ARROW-18402: [C++] Expose DeclarationInfo

cpp/src/arrow/engine/api.h

@@ -21,4 +21,5 @@
 
 #include "arrow/engine/substrait/extension_set.h"
 #include "arrow/engine/substrait/extension_types.h"
+#include "arrow/engine/substrait/relation.h"
 #include "arrow/engine/substrait/serde.h"

cpp/src/arrow/engine/substrait/relation.h

@@ -0,0 +1,37 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#pragma once
+
+#include <memory>
+
+#include "arrow/compute/exec/exec_plan.h"
+#include "arrow/type_fwd.h"
+
+namespace arrow {
+namespace engine {
+
+/// Information resulting from converting a Substrait relation.
+struct ARROW_ENGINE_EXPORT DeclarationInfo {
+  /// The compute declaration produced thus far.
+  compute::Declaration declaration;
+
+  std::shared_ptr<Schema> output_schema;
+};
+
+}  // namespace engine
+}  // namespace arrow

cpp/src/arrow/engine/substrait/relation_internal.h

@@ -22,6 +22,7 @@
 #include "arrow/compute/exec/exec_plan.h"
 #include "arrow/engine/substrait/extension_types.h"
 #include "arrow/engine/substrait/options.h"
+#include "arrow/engine/substrait/relation.h"
 #include "arrow/engine/substrait/serde.h"
 #include "arrow/engine/substrait/visibility.h"
 #include "arrow/type_fwd.h"
@@ -31,14 +32,6 @@
 namespace arrow {
 namespace engine {
 
-/// Information resulting from converting a Substrait relation.
-struct DeclarationInfo {
-  /// The compute declaration produced thus far.
-  compute::Declaration declaration;
-
-  std::shared_ptr<Schema> output_schema;
-};
-
 /// \brief Convert a Substrait Rel object to an Acero declaration
 ARROW_ENGINE_EXPORT
 Result<DeclarationInfo> FromProto(const substrait::Rel&, const ExtensionSet&,