Tidy up the handling of wild-card binders, and make Lint check it

See Note [WildCard binders] in SimplEnv.  Spotted by Roman.

compiler/coreSyn/CoreLint.lhs

@@ -227,7 +227,11 @@ lintCoreExpr (Var var)
   = do	{ checkL (not (var == oneTupleDataConId))
 		 (ptext (sLit "Illegal one-tuple"))
 
-	; checkDeadIdOcc var
+        ; checkL (not (var `hasKey` wildCardKey))
+                 (ptext (sLit "Occurence of a wild-card binder") <+> ppr var)
+                 -- See Note [WildCard binders] in SimplEnv
+
+        ; checkDeadIdOcc var
 	; var' <- lookupIdInScope var
         ; return (idType var') }
 

compiler/coreSyn/MkCore.lhs

@@ -65,7 +65,6 @@ import Name
 import Outputable
 import FastString
 import UniqSupply
-import Unique		( mkBuiltinUnique )
 import BasicTypes
 import Util             ( notNull, zipEqual )
 import Constants
@@ -156,8 +155,9 @@ mkWildEvBinder pred             = mkWildValBinder (mkPredTy pred)
 -- that you expect to use only at a *binding* site.  Do not use it at
 -- occurrence sites because it has a single, fixed unique, and it's very
 -- easy to get into difficulties with shadowing.  That's why it is used so little.
+-- See Note [WildCard binders] in SimplEnv
 mkWildValBinder :: Type -> Id
-mkWildValBinder ty = mkSysLocal (fsLit "wild") (mkBuiltinUnique 1) ty
+mkWildValBinder ty = mkLocalId wildCardName ty
 
 mkWildCase :: CoreExpr -> Type -> Type -> [CoreAlt] -> CoreExpr
 -- Make a case expression whose case binder is unused

compiler/prelude/PrelNames.lhs

@@ -58,7 +58,7 @@ import Unique	  ( Unique, Uniquable(..), hasKey,
 		    mkTupleTyConUnique
 		  ) 
 import BasicTypes ( Boxity(..), Arity )
-import Name	  ( Name, mkInternalName, mkExternalName )
+import Name       ( Name, mkInternalName, mkExternalName, mkSystemVarName )
 import SrcLoc
 import FastString
 \end{code}
@@ -542,6 +542,9 @@ and it's convenient to write them all down in one place.
 
 
 \begin{code}
+wildCardName :: Name
+wildCardName = mkSystemVarName wildCardKey (fsLit "wild")
+
 runMainIOName :: Name
 runMainIOName = varQual gHC_TOP_HANDLER (fsLit "runMainIO") runMainKey
 
@@ -1127,10 +1130,11 @@ absentErrorIdKey, augmentIdKey, appendIdKey, buildIdKey, errorIdKey,
     noMethodBindingErrorIdKey, nonExhaustiveGuardsErrorIdKey,
     runtimeErrorIdKey, parErrorIdKey, parIdKey, patErrorIdKey,
     realWorldPrimIdKey, recConErrorIdKey, recUpdErrorIdKey,
-    traceIdKey,
+    traceIdKey, wildCardKey,
     unpackCStringUtf8IdKey, unpackCStringAppendIdKey,
     unpackCStringFoldrIdKey, unpackCStringIdKey :: Unique
-absentErrorIdKey	      = mkPreludeMiscIdUnique  1
+wildCardKey                   = mkPreludeMiscIdUnique  0  -- See Note [WildCard]
+absentErrorIdKey              = mkPreludeMiscIdUnique  1
 augmentIdKey		      = mkPreludeMiscIdUnique  3
 appendIdKey		      = mkPreludeMiscIdUnique  4
 buildIdKey		      = mkPreludeMiscIdUnique  5

compiler/simplCore/SimplEnv.lhs

@@ -46,6 +46,8 @@ import VarEnv
 import VarSet
 import OrdList
 import Id
+import MkCore
+import TysWiredIn
 import qualified CoreSubst
 import qualified Type		( substTy, substTyVarBndr, substTyVar )
 import Type hiding		( substTy, substTyVarBndr, substTyVar )
@@ -220,13 +222,31 @@ seIdSubst:
 \begin{code}
 mkSimplEnv :: SimplifierMode -> SimplEnv
 mkSimplEnv mode
-  = SimplEnv { seCC = subsumedCCS,
-	       seMode = mode, seInScope = emptyInScopeSet, 
-	       seFloats = emptyFloats,
-	       seTvSubst = emptyVarEnv, seIdSubst = emptyVarEnv }
+  = SimplEnv { seCC = subsumedCCS
+             , seMode = mode
+             , seInScope = init_in_scope
+             , seFloats = emptyFloats
+             , seTvSubst = emptyVarEnv
+             , seIdSubst = emptyVarEnv }
 	-- The top level "enclosing CC" is "SUBSUMED".
 
----------------------
+init_in_scope :: InScopeSet
+init_in_scope = mkInScopeSet (unitVarSet (mkWildValBinder unitTy))
+              -- See Note [WildCard binders]
+\end{code}
+
+Note [WildCard binders]
+~~~~~~~~~~~~~~~~~~~~~~~
+The program to be simplified may have wild binders
+    case e of wild { p -> ... }
+We want to *rename* them away, so that there are no
+occurrences of 'wild' (with wildCardKey).  The easy
+way to do that is to start of with a representative
+Id in the in-scope set
+
+There should be no *occurrences* of wild.
+
+\begin{code}
 getMode :: SimplEnv -> SimplifierMode
 getMode env = seMode env