Bug 1129247 - Introduce MOZ_DIAGNOSTIC_ASSERT. r=Waldo

mfbt/Assertions.h

@@ -294,6 +294,13 @@ __declspec(noreturn) __inline void MOZ_NoReturn() {}
  * MOZ_ASSERT has no effect in non-debug builds.  It is designed to catch bugs
  * *only* during debugging, not "in the field". If you want the latter, use
  * MOZ_RELEASE_ASSERT, which applies to non-debug builds as well.
+ *
+ * MOZ_DIAGNOSTIC_ASSERT works like MOZ_RELEASE_ASSERT in Nightly/Aurora and
+ * MOZ_ASSERT in Beta/Release - use this when a condition is potentially rare
+ * enough to require real user testing to hit, but is not security-sensitive.
+ * This can cause user pain, so use it sparingly. If a MOZ_DIAGNOSTIC_ASSERT
+ * is firing, it should promptly be converted to a MOZ_ASSERT while the failure
+ * is being investigated, rather than letting users suffer.
  */
 
 /*
@@ -378,6 +385,12 @@ struct AssertionConditionType
 #  define MOZ_ASSERT(...) do { } while (0)
 #endif /* DEBUG */
 
+#ifdef RELEASE_BUILD
+#  define MOZ_DIAGNOSTIC_ASSERT MOZ_ASSERT
+#else
+#  define MOZ_DIAGNOSTIC_ASSERT MOZ_RELEASE_ASSERT
+#endif
+
 /*
  * MOZ_ASSERT_IF(cond1, cond2) is equivalent to MOZ_ASSERT(cond2) if cond1 is
  * true.