debug: add RETURN_UNLESS convenience macro

returns a given value unless a condition is true. primarily useful for
computing is_valid and breaking if one of the preconditions isn't valid.

debug.hpp

@@ -93,6 +93,21 @@
 } while (0)
 
 
+#define RETURN_UNLESS(VALUE,CONDITION) do {                             \
+    if (const auto &__return_unless = (CONDITION); !__return_unless) {  \
+        if constexpr (debug_enabled) {                                  \
+            std::clog << __FILE__ << ':'                                \
+                      << __LINE__ << ':'                                \
+                      << __PRETTY_FUNCTION__ << "; "                    \
+                      << #CONDITION << '\n';                            \
+            breakpoint ();                                              \
+        }                                                               \
+                                                                        \
+        return (VALUE);                                                 \
+    }                                                                   \
+} while (0)
+
+
 ///////////////////////////////////////////////////////////////////////////////
 #define _CHECK_PANIC(FMT,...) do {                          \
     panic ("%s:%s:%i:%s\n" FMT,                             \