Re-do rubbish literals

As #19882 pointed out, we were simply doing rubbish literals wrong.
(I'll refrain from explaining the wrong-ness here -- see the ticket.)

This patch fixes it by adding a Type (of kind RuntimeRep) as field of
LitRubbish, rather than [PrimRep].

The Note [Rubbish literals] in GHC.Types.Literal explains the details.

1 files changed, 18 insertions, 0 deletions

diff --git a/compiler/GHC/Core/Make.hs b/compiler/GHC/Core/Make.hs
index 129120139b..e0897670fc 100644
--- a/compiler/GHC/Core/Make.hs
+++ b/compiler/GHC/Core/Make.hs
@@ -13,6 +13,7 @@ module GHC.Core.Make (
         sortQuantVars, castBottomExpr,
 
         -- * Constructing boxed literals
+        mkLitRubbish,
         mkWordExpr,
         mkIntExpr, mkIntExprInt, mkUncheckedIntExpr,
         mkIntegerExpr, mkNaturalExpr,
@@ -243,6 +244,23 @@ castBottomExpr e res_ty
   where
     e_ty = exprType e
 
+mkLitRubbish :: Type -> Maybe CoreExpr
+-- Make a rubbish-literal CoreExpr of the given type.
+-- Fail (returning Nothing) if
+--    * the RuntimeRep of the Type is not monomorphic;
+--    * the type is (a ~# b), the type of coercion
+-- See INVARIANT 1 and 2 of item (2) in Note [Rubbish literals]
+-- in GHC.Types.Literal
+mkLitRubbish ty
+  | not (noFreeVarsOfType rep)
+  = Nothing   -- Satisfy INVARIANT 1
+  | isCoVarType ty
+  = Nothing   -- Satisfy INVARIANT 2
+  | otherwise
+  = Just (Lit (LitRubbish rep) `mkTyApps` [ty])
+  where
+    rep  = getRuntimeRep ty
+
 {-
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