Don't ever auto-define NDEBUG. Just decide KJ_DEBUG based on optimization mode…

Don't ever auto-define NDEBUG.  Just decide KJ_DEBUG based on optimization mode if neither DEBUG nor NDEBUG is defined.

c++/src/kj/common.h

@@ -98,16 +98,18 @@ typedef unsigned char byte;
   #endif
 #endif
 
-#if __OPTIMIZE__ && !defined(NDEBUG) && !defined(DEBUG) && !defined(KJ_DEBUG)
-#warning "You've enabled optimization but not NDEBUG. Usually optimized builds should #define \
-NDEBUG to disable debug asserts, so I am #defining it for you. If you actually want debug asserts, \
-please #define DEBUG or KJ_DEBUG. To make this warning go away, #define NDEBUG yourself, e.g. with \
-the compiler flag -DNDEBUG."
-#define NDEBUG 1
-#endif
-
-#if !defined(NDEBUG) && !defined(KJ_DEBUG)
+#if !defined(KJ_DEBUG) && !defined(KJ_NDEBUG)
+// Heuristically decide whether to enable debug mode.  If DEBUG or NDEBUG is defined, use that.
+// Otherwise, fall back to checking whether optimization is enabled.
+#if defined(DEBUG)
 #define KJ_DEBUG
+#elif defined(NDEBUG)
+#define KJ_NDEBUG
+#elif __OPTIMIZE__
+#define KJ_NDEBUG
+#else
+#define KJ_DEBUG
+#endif
 #endif
 
 #define KJ_DISALLOW_COPY(classname) \

c++/src/kj/debug.h

@@ -101,7 +101,10 @@
 //   `FAIL_RECOVERABLE_SYSCALL` take a string and an OS error number as the first two parameters.
 //   The string should be the name of the failed system call.
 // * For every macro `FOO` above, there is a `DFOO` version (or `RECOVERABLE_DFOO`) which is only
-//   executed in debug mode.  When `NDEBUG` is defined, these macros expand to nothing.
+//   executed in debug mode, i.e. when KJ_DEBUG is defined.  KJ_DEBUG is defined automatically
+//   by common.h when compiling without optimization (unless NDEBUG is defined), but you can also
+//   define it explicitly (e.g. -DKJ_DEBUG).  Generally, production builds should NOT use KJ_DEBUG
+//   as it may enable expensive checks that are unlikely to fail.
 
 #ifndef KJ_DEBUG_H_
 #define KJ_DEBUG_H_