test(smir): reproduce _projected_ty Subslice drop-glue pruning bug

Add unit test demonstrating that _projected_ty() returns the original
array type for Subslice projections, causing call_edges to miss the
drop_in_place::<[T; M]> edge and reduce_to() to incorrectly prune
the required drop glue.

Add integration test subslice-drop-partial-move.rs that generates
Drop(arr.Subslice(1,3,false)) in the MIR — the exact pattern
described in the Codex review comment.

Author: Stevengre

kmir/src/tests/integration/data/prove-rs/subslice-drop-partial-move.rs

@@ -0,0 +1,20 @@
+// Subslice projection in a Drop terminator's place.
+//
+// `let [first, ..] = arr` moves only `first`; the remaining elements
+// are dropped in place via Drop(arr.Subslice(1, 3, false)).
+// This exercises _projected_ty() resolving the Subslice to the correct
+// [Droppable; 2] type so reduce_to() preserves the drop glue.
+
+struct Droppable(u8);
+
+impl Drop for Droppable {
+    fn drop(&mut self) {}
+}
+
+fn consume(_: Droppable) {}
+
+fn main() {
+    let arr = [Droppable(1), Droppable(2), Droppable(3)];
+    let [first, ..] = arr;
+    consume(first);
+}

kmir/src/tests/unit/test_smir.py

@@ -258,3 +258,125 @@ def test_drop_function_tys_accept_core_drop_in_place_names() -> None:
     )
 
     assert smir_info.drop_function_tys == {2: 12}
+
+
+def test_call_edges_preserve_drop_glue_for_subslice_projection() -> None:
+    # This SMIR models the pattern from `let [first, ..] = arr` where
+    # arr: [Inner; 3].  The compiler generates:
+    #   Drop(local 1 . Subslice { from: 1, to: 3, from_end: false })
+    # to drop the remaining [Inner; 2] subslice.
+    #
+    # _projected_ty() must resolve the Subslice to [Inner; 2] (ty 5),
+    # not return the original [Inner; 3] (ty 1), so call_edges finds the
+    # drop_in_place::<[Inner; 2]> edge.
+    smir_info = SMIRInfo(
+        {
+            'name': 'drop-subslice',
+            'allocs': [],
+            'types': [
+                [
+                    1,
+                    {
+                        'ArrayType': {
+                            'elem_type': 2,
+                            'size': {
+                                'kind': {'Value': [0, {'bytes': [3, 0, 0, 0, 0, 0, 0, 0], 'provenance': {'ptrs': []}, 'align': 8, 'mutability': 'Mut'}]},
+                                'id': 1,
+                            },
+                        }
+                    },
+                ],  # [Inner; 3]
+                [
+                    2,
+                    {
+                        'StructType': {
+                            'name': 'Inner',
+                            'adt_def': 2,
+                            'fields': [],
+                            'layout': None,
+                        }
+                    },
+                ],
+                [3, {'PtrType': {'pointee_type': 2}}],
+                [
+                    5,
+                    {
+                        'ArrayType': {
+                            'elem_type': 2,
+                            'size': {
+                                'kind': {'Value': [0, {'bytes': [2, 0, 0, 0, 0, 0, 0, 0], 'provenance': {'ptrs': []}, 'align': 8, 'mutability': 'Mut'}]},
+                                'id': 2,
+                            },
+                        }
+                    },
+                ],  # [Inner; 2]
+                [6, {'PtrType': {'pointee_type': 5}}],
+            ],
+            'functions': [
+                [10, {'NormalSym': 'caller'}],
+                [12, {'NormalSym': 'drop_inner'}],
+                [14, {'NormalSym': 'drop_inner_array2'}],
+            ],
+            'items': [
+                {
+                    'symbol_name': 'caller',
+                    'mono_item_kind': {
+                        'MonoItemFn': {
+                            'name': 'caller',
+                            'body': {
+                                'arg_count': 0,
+                                'locals': [{'ty': 0}, {'ty': 1}],
+                                'blocks': [
+                                    {
+                                        'terminator': {
+                                            'kind': {
+                                                'Drop': {
+                                                    'place': {
+                                                        'local': 1,
+                                                        'projection': [
+                                                            {'Subslice': {'from': 1, 'to': 3, 'from_end': False}}
+                                                        ],
+                                                    },
+                                                    'target': 0,
+                                                    'unwind': 'Continue',
+                                                }
+                                            }
+                                        }
+                                    }
+                                ],
+                            },
+                        }
+                    },
+                },
+                {
+                    'symbol_name': 'drop_inner',
+                    'mono_item_kind': {
+                        'MonoItemFn': {
+                            'name': 'std::ptr::drop_in_place::<Inner>',
+                            'body': {
+                                'arg_count': 1,
+                                'locals': [{'ty': 0}, {'ty': 3}],
+                                'blocks': [],
+                            },
+                        }
+                    },
+                },
+                {
+                    'symbol_name': 'drop_inner_array2',
+                    'mono_item_kind': {
+                        'MonoItemFn': {
+                            'name': 'std::ptr::drop_in_place::<[Inner; 2]>',
+                            'body': {
+                                'arg_count': 1,
+                                'locals': [{'ty': 0}, {'ty': 6}],
+                                'blocks': [],
+                            },
+                        }
+                    },
+                },
+            ],
+            'spans': [],
+        }
+    )
+
+    assert smir_info.call_edges == {10: {14}, 12: set(), 14: set()}