Remove TCvSubst and use Subst for both term and type-level subst

This patch removes the TCvSubst data type and instead uses Subst as
the environment for both term and type level substitution. This
change is partially motivated by the existential type proposal,
which will introduce types that contain expressions and therefore
forces us to carry around an "IdSubstEnv" even when substituting for
types. It also reduces the amount of code because "Subst" and
"TCvSubst" share a lot of common operations. There isn't any
noticeable impact on performance (geo. mean for ghc/alloc is around
0.0% but we have -94 loc and one less data type to worry abount).

Currently, the "TCvSubst" data type for substitution on types is
identical to the "Subst" data type except the former doesn't store
"IdSubstEnv". Using "Subst" for type-level substitution means there
will be a redundant field stored in the data type. However, in cases
where the substitution starts from the expression, using "Subst" for
type-level substitution saves us from having to project "Subst" into a
"TCvSubst". This probably explains why the allocation is mostly even
despite the redundant field.

The patch deletes "TCvSubst" and moves "Subst" and its relevant
functions from "GHC.Core.Subst" into "GHC.Core.TyCo.Subst".
Substitution on expressions is still defined in "GHC.Core.Subst" so we
don't have to expose the definition of "Expr" in the hs-boot file that
"GHC.Core.TyCo.Subst" must import to refer to "IdSubstEnv" (whose
codomain is "CoreExpr"). Most functions named fooTCvSubst are renamed
into fooSubst with a few exceptions (e.g. "isEmptyTCvSubst" is a
distinct function from "isEmptySubst"; the former ignores the
emptiness of "IdSubstEnv"). These exceptions mainly exist for
performance reasons and will go away when "Expr" and "Type" are
mutually recursively defined (we won't be able to take those
shortcuts if we can't make the assumption that expressions don't
appear in types).

1 files changed, 3 insertions, 3 deletions

diff --git a/compiler/GHC/Runtime/Eval.hs b/compiler/GHC/Runtime/Eval.hs
index e4f4de3fc5..2a3099e08b 100644
--- a/compiler/GHC/Runtime/Eval.hs
+++ b/compiler/GHC/Runtime/Eval.hs
@@ -621,10 +621,10 @@ bindLocalsAtBreakpoint hsc_env apStack_fhv (Just BreakInfo{..}) = do
      = do { name <- newInteractiveBinder hsc_env occ (getSrcSpan old_id)
           ; return (Id.mkVanillaGlobalWithInfo name ty (idInfo old_id)) }
 
-   newTyVars :: UniqSupply -> [TcTyVar] -> TCvSubst
+   newTyVars :: UniqSupply -> [TcTyVar] -> Subst
      -- Similarly, clone the type variables mentioned in the types
      -- we have here, *and* make them all RuntimeUnk tyvars
-   newTyVars us tvs = foldl' new_tv emptyTCvSubst (tvs `zip` uniqsFromSupply us)
+   newTyVars us tvs = foldl' new_tv emptySubst (tvs `zip` uniqsFromSupply us)
    new_tv subst (tv,uniq) = extendTCvSubstWithClone subst tv new_tv
     where
      new_tv = mkRuntimeUnkTyVar (setNameUnique (tyVarName tv) uniq)
@@ -1146,7 +1146,7 @@ checkForExistence clsInst mb_inst_tys = do
       Just (_, tys@(_:_)) -> all isTyVarTy tys
       _                   -> isTyVarTy ty
 
-  empty_subst = mkEmptyTCvSubst (mkInScopeSet (tyCoVarsOfType (idType $ is_dfun clsInst)))
+  empty_subst = mkEmptySubst (mkInScopeSet (tyCoVarsOfType (idType $ is_dfun clsInst)))
 
   {- Create a ClsInst with instantiated arguments and constraints.