Remove TcType.toTcType

In the olden days we insisted that only TcTyVars could appear
in a TcType.  But now we are more accommodating; see TcType
  Note [TcTyVars and TyVars in the typechecker]

This patch removes a function that converted a Type to a TcType.
It didn't do anything useful except statisfy an invariant that
we no longer have.  Now it's gone.

4 files changed, 20 insertions, 69 deletions

diff --git a/compiler/deSugar/Check.hs b/compiler/deSugar/Check.hs
index b383fb2f5d..39f585394a 100644
--- a/compiler/deSugar/Check.hs
+++ b/compiler/deSugar/Check.hs
@@ -44,7 +44,7 @@ import HscTypes (CompleteMatch(..))
 
 import DsMonad
 import TcSimplify    (tcCheckSatisfiability)
-import TcType        (toTcType, isStringTy, isIntTy, isWordTy)
+import TcType        (isStringTy, isIntTy, isWordTy)
 import Bag
 import ErrUtils
 import Var           (EvVar)
@@ -624,12 +624,12 @@ inhabitationCandidates fam_insts ty
       Just (tc, _)
         | tc `elem` trivially_inhabited -> case dcs of
             []    -> return (Left src_ty)
-            (_:_) -> do var <- liftD $ mkPmId (toTcType core_ty)
+            (_:_) -> do var <- liftD $ mkPmId core_ty
                         let va = build_tm (PmVar var) dcs
                         return $ Right [(va, mkIdEq var, emptyBag)]
 
         | pmIsClosedType core_ty -> liftD $ do
-            var  <- mkPmId (toTcType core_ty) -- it would be wrong to unify x
+            var  <- mkPmId core_ty -- it would be wrong to unify x
             alts <- mapM (mkOneConFull var . RealDataCon) (tyConDataCons tc)
             return $ Right [(build_tm va dcs, eq, cs) | (va, eq, cs) <- alts]
       -- For other types conservatively assume that they are inhabited.
@@ -1330,7 +1330,7 @@ allCompleteMatches cl tys = do
 -- * Types and constraints
 
 newEvVar :: Name -> Type -> EvVar
-newEvVar name ty = mkLocalId name (toTcType ty)
+newEvVar name ty = mkLocalId name ty
 
 nameType :: String -> Type -> DsM EvVar
 nameType name ty = do
diff --git a/compiler/deSugar/DsBinds.hs b/compiler/deSugar/DsBinds.hs
index ad666a2ce2..4684d436a4 100644
--- a/compiler/deSugar/DsBinds.hs
+++ b/compiler/deSugar/DsBinds.hs
@@ -181,7 +181,7 @@ dsHsBind dflags (AbsBinds { abs_tvs = tyvars, abs_ev_vars = dicts
                           , abs_exports = exports
                           , abs_ev_binds = ev_binds
                           , abs_binds = binds, abs_sig = has_sig })
-  = do { ds_binds <- addDictsDs (toTcTypeBag (listToBag dicts)) $
+  = do { ds_binds <- addDictsDs (listToBag dicts) $
                      dsLHsBinds binds
                                    -- addDictsDs: push type constraints deeper
                                    --             for inner pattern match check
diff --git a/compiler/deSugar/Match.hs b/compiler/deSugar/Match.hs
index 0044cbe49f..6b548a4f5a 100644
--- a/compiler/deSugar/Match.hs
+++ b/compiler/deSugar/Match.hs
@@ -39,7 +39,6 @@ import MatchCon
 import MatchLit
 import Type
 import Coercion ( eqCoercion )
-import TcType ( toTcTypeBag )
 import TyCon( isNewTyCon )
 import TysWiredIn
 import SrcLoc
@@ -733,7 +732,7 @@ matchWrapper ctxt mb_scr (MG { mg_alts = L _ matches
     mk_eqn_info vars (L _ (Match { m_pats = pats, m_grhss = grhss }))
       = do { dflags <- getDynFlags
            ; let upats = map (unLoc . decideBangHood dflags) pats
-                 dicts = toTcTypeBag (collectEvVarsPats upats) -- Only TcTyVars
+                 dicts = collectEvVarsPats upats
            ; tm_cs <- genCaseTmCs2 mb_scr upats vars
            ; match_result <- addDictsDs dicts $ -- See Note [Type and Term Equality Propagation]
                              addTmCsDs tm_cs  $ -- See Note [Type and Term Equality Propagation]
diff --git a/compiler/typecheck/TcType.hs b/compiler/typecheck/TcType.hs
index 9abd264949..f5f7532075 100644
--- a/compiler/typecheck/TcType.hs
+++ b/compiler/typecheck/TcType.hs
@@ -176,10 +176,6 @@ module TcType (
   noFreeVarsOfType,
 
   --------------------------------
-  -- Transforming Types to TcTypes
-  toTcType,    -- :: Type -> TcType
-  toTcTypeBag, -- :: Bag EvVar -> Bag EvVar
-
   pprKind, pprParendKind, pprSigmaType,
   pprType, pprParendType, pprTypeApp, pprTyThingCategory, tyThingCategory,
   pprTheta, pprParendTheta, pprThetaArrowTy, pprClassPred,
@@ -222,7 +218,6 @@ import TysWiredIn( coercibleClass, unitTyCon, unitTyConKey
                  , listTyCon, constraintKind )
 import BasicTypes
 import Util
-import Bag
 import Maybes
 import ListSetOps ( getNth, findDupsEq )
 import Outputable
@@ -233,7 +228,6 @@ import qualified GHC.LanguageExtensions as LangExt
 import Data.List  ( mapAccumL )
 import Data.IORef
 import Data.List.NonEmpty( NonEmpty(..) )
-import Data.Functor.Identity
 import qualified Data.Semigroup as Semi
 
 {-
@@ -272,13 +266,20 @@ tau ::= tyvar
 -- In all cases, a (saturated) type synonym application is legal,
 -- provided it expands to the required form.
 
-Note [TcTyVars in the typechecker]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Note [TcTyVars and TyVars in the typechecker]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 The typechecker uses a lot of type variables with special properties,
 notably being a unification variable with a mutable reference.  These
 use the 'TcTyVar' variant of Var.Var.
 
-However, the type checker and constraint solver can encounter type
+Note, though, that a /bound/ type variable can (and probably should)
+be a TyVar.  E.g
+    forall a. a -> a
+Here 'a' is really just a deBruijn-number; it certainly does not have
+a signficant TcLevel (as every TcTyVar does).  So a forall-bound type
+variable should be TyVars; and hence a TyVar can appear free in a TcType.
+
+The type checker and constraint solver can also encounter /free/ type
 variables that use the 'TyVar' variant of Var.Var, for a couple of
 reasons:
 
@@ -299,7 +300,8 @@ reasons:
     long afer TcTyVars have been zonked away
 
 It's convenient to simply treat these TyVars as skolem constants,
-which of course they are.  So
+which of course they are.  We give them a level number of "outermost",
+so they behave as global constants.  Specifically:
 
 * Var.tcTyVarDetails succeeds on a TyVar, returning
   vanillaSkolemTv, as well as on a TcTyVar.
@@ -326,7 +328,7 @@ for coercion variables--on the variable. Failing to do so led to
 GHC Trac #12785.
 -}
 
--- See Note [TcTyVars in the typechecker]
+-- See Note [TcTyVars and TyVars in the typechecker]
 type TcCoVar = CoVar    -- Used only during type inference
 type TcType = Type      -- A TcType can have mutable type variables
 type TcTyCoVar = Var    -- Either a TcTyVar or a CoVar
@@ -1172,7 +1174,7 @@ candidateQTyVarsOfTypes = foldl (split_dvs emptyVarSet) mempty
 -}
 
 tcIsTcTyVar :: TcTyVar -> Bool
--- See Note [TcTyVars in the typechecker]
+-- See Note [TcTyVars and TyVars in the typechecker]
 tcIsTcTyVar tv = isTyVar tv
 
 isTouchableMetaTyVar :: TcLevel -> TcTyVar -> Bool
@@ -2319,56 +2321,6 @@ isRigidTy ty
   | isForAllTy ty                           = True
   | otherwise                               = False
 
-{-
-************************************************************************
-*                                                                      *
-\subsection{Transformation of Types to TcTypes}
-*                                                                      *
-************************************************************************
--}
-
-toTcType :: Type -> TcType
--- The constraint solver expects EvVars to have TcType, in which the
--- free type variables are TcTyVars. So we convert from Type to TcType here
--- A bit tiresome; but one day I expect the two types to be entirely separate
--- in which case we'll definitely need to do this
-toTcType = runIdentity . to_tc_type emptyVarSet
-
-toTcTypeBag :: Bag EvVar -> Bag EvVar -- All TyVars are transformed to TcTyVars
-toTcTypeBag evvars = mapBag (\tv -> setTyVarKind tv (toTcType (tyVarKind tv))) evvars
-
-to_tc_mapper :: TyCoMapper VarSet Identity
-to_tc_mapper
-  = TyCoMapper { tcm_smart    = False   -- more efficient not to use smart ctors
-               , tcm_tyvar    = tyvar
-               , tcm_covar    = covar
-               , tcm_hole     = hole
-               , tcm_tybinder = tybinder }
-  where
-    tyvar :: VarSet -> TyVar -> Identity Type
-    tyvar ftvs tv
-      | Just var <- lookupVarSet ftvs tv = return $ TyVarTy var
-      | isTcTyVar tv = TyVarTy <$> updateTyVarKindM (to_tc_type ftvs) tv
-      | otherwise
-      = do { kind' <- to_tc_type ftvs (tyVarKind tv)
-           ; return $ TyVarTy $ mkTcTyVar (tyVarName tv) kind' vanillaSkolemTv }
-
-    covar :: VarSet -> CoVar -> Identity Coercion
-    covar ftvs cv
-      | Just var <- lookupVarSet ftvs cv = return $ CoVarCo var
-      | otherwise = CoVarCo <$> updateVarTypeM (to_tc_type ftvs) cv
-
-    hole :: VarSet -> CoercionHole -> Identity Coercion
-    hole _ hole = pprPanic "toTcType: found a coercion hole" (ppr hole)
-
-    tybinder :: VarSet -> TyVar -> ArgFlag -> Identity (VarSet, TyVar)
-    tybinder ftvs tv _vis = do { kind' <- to_tc_type ftvs (tyVarKind tv)
-                               ; let tv' = mkTcTyVar (tyVarName tv) kind'
-                                                     vanillaSkolemTv
-                               ; return (ftvs `extendVarSet` tv', tv') }
-
-to_tc_type :: VarSet -> Type -> Identity TcType
-to_tc_type = mapType to_tc_mapper
 
 {-
 ************************************************************************