fix(decoding): decode fieldless signed enum tags for allocs

Author: Stevengre

kmir/src/kmir/decoding.py

@@ -402,10 +402,14 @@ def _decode_enum_multiple(
     # ---
     types: Mapping[Ty, TypeMetadata],
 ) -> Value:
-    assert len(offsets) == 1, 'Assumed offsets to only contain the tag offset'
-    assert tag_field == 0, 'Assumed tag field to be zero accordingly'
-    tag_offset = offsets[tag_field]
-    tag_value, width = _extract_tag(data=data, tag_offset=tag_offset, tag=tag)
+    if offsets:
+        assert 0 <= tag_field < len(offsets), 'Expected tag_field to point to an existing offset'
+        tag_offset_in_bytes = offsets[tag_field].in_bytes
+    else:
+        # Fieldless enums can omit offsets in Stable MIR; their tag still starts at byte 0.
+        assert tag_field == 0, 'Expected empty offsets only when tag_field is zero'
+        tag_offset_in_bytes = 0
+    tag_value, width = _extract_tag(data=data, tag_offset_in_bytes=tag_offset_in_bytes, tag=tag)
     discriminant = tag_encoding.decode(tag_value, width=width)
 
     try:
@@ -440,7 +444,7 @@ def _decode_fields(
     return res
 
 
-def _extract_tag(*, data: bytes, tag_offset: MachineSize, tag: Scalar) -> tuple[int, IntegerLength]:
+def _extract_tag(*, data: bytes, tag_offset_in_bytes: int, tag: Scalar) -> tuple[int, IntegerLength]:
     match tag:
         case Initialized(
             value=PrimitiveInt(
@@ -449,9 +453,8 @@ def _extract_tag(*, data: bytes, tag_offset: MachineSize, tag: Scalar) -> tuple[
             ),
             valid_range=_,
         ):
-            # Stable MIR enum discriminants are represented against the raw tag bits.
-            # Use unsigned decoding even when the scalar metadata marks the tag as signed.
-            tag_data = data[tag_offset.in_bytes : tag_offset.in_bytes + length.value]
+            tag_data = data[tag_offset_in_bytes : tag_offset_in_bytes + length.value]
+            # For enum-tag decoding we match Stable MIR discriminants in wrapped form.
             tag_value = int.from_bytes(tag_data, byteorder='little', signed=False)
             return tag_value, length
         # special case: niche-encoded optional pointer, None == 0x00000000

kmir/src/tests/integration/data/prove-rs/show/iter-eq-copied-take-dereftruncate-fail.repro.expected

@@ -3,13 +3,14 @@
 │   #execTerminator ( terminator ( ... kind: terminatorKindCall ( ... func: operandC
 │   span: 0
 │
-│  (5553 steps)
-└─ 3 (stuck, leaf)
-    #traverseProjection ( toLocal ( 12 ) , thunk ( #decodeConstant ( constantKindAll
-    span: 282
-
-
-┌─ 2 (root, leaf, target, terminal)
+│  (5601 steps)
+├─ 3 (terminal)
 │   #EndProgram ~> .K
+│
+┊  constraint: true
+┊  subst: ...
+└─ 2 (leaf, target, terminal)
+    #EndProgram ~> .K
+