GH-39049: [C++] Use Cast() instead of CastTo() for Dictionary Scalar in test

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

@@ -36,14 +36,22 @@ Status CastToDictionary(KernelContext* ctx, const ExecSpan& batch, ExecResult* o
   const CastOptions& options = CastState::Get(ctx);
   const auto& out_type = checked_cast<const DictionaryType&>(*out->type());
 
+  std::shared_ptr<ArrayData> in_array = batch[0].array.ToArrayData();
+
   // if out type is same as in type, return input
   if (out_type.Equals(*batch[0].type())) {
     /// XXX: This is the wrong place to do a zero-copy optimization
-    out->value = batch[0].array.ToArrayData();
+    out->value = in_array;
     return Status::OK();
   }
 
-  std::shared_ptr<ArrayData> in_array = batch[0].array.ToArrayData();
+  // If the input type is STRING, it is first encoded as a dictionary to facilitate
+  // processing. This approach allows the subsequent code to uniformly handle STRING
+  // inputs as if they were originally provided in dictionary format. Encoding as a
+  // dictionary helps in reusing the same logic for dictionary operations.
+  if (batch[0].type()->id() == Type::STRING) {
+    in_array = DictionaryEncode(in_array)->array();
+  }
   const auto& in_type = checked_cast<const DictionaryType&>(*in_array->type);
 
   ArrayData* out_array = out->array_data().get();
@@ -77,17 +85,21 @@ Status CastToDictionary(KernelContext* ctx, const ExecSpan& batch, ExecResult* o
   return Status::OK();
 }
 
-std::vector<std::shared_ptr<CastFunction>> GetDictionaryCasts() {
-  auto func = std::make_shared<CastFunction>("cast_dictionary", Type::DICTIONARY);
-
-  AddCommonCasts(Type::DICTIONARY, kOutputTargetType, func.get());
-  ScalarKernel kernel({InputType(Type::DICTIONARY)}, kOutputTargetType, CastToDictionary);
+template <typename SrcType>
+void AddDictionaryCast(CastFunction* func) {
+  ScalarKernel kernel({InputType(SrcType::type_id)}, kOutputTargetType, CastToDictionary);
   kernel.null_handling = NullHandling::COMPUTED_NO_PREALLOCATE;
   kernel.mem_allocation = MemAllocation::NO_PREALLOCATE;
+  DCHECK_OK(func->AddKernel(SrcType::type_id, std::move(kernel)));
+}
 
-  DCHECK_OK(func->AddKernel(Type::DICTIONARY, std::move(kernel)));
+std::vector<std::shared_ptr<CastFunction>> GetDictionaryCasts() {
+  auto cast_dict = std::make_shared<CastFunction>("cast_dictionary", Type::DICTIONARY);
+  AddCommonCasts(Type::DICTIONARY, kOutputTargetType, cast_dict.get());
+  AddDictionaryCast<DictionaryType>(cast_dict.get());
+  AddDictionaryCast<StringType>(cast_dict.get());
 
-  return {func};
+  return {cast_dict};
 }
 
 }  // namespace internal

cpp/src/arrow/dataset/partition_test.cc

@@ -316,7 +316,7 @@ TEST_F(TestPartitioning, DirectoryPartitioningFormatDictionary) {
                                                           ArrayVector{dictionary});
   written_schema_ = partitioning_->schema();
 
-  ASSERT_OK_AND_ASSIGN(auto dict_hello, MakeScalar("hello")->CastTo(DictStr("")->type()));
+  ASSERT_OK_AND_ASSIGN(auto dict_hello, Cast(MakeScalar("hello"), DictStr("")->type()));
   AssertFormat(equal(field_ref("alpha"), literal(dict_hello)), "hello");
 }
 
@@ -329,7 +329,7 @@ TEST_F(TestPartitioning, DirectoryPartitioningFormatDictionaryCustomIndex) {
       schema({field("alpha", dict_type)}), ArrayVector{dictionary});
   written_schema_ = partitioning_->schema();
 
-  ASSERT_OK_AND_ASSIGN(auto dict_hello, MakeScalar("hello")->CastTo(dict_type));
+  ASSERT_OK_AND_ASSIGN(auto dict_hello, Cast(MakeScalar("hello"), dict_type));
   AssertFormat(equal(field_ref("alpha"), literal(dict_hello)), "hello");
 }
 

cpp/src/arrow/scalar_test.cc

@@ -1479,7 +1479,8 @@ TEST(TestDictionaryScalar, Cast) {
       auto alpha =
           dict->IsValid(i) ? MakeScalar(dict->GetString(i)) : MakeNullScalar(utf8());
       // Cast string to dict(..., string)
-      ASSERT_OK_AND_ASSIGN(auto cast_alpha, alpha->CastTo(ty));
+      ASSERT_OK_AND_ASSIGN(auto cast_alpha_datum, Cast(alpha, ty));
+      const auto& cast_alpha = cast_alpha_datum.scalar();
       ASSERT_OK(cast_alpha->ValidateFull());
       ASSERT_OK_AND_ASSIGN(
           auto roundtripped_alpha,