Fix BuiltinCasing crash on GHC.Prim.Addr# (#7716)

[Unisay]

Summary

• Add unreachable second constructor to BuiltinData to prevent GHC's case-of-single-constructor optimization from exposing PlutusCore.Data.Data in join point types
• Add COMPLETE pragma so existing pattern matches remain exhaustive
• Add failing test (BuiltinCasing/Lib.hs) that reproduces the crash

Closes #7716

The problem

data BuiltinData = BuiltinData ~Data has one constructor. GHC's simplifier always unwraps single-constructor types, producing case bd of { BuiltinData d -> ... } in Core. This leaks d :: Data into join point type signatures. When the plugin compiles with BuiltinCasing, it tries to compile Data as a regular ADT, follows B ByteString to BS Addr# Int, and crashes — Addr# has no Plutus Core equivalent.

The fix

Add a second constructor BuiltinDataUnreachable that is never constructed. GHC won't case-simplify multi-constructor types, so Data stays behind the BuiltinData wrapper. A COMPLETE pragma marks the original constructor as exhaustive.

Details in Note [Opaque builtin types] in PlutusTx.Builtins.Internal.

Alternatives I tried

• Hiding the constructor via export lists — GHC includes all constructors in .hi files regardless of what the module exports
• newtype BuiltinData = UnsafeBuiltinData Any — newtypes are transparent in Core (coercions), so the plugin just crashes on Kind: forall k. k instead
• Mapping Data to the builtin type in the plugin — fixes the Addr# crash but exposes a second error (Cannot construct BuiltinData); would need deeper changes to the expression compiler

[zliu41]

I'll need to understand what happened a bit more. There may be simpler solutions.

The issue description says "But the plugin's own simplifier pass (mkSimplPass in Plugin/Common.hs) can produce join points" - that's false. The plugin cannot produce join points.

In general I'd recommend against copy-pasting large amount of AI text. I generally find it verbose, with low signal-to-noise ratio, and unpleasant to read. It's better to use AI to understand the issue, then write the description on your own.

In this particular case, what would be a useful issue description is: "for this Haskell function, without builtin casing, it generates this GHC Core, which the plugin can compile. But with builtin casing, the GHC Core becomes this, which is problematic". It's very useful to include GHC Core, and anything else is unnecessary.

[github-actions]

Execution Budget Golden Diff

90dc9d6 (master) vs 6d710a2

output

plutus-tx-plugin/test-ledger-api/Spec/Data/Budget/9.12/geq1.golden.eval

Metric	Old	New	Δ%
CPU	337_547_895	341_387_895	+1.14%
Memory	988_745	1_012_745	+2.43%
Flat Size	949	939	-1.05%

plutus-tx-plugin/test-ledger-api/Spec/Data/Budget/9.12/geq2.golden.eval

Metric	Old	New	Δ%
CPU	355_097_594	359_033_594	+1.11%
Memory	1_055_015	1_079_615	+2.33%
Flat Size	1_000	990	-1.00%

plutus-tx-plugin/test-ledger-api/Spec/Data/Budget/9.12/geq3.golden.eval

Metric	Old	New	Δ%
CPU	368_720_887	372_656_887	+1.07%
Memory	1_100_329	1_124_929	+2.24%
Flat Size	1_000	990	-1.00%

plutus-tx-plugin/test-ledger-api/Spec/Data/Budget/9.12/geq4.golden.eval

Metric	Old	New	Δ%
CPU	331_212_553	335_148_553	+1.19%
Memory	946_336	970_936	+2.60%
Flat Size	956	946	-1.05%

plutus-tx-plugin/test-ledger-api/Spec/Data/Budget/9.12/geq5.golden.eval

Metric	Old	New	Δ%
CPU	349_612_383	353_548_383	+1.13%
Memory	1_021_500	1_046_100	+2.41%
Flat Size	956	946	-1.05%

plutus-tx-plugin/test-ledger-api/Spec/Data/Budget/9.12/gt1.golden.eval

Metric	Old	New	Δ%
CPU	388_148_300	391_988_300	+0.99%
Memory	1_166_660	1_190_660	+2.06%
Flat Size	1_324	1_314	-0.76%

plutus-tx-plugin/test-ledger-api/Spec/Data/Budget/9.12/gt2.golden.eval

Metric	Old	New	Δ%
CPU	355_465_594	359_401_594	+1.11%
Memory	1_057_315	1_081_915	+2.33%
Flat Size	1_375	1_365	-0.73%

plutus-tx-plugin/test-ledger-api/Spec/Data/Budget/9.12/gt3.golden.eval

Metric	Old	New	Δ%
CPU	420_042_992	423_978_992	+0.94%
Memory	1_282_209	1_306_809	+1.92%
Flat Size	1_375	1_365	-0.73%

plutus-tx-plugin/test-ledger-api/Spec/Data/Budget/9.12/gt4.golden.eval

Metric	Old	New	Δ%
CPU	331_580_553	335_516_553	+1.19%
Memory	948_636	973_236	+2.59%
Flat Size	1_331	1_321	-0.75%

plutus-tx-plugin/test-ledger-api/Spec/Data/Budget/9.12/gt5.golden.eval

Metric	Old	New	Δ%
CPU	373_819_011	377_755_011	+1.05%
Memory	1_110_324	1_134_924	+2.22%
Flat Size	1_331	1_321	-0.75%

plutus-tx-plugin/test/Budget/9.12/map2.golden.eval

Metric	Old	New	Δ%
CPU	67_827_382	68_307_382	+0.71%
Memory	197_790	200_790	+1.52%
Flat Size	458	453	-1.09%

plutus-tx-plugin/test/Budget/9.12/map3.golden.eval

Metric	Old	New	Δ%
CPU	111_907_732	112_771_732	+0.77%
Memory	333_684	339_084	+1.62%
Flat Size	705	695	-1.42%

This comment will get updated when changes are made.

[Unisay]

Thanks for the feedback — you're right, I was wrong about mkSimplPass producing join points. Fixed the issue and PR description.

Here's the minimal Core showing the problem. Test function:

useTwiceData :: BuiltinData -> BuiltinUnit
useTwiceData bd =
  case toBuiltinData (firstOf items) of
    _ -> case toBuiltinData (firstOf items) of
      _ -> unitval
  where
    items = unsafeFromBuiltinData bd
    firstOf = caseList' Nothing (\(h :: BuiltinData) _t -> Just h)

GHC Core produced (without BuiltinCasing, stored in the .hi file):

useTwiceData :: BuiltinData -> BuiltinUnit
useTwiceData
  = \ (bd :: BuiltinData) ->
      case bd of bd1 { BuiltinData ipv ->        -- ← BuiltinData unwrapped
      case unsafeDataAsList of g1 { __DEFAULT ->
      case g1 bd1 of nt { BuiltinList ipv1 ->
      join {
        $j :: Data -> BuiltinUnit                 -- ← Data in join point type
        $j _
          = case caseList'
                   @BuiltinData
                   @(Maybe BuiltinData)
                   (Nothing @BuiltinData)
                   (\ (x :: BuiltinData) (eta :: BuiltinList BuiltinData) ->
                      case x of x1 { BuiltinData ipv3 ->
                      case eta of { BuiltinList ipv4 ->
                      Just @BuiltinData x1
                      }})
                   nt
            of {
              Nothing -> case mkConstr (IS 1#) (mkNilData unitval) of
                           { BuiltinData ipv3 -> unitval };
              Just arg -> case mkConstr (IS 0#) (mkCons @BuiltinData arg ...) of
                            { BuiltinData ipv3 -> unitval }
            }
      } in ...

The plugin (with BuiltinCasing) reads this Core via the interface file and tries to compile the type Data that appears naked in $j :: Data -> BuiltinUnit. It hits B ByteString -> BS Addr# Int and crashes.

Without the second constructor, GHC's simplifier unwraps BuiltinData via case-of-single-constructor at every use site, and $j's captured free variables pull the unwrapped Data into the join point's type signature. With two constructors, GHC can't apply case-of-single-constructor, so Data stays hidden behind the BuiltinData wrapper and the plugin never sees it.