Rubbish literals for all representations (#18983)

This patch cleans up the complexity around WW's `mk_absent_let` by
broadening the scope of `LitRubbish`. Rubbish literals now store the
`PrimRep` they represent and are ultimately lowered in Cmm.
This in turn allows absent literals of `VecRep` or `VoidRep`. The latter
allows absent literals for unlifted coercions, as requested in #18983.

I took the liberty to rewrite and clean up `Note [Absent fillers]` and
`Note [Rubbish values]` to account for the new implementation and to
make them more orthogonal in their description.

I didn't add a new regression test, as `T18982` already contains the
test in the ticket and its test output changes as expected.

Fixes #18983.

1 files changed, 13 insertions, 1 deletions

diff --git a/compiler/GHC/Utils/Misc.hs b/compiler/GHC/Utils/Misc.hs
index f7168190e4..abd85b6b66 100644
--- a/compiler/GHC/Utils/Misc.hs
+++ b/compiler/GHC/Utils/Misc.hs
@@ -43,7 +43,7 @@ module GHC.Utils.Misc (
         listLengthCmp, atLength,
         equalLength, compareLength, leLength, ltLength,
 
-        isSingleton, only, GHC.Utils.Misc.singleton,
+        isSingleton, only, expectOnly, GHC.Utils.Misc.singleton,
         notNull, snocView,
 
         isIn, isn'tIn,
@@ -563,6 +563,18 @@ only (a:_) = a
 #endif
 only _ = panic "Util: only"
 
+-- | Extract the single element of a list and panic with the given message if
+-- there are more elements or the list was empty.
+-- Like 'expectJust', but for lists.
+expectOnly :: HasCallStack => String -> [a] -> a
+{-# INLINE expectOnly #-}
+#if defined(DEBUG)
+expectOnly _   [a]   = a
+#else
+expectOnly _   (a:_) = a
+#endif
+expectOnly msg _     = panic ("expectOnly: " ++ msg)
+
 -- Debugging/specialising versions of \tr{elem} and \tr{notElem}
 
 # if !defined(DEBUG)