support for memory_resource

Summary:
This diff adds folly/memory/MemoryResource.h, which includes
<experimental/memory_resource> or <memory_resource> if they are available.

Reviewed By: yfeldblum

Differential Revision: D15789182

fbshipit-source-id: ff8410b59354af7124a50ca6000c072372397cda

folly/memory/MemoryResource.h

+/*
+ * Copyright 2019-present Facebook, Inc.
+ *
+ * Licensed 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
+
+#ifdef __has_include
+
+#if __has_include(<memory_resource>)
+
+#define FOLLY_HAS_MEMORY_RESOURCE 1
+#include <memory_resource> // @manual
+namespace folly {
+namespace detail {
+namespace std_pmr = ::std::pmr;
+} // namespace detail
+} // namespace folly
+
+#elif __has_include(<experimental/memory_resource>)
+
+#define FOLLY_HAS_MEMORY_RESOURCE 1
+#include <experimental/memory_resource> // @manual
+namespace folly {
+namespace detail {
+namespace std_pmr = ::std::experimental::pmr;
+} // namespace detail
+} // namespace folly
+
+#else
+
+#define FOLLY_HAS_MEMORY_RESOURCE 0
+
+#endif
+
+#else // __has_include
+
+#define FOLLY_HAS_MEMORY_RESOURCE 0
+
+#endif // __has_include

folly/memory/test/MemoryResourceTest.cpp

+/*
+ * Copyright 2019-present Facebook, Inc.
+ *
+ * Licensed 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.
+ */
+
+#include <folly/memory/MemoryResource.h>
+
+#include <folly/portability/GTest.h>
+
+#if FOLLY_HAS_MEMORY_RESOURCE
+
+TEST(MemoryResource, simple) {
+  using Alloc = folly::detail::std_pmr::polymorphic_allocator<char>;
+
+  std::vector<char, Alloc> v{
+      Alloc{folly::detail::std_pmr::null_memory_resource()}};
+  EXPECT_THROW(v.push_back('x'), std::bad_alloc);
+}
+
+#endif // FOLLY_HAS_MEMORY_RESOURCE