lookupIdSubst warning

The function lookupIdSubst

lookupIdSubst :: HasDebugCallStack => Subst -> Id -> CoreExpr
lookupIdSubst (Subst in_scope ids _ _) v
  | not (isLocalId v) = Var v
  | Just e  <- lookupVarEnv ids       v = e
  | Just v' <- lookupInScope in_scope v = Var v'
        -- Vital! See Note [Extending the Subst]
  | otherwise = warnPprTrace True (text "GHC.Core.Subst.lookupIdSubst" <+> ppr v
                            $$ ppr in_scope) $
                Var v

prints a warning message if it cannot find the variable. Such message is a sign of a bug in GHC - as the Note [Extending the Subst] says, the in-scope set might contain an updated version of the variable which will be missed.

I've tried building GHC when this warning is replaced by a panic. There's only one problematic call to lookupIdSubst, in GHC.Core.Rules. Here's a reproducer, needs -O:

module GHC.Utils.TmpFs where

import qualified Data.Map as Map

cleanTempDirs :: ([String] -> a) -> Map.Map String String -> a
cleanTempDirs logger ds = logger (Map.elems ds)

Since all other calls seem fine, I've created a MR !6302 (closed) with a function lookupIdSubstUnchecked that is used only in Rules.

The goal of this ticket is to fix the problem in Rules.