a test for memcpy-use

Summary: Proves the internal build rule replaces `memcpy`.

Reviewed By: LoganEvans

Differential Revision: D29665797

fbshipit-source-id: 7f3a8550853261de94cedf9545b6445d11297b2b

folly/test/MemcpyUseTest.cpp

+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * 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/FollyMemcpy.h>
+
+#include <cstring>
+
+#include <folly/lang/Keep.h>
+#include <folly/portability/GTest.h>
+
+extern "C" FOLLY_KEEP void check_c_memcpy(void* d, void* s, size_t n) {
+  memcpy(d, s, n);
+}
+
+extern "C" FOLLY_KEEP void check_std_memcpy(void* d, void* s, size_t n) {
+  std::memcpy(d, s, n);
+}
+
+extern "C" FOLLY_KEEP void check_folly_memcpy(void* d, void* s, size_t n) {
+  __folly_memcpy(d, s, n);
+}
+
+TEST(MemcpyUseTest, empty) {
+  // this empty test existing forces the keeps above, the native code of which
+  // may be inspected to prove that the internal build rules actually replace
+  // what would otherwise be calls to memcpy with calls to __folly_memcpy
+}