[C++][Compute] Ignore null view sizes when casting binary_view to binary

cpp/src/arrow/compute/kernels/scalar_cast_string.cc

@@ -367,8 +367,14 @@ BinaryToBinaryCastExec(KernelContext* ctx, const ExecSpan& batch, ExecResult* ou
   OffsetBuilder offset_builder(ctx->memory_pool());
   RETURN_NOT_OK(offset_builder.Reserve(input.length + 1));
   offset_builder.UnsafeAppend(0);  // offsets start at 0
-  const int64_t sum_of_binary_view_sizes = util::SumOfBinaryViewSizes(
-      input.GetValues<BinaryViewType::c_type>(1), input.length);
+  // Null views are undefined and may hold any size, so only sum the valid ones
+  const auto* views = input.GetValues<BinaryViewType::c_type>(1);
+  int64_t sum_of_binary_view_sizes = 0;
+  arrow::internal::VisitSetBitRunsVoid(
+      input.buffers[0].data, input.offset, input.length,
+      [&](int64_t position, int64_t length) {
+        sum_of_binary_view_sizes += util::SumOfBinaryViewSizes(views + position, length);
+      });
   DataBuilder data_builder(ctx->memory_pool());
   RETURN_NOT_OK(data_builder.Reserve(sum_of_binary_view_sizes));
   VisitArraySpanInline<I>(

cpp/src/arrow/compute/kernels/scalar_cast_test.cc

@@ -3450,6 +3450,26 @@ TEST(Cast, StringToString) {
   }
 }
 
+TEST(Cast, BinaryViewToBinaryNullViewSize) {
+  // Views under a null bit are undefined and may hold a negative size; it must
+  // not shrink the output data buffer.
+  auto json = R"(["foofoofoofoofoo", null, "barbarbarbarbarbarbar"])";
+  for (auto from_type : {utf8_view(), binary_view()}) {
+    auto data = ArrayFromJSON(from_type, json)->data()->Copy();
+    ASSERT_OK_AND_ASSIGN(data->buffers[1],
+                         data->buffers[1]->CopySlice(0, data->buffers[1]->size()));
+    data->GetMutableValues<BinaryViewType::c_type>(1)[1].inlined.size = -36;
+    auto from = MakeArray(data);
+    ASSERT_OK(from->ValidateFull());
+
+    for (auto to_type : {utf8(), large_utf8(), binary(), large_binary()}) {
+      ASSERT_OK_AND_ASSIGN(auto out, Cast(*from, to_type));
+      ValidateOutput(*out);
+      AssertArraysEqual(*ArrayFromJSON(to_type, json), *out, /*verbose=*/true);
+    }
+  }
+}
+
 TEST(Cast, StringToStringWithOffset) {
   // GH-43660: Check casting String Arrays with nonzero offset
   std::vector<int64_t> offsets = {3, 8, 10, 12};