Make it a compile error to use a crash key with no variable name.

base/debug/crash_logging.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_DEBUG_CRASH_LOGGING_H_
 #define BASE_DEBUG_CRASH_LOGGING_H_
 
 #include <stddef.h>
 
 #include <string>
@@ skipping 31 matching lines @@
                                           const void* const* addresses,
                                           size_t count);
 
 // A scoper that sets the specified key to value for the lifetime of the
 // object, and clears it on destruction.
 class BASE_EXPORT ScopedCrashKey {
  public:
   ScopedCrashKey(const base::StringPiece& key, const base::StringPiece& value);
   ~ScopedCrashKey();
 
+  // Helper to force a static_assert when instantiating a ScopedCrashKey
+  // temporary without a name. The usual idiom is to just #define a macro that
+  // static_asserts with the message; however, that doesn't work well when the
+  // type is in a namespace.
+  //
+  // Instead, we use a templated helper to trigger the static_assert, observing
+  //   two rules:
+  // - The static_assert needs to be in a normally uninstantiated template;
+  //   otherwise, it will fail to compile =)
+  // - Similarly, the static_assert must be dependent on the template argument,
+  //   to prevent it from being evaluated until the template is instantiated.
+  template<typename T>
+  ScopedCrashKey(const T&) {

[danakj, 2016/07/12 19:59:26]

Why a constructor and not just a free CrashFromBadScopedCrashKey() function or
something?

[dcheng, 2016/07/13 00:34:11]

This prevents me from having to "using base::debug::HelperFunction" in Blink.

[Robert Sesek, 2016/07/13 19:46:22]

What about something like this:

enum class UnnamedScopedCrashKey { ERROR };
template<typename T>
ScopedCrashKey(const T&) {
  static_assert(std::is_same<T, UnnamedScopedCrashKey>::value == false, "Unnamed
ScopedCrashKey");
  static_assert(std::is_same<T, UnnamedScopedCrashKey>::value == true, "Invalid
constructor");
}

[danakj, 2016/07/13 19:48:16]

I was just thinking similar things.. but more like

enum class UnnamedScopedCrashKey { ERROR };
template<typename T>
explicit ScopedCrashKey(const T&, UnnamedScopedCrashKey) {
  static_assert(...);
}

So that people can't really call this constructor as easily, they'd have to
really try to.

+    static_assert(
+        sizeof(T) == 0,
+        "scoped crash key objects should not be unnamed temporaries.");
+  }
+
  private:
   std::string key_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedCrashKey);
 };
 
+// Disallow an instantation of ScopedCrashKey without a name, since this results
+// in a temporary that is immediately destroyed. Doing so will trigger the
+// static_assert in the templated constructor helper in ScopedCrashKey.
+#define ScopedCrashKey(key, value) ScopedCrashKey(0xbad)
+
 // Before setting values for a key, all the keys must be registered.
 struct BASE_EXPORT CrashKey {
   // The name of the crash key, used in the above functions.
   const char* key_name;
 
   // The maximum length for a value. If the value is longer than this, it will
   // be truncated. If the value is larger than the |chunk_max_length| passed to
   // InitCrashKeys() but less than this value, it will be split into multiple
   // numbered chunks.
   size_t max_length;
@@ skipping 29 matching lines @@
     const base::StringPiece& value,
     size_t chunk_max_length);
 
 // Resets the crash key system so it can be reinitialized. For testing only.
 BASE_EXPORT void ResetCrashLoggingForTesting();
 
 }  // namespace debug
 }  // namespace base
 
 #endif  // BASE_DEBUG_CRASH_LOGGING_H_