Fix substitution in bindAuxiliaryDictwip/T21391

In GHC.Core.Opt.Specialise.bindAuxiliaryDict we were unnecessarily
calling `extendInScope` to bring into scope variables that were
/already/ in scope.  Worse, GHC.Core.Subst.extendInScope strangely
deleted the newly-in-scope variables from the substitution -- and that
was fatal in #21391.

I removed the redundant calls to extendInScope.

More ambitiously, I changed GHC.Core.Subst.extendInScope (and cousins)
to stop deleting variables from the substitution.  I even changed the
names of the function to extendSubstInScope (and cousins) and audited
all the calls to check that deleting from the substitution was wrong.
In fact there are very few such calls, and they are all about
introducing a fresh non-in-scope variable.  These are "OutIds"; it is
utterly wrong to mess with the "InId" substitution.

I have not added a Note, because I'm deleting wrong code, and it'd be
distracting to document a bug.

1 files changed, 11 insertions, 22 deletions

diff --git a/compiler/GHC/Core/Subst.hs b/compiler/GHC/Core/Subst.hs
index 2c470c5dcb..e6f0237f32 100644
--- a/compiler/GHC/Core/Subst.hs
+++ b/compiler/GHC/Core/Subst.hs
@@ -24,7 +24,7 @@ module GHC.Core.Subst (
         emptySubst, mkEmptySubst, mkSubst, mkOpenSubst, substInScope, isEmptySubst,
         extendIdSubst, extendIdSubstList, extendTCvSubst, extendTvSubstList,
         extendSubst, extendSubstList, extendSubstWithVar, zapSubstEnv,
-        extendInScope, extendInScopeList, extendInScopeIds, GHC.Core.Subst.extendInScopeSet,
+        extendSubstInScope, extendSubstInScopeList, extendSubstInScopeSet,
         isInScope, setInScope, getTCvSubst, extendTvSubst, extendCvSubst,
         delBndr, delBndrs,
 
@@ -57,7 +57,6 @@ import GHC.Types.Var
 import GHC.Types.Tickish
 import GHC.Types.Id.Info
 import GHC.Types.Unique.Supply
-import GHC.Types.Unique.Set
 
 import GHC.Builtin.Names
 import GHC.Data.Maybe
@@ -285,33 +284,23 @@ mkOpenSubst in_scope pairs = Subst in_scope
 isInScope :: Var -> Subst -> Bool
 isInScope v (Subst in_scope _ _ _) = v `elemInScopeSet` in_scope
 
--- | Add the 'Var' to the in-scope set: as a side effect,
--- and remove any existing substitutions for it
-extendInScope :: Subst -> Var -> Subst
-extendInScope (Subst in_scope ids tvs cvs) v
+-- | Add the 'Var' to the in-scope set
+extendSubstInScope :: Subst -> Var -> Subst
+extendSubstInScope (Subst in_scope ids tvs cvs) v
   = Subst (in_scope `InScopeSet.extendInScopeSet` v)
-          (ids `delVarEnv` v) (tvs `delVarEnv` v) (cvs `delVarEnv` v)
+          ids tvs cvs
 
 -- | Add the 'Var's to the in-scope set: see also 'extendInScope'
-extendInScopeList :: Subst -> [Var] -> Subst
-extendInScopeList (Subst in_scope ids tvs cvs) vs
+extendSubstInScopeList :: Subst -> [Var] -> Subst
+extendSubstInScopeList (Subst in_scope ids tvs cvs) vs
   = Subst (in_scope `extendInScopeSetList` vs)
-          (ids `delVarEnvList` vs) (tvs `delVarEnvList` vs) (cvs `delVarEnvList` vs)
-
--- | Optimized version of 'extendInScopeList' that can be used if you are certain
--- all the things being added are 'Id's and hence none are 'TyVar's or 'CoVar's
-extendInScopeIds :: Subst -> [Id] -> Subst
-extendInScopeIds (Subst in_scope ids tvs cvs) vs
-  = Subst (in_scope `extendInScopeSetList` vs)
-          (ids `delVarEnvList` vs) tvs cvs
+          ids tvs cvs
 
 -- | Add the 'Var's to the in-scope set: see also 'extendInScope'
-extendInScopeSet :: Subst -> VarSet -> Subst
-extendInScopeSet (Subst in_scope ids tvs cvs) vs
+extendSubstInScopeSet :: Subst -> VarSet -> Subst
+extendSubstInScopeSet (Subst in_scope ids tvs cvs) vs
   = Subst (in_scope `extendInScopeSetSet` vs)
-          (ids `minus` vs) (tvs `minus` vs) (cvs `minus` vs)
-  where
-    minus env set = minusVarEnv env (getUniqSet set)
+          ids tvs cvs
 
 setInScope :: Subst -> InScopeSet -> Subst
 setInScope (Subst _ ids tvs cvs) in_scope = Subst in_scope ids tvs cvs