New macro: RTC_DEPRECATED (for annotating deprecated functions)

Review URL: https://codereview.webrtc.org/1494133003

Cr-Commit-Position: refs/heads/master@{#11042}

webrtc/base/BUILD.gn

@@ -119,6 +119,7 @@ static_library("rtc_base_approved") {
     "constructormagic.h",
     "criticalsection.cc",
     "criticalsection.h",
+    "deprecation.h",
     "event.cc",
     "event.h",
     "event_tracer.cc",

webrtc/base/base.gyp

@@ -85,6 +85,7 @@
         'constructormagic.h',
         'criticalsection.cc',
         'criticalsection.h',
+        'deprecation.h',
         'event.cc',
         'event.h',
         'event_tracer.cc',

webrtc/base/deprecation.h

@@ -0,0 +1,45 @@
+/*
+ *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_BASE_DEPRECATION_H_
+#define WEBRTC_BASE_DEPRECATION_H_
+
+// Annotate the declarations of deprecated functions with this to cause a
+// compiler warning when they're used. Like so:
+//
+//   RTC_DEPRECATED std::pony PonyPlz(const std::pony_spec& ps);
+//
+// NOTE 1: The annotation goes on the declaration in the .h file, not the
+// definition in the .cc file!
+//
+// NOTE 2: In order to keep unit testing the deprecated function without
+// getting warnings, do something like this:
+//
+//   std::pony DEPRECATED_PonyPlz(const std::pony_spec& ps);
+//   RTC_DEPRECATED inline std::pony PonyPlz(const std::pony_spec& ps) {
+//     return DEPRECATED_PonyPlz(ps);
+//   }
+//
+// In other words, rename the existing function, and provide an inline wrapper
+// using the original name that calls it. That way, callers who are willing to
+// call it using the DEPRECATED_-prefixed name don't get the warning.
+//
+// TODO(kwiberg): Remove this when we can use [[deprecated]] from C++14.
+#if defined(_MSC_VER)
+// Note: Deprecation warnings seem to fail to trigger on Windows
+// (https://bugs.chromium.org/p/webrtc/issues/detail?id=5368).
+#define RTC_DEPRECATED __declspec(deprecated)
+#elif defined(__GNUC__)
+#define RTC_DEPRECATED __attribute__((deprecated))
+#else
+#define RTC_DEPRECATED
+#endif
+
+#endif  // WEBRTC_BASE_DEPRECATION_H_