[C++]: support casting nullable fields to non-nullable if there are n…

…o null values

Fixes #33592

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

@@ -363,10 +363,6 @@ struct CastStruct {
         const auto& in_field = in_type.field(in_field_index);
         // If there are more in_fields check if they match the out_field.
         if (in_field->name() == out_field->name()) {
-          if (in_field->nullable() && !out_field->nullable()) {
-            return Status::TypeError("cannot cast nullable field to non-nullable field: ",
-                                     in_type.ToString(), " ", out_type.ToString());
-          }
           // Found matching in_field and out_field.
           fields_to_select[out_field_index++] = in_field_index;
           // Using the same in_field for multiple out_fields is not allowed.
@@ -412,8 +408,14 @@ struct CastStruct {
       } else {
         const auto& values = (in_array.child_data[field_index].ToArrayData()->Slice(
             in_array.offset, in_array.length));
-        ARROW_ASSIGN_OR_RAISE(Datum cast_values,
-                              Cast(values, target_type, options, ctx->exec_context()));
+        const auto& target_field = out->type()->field(out_field_index++);
+        if (!target_field->nullable() && values->null_count > 0) {
+          return Status::TypeError("field '", target_field->name(),
+                                   "' has nulls. Can't cast to non-nullable type ",
+                                   target_field->type()->ToString());
+        }
+        ARROW_ASSIGN_OR_RAISE(Datum cast_values, Cast(values, target_field->type(),
+                                                      options, ctx->exec_context()));
         DCHECK(cast_values.is_array());
         out_array->child_data.push_back(cast_values.array());
       }

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

@@ -2943,7 +2943,7 @@ TEST(Cast, StructToDifferentNullabilityStruct) {
     CheckCast(src_non_nullable, dest3_nullable);
   }
   {
-    // But NOT OK to go from nullable to non-nullable...
+    // But when going from nullable to non-nullable, all data must be non-null...
     std::vector<std::shared_ptr<Field>> fields_src_nullable = {
         std::make_shared<Field>("a", int8(), true),
         std::make_shared<Field>("b", int8(), true),
@@ -2965,27 +2965,30 @@ TEST(Cast, StructToDifferentNullabilityStruct) {
     const auto options1_non_nullable = CastOptions::Safe(dest1_non_nullable);
     EXPECT_RAISES_WITH_MESSAGE_THAT(
         TypeError,
-        ::testing::HasSubstr("cannot cast nullable field to non-nullable field"),
+        ::testing::HasSubstr(
+            "field 'a' has nulls. Can't cast to non-nullable type int64"),
         Cast(src_nullable, options1_non_nullable));
 
     std::vector<std::shared_ptr<Field>> fields_dest2_non_nullable = {
         std::make_shared<Field>("a", int64(), false),
         std::make_shared<Field>("c", int64(), false)};
     const auto dest2_non_nullable = arrow::struct_(fields_dest2_non_nullable);
-    const auto options2_non_nullable = CastOptions::Safe(dest2_non_nullable);
+    const auto options2_non_nullable = CastOptions::Unsafe(dest2_non_nullable);
     EXPECT_RAISES_WITH_MESSAGE_THAT(
         TypeError,
-        ::testing::HasSubstr("cannot cast nullable field to non-nullable field"),
+        ::testing::HasSubstr(
+            "field 'a' has nulls. Can't cast to non-nullable type int64"),
         Cast(src_nullable, options2_non_nullable));
 
-    std::vector<std::shared_ptr<Field>> fields_dest3_non_nullable = {
+    std::shared_ptr<Array> c_dest_no_nulls;
+    c_dest_no_nulls = ArrayFromJSON(int64(), "[9, 11, 44]");
+    std::vector<std::shared_ptr<Field>> fields_dest_no_nulls = {
         std::make_shared<Field>("c", int64(), false)};
-    const auto dest3_non_nullable = arrow::struct_(fields_dest3_non_nullable);
-    const auto options3_non_nullable = CastOptions::Safe(dest3_non_nullable);
-    EXPECT_RAISES_WITH_MESSAGE_THAT(
-        TypeError,
-        ::testing::HasSubstr("cannot cast nullable field to non-nullable field"),
-        Cast(src_nullable, options3_non_nullable));
+    ASSERT_OK_AND_ASSIGN(auto dest_no_nulls,
+                         StructArray::Make({c_dest_no_nulls}, fields_dest_no_nulls));
+    const auto options3_non_nullable =
+        CastOptions::Unsafe(arrow::struct_(fields_dest_no_nulls));
+    CheckCast(src_nullable, dest_no_nulls, options3_non_nullable);
   }
 }