Improve helpers.h

Use C11 standard _Noreturn instead of attributes (except, of course, MSVC)
Remove unused helpers
Avoid trapping in release builds, in favor of just unreachability
Improve shim when __builtin_* are not available

include/helpers.h

@@ -9,21 +9,30 @@
 #ifndef HELPERS_H
 #define HELPERS_H
 
-#ifdef __GNUC__
-	/* GCC or compatible */
-	#define format_(archetype, str_index, first_arg) \
-		__attribute__ ((format (archetype, str_index, first_arg)))
-	#define noreturn_	__attribute__ ((noreturn))
-	#define trap_		__builtin_trap()
-#else
-	/* Unsupported, but no need to throw a fit */
-	#define format_(archetype, str_index, first_arg)
-	#define noreturn_
-	#define unused_
-	#define trap_
+// Of course, MSVC does not support C11, so no _Noreturn there...
+#ifdef _MSC_VER
+	#define _Noreturn __declspec(noreturn)
 #endif
 
-/* Macros for stringification */
+// Ideally, we'd use `__has_attribute` and `__has_builtin`, but these were only introduced in GCC 9
+#ifdef __GNUC__ // GCC or compatible
+	#define format_(archetype, str_index, first_arg) \
+		__attribute__ ((format (archetype, str_index, first_arg)))
+	// In release builds, define "unreachable" as such, but trap in debug builds
+	#ifdef NDEBUG
+		#define unreachable_	__builtin_unreachable
+	#else
+		#define unreachable_	__builtin_trap
+	#endif
+#else
+	// Unsupported, but no need to throw a fit
+	#define format_(archetype, str_index, first_arg)
+	// This seems to generate similar code to __builtin_unreachable, despite different semantics
+	// Note that executing this is undefined behavior (declared _Noreturn, but does return)
+	static inline _Noreturn unreachable_(void) {}
+#endif
+
+// Macros for stringification
 #define STR(x) #x
 #define EXPAND_AND_STR(x) STR(x)