caseCon introduces free variables because of evaluator changes

[leonschoorl]

topEntity = foo @System

foo :: forall dom. Signal dom (Vec 1 (Signed 2)) -> Signal dom Bool
foo = \input ->
    let
      scs :: Signal dom (Vec 1 Bool)
      scs = bundle $ map f $ unbundle input
    in
      fmap bar scs


bar :: (KnownNat n) => Vec (n+1) Bool -> Bool
bar = fold (&&) . map (id)

f :: Signal dom a -> Signal dom Bool
f = const $ pure $ True

When compiled with -fclash-debug-invariants (DebugSilent or higher) this crashes with:

    Clash error call:
    Clash.Rewrite.Util(226): Error when applying rewrite caseCon to:
    case Clash.Sized.Vector.map @GHC.Types.Bool[3674937295934324744]
           @GHC.Types.Bool[3674937295934324744]
           @(GHC.TypeNats.+[3674937295934325540] 0 1)
           (GHC.Base.id[8214565720323788222][GlobalId]
              @GHC.Types.Bool[3674937295934324744])
           x[6989586621679027447][LocalId] of
      Clash.Sized.Vector.Cons[8214565720323790062] m[12]
        (_co_[7] :: GHC.Prim.~#[3674937295934324842]
                      GHC.Num.Natural.Natural[3674937295934324786]
                      GHC.Num.Natural.Natural[3674937295934324786]
                      1
                      (GHC.TypeNats.+[3674937295934325540] 0 1))
          (el[6] :: GHC.Types.Bool[3674937295934324744])
          (rest[14] :: Clash.Sized.Vector.Vec[8214565720323790059]
                         0
                         GHC.Types.Bool[3674937295934324744]) ->
        el[6][LocalId]
    Result:
    let
      el[18] :: GHC.Types.Bool[3674937295934324744]
      = (λ(x[8214565720323804840] :: GHC.Types.Bool[3674937295934324744]) ->
          x[8214565720323804840][LocalId])
          ((λ(ds[6341068275337658375] :: Clash.Signal.Internal.Signal[8214565720323790017]
                                           "System"
                                           (Clash.Sized.Internal.Signed.Signed[8214565720323790051] 2)) ->
             Main.f_x[8214565720323803592][GlobalId] @"System")
             (Clash.Sized.Vector.head @0
                @(Clash.Sized.Internal.Signed.Signed[8214565720323790051] 2)
                ds[6341068275337658369][LocalId]))
    in el[18][LocalId]
    It introduces free variables.
    Before: x[6989586621679027447] :: Clash.Sized.Vector.Vec[8214565720323790059]
                                (GHC.TypeNats.+[3674937295934325540] 0 1)
                                GHC.Types.Bool[3674937295934324744]
    After: ds[6341068275337658369] :: Clash.Signal.Internal.Signal[8214565720323790017]
                                 "System"
                                 (Clash.Sized.Vector.Vec[8214565720323790059]
                                    1
                                    (Clash.Sized.Internal.Signed.Signed[8214565720323790051] 2))
    CallStack (from HasCallStack):
      error, called at src/Clash/Rewrite/Util.hs:226:15 in clash-lib-1.9.0-inplace:Clash.Rewrite.Util

The problem is that since Give evaluator acces to inscope let-bindings #2571 the evaluator has access to the let bindings, and they may reference new free variables.

In this case it looks like:

\ds[6341068275337658369] ->
  let x[6989586621679027447] = ... ds[6341068275337658369] ...
  in .................. case map id x[6989586621679027447] of ...
                    --  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  term being rewritten

The evaluator transforms this into some transformed form of x which contains ds, and then checkDebug complains there is now a new free variable in the expression: ds