypeable.hs
+++ b/compiler/typecheck/TcTypeable.hs
@@ -31,7 +31,6 @@ import Type
 import Kind ( isTYPEApp )
 import TyCon
 import DataCon
-import Name ( getOccName )
 import Module
 import HsSyn
 import DynFlags