LibJS: Add a MUST() macro, like TRY() but for the spec's ! shortcut

Author: https://github.com/linusg
Commit: 7cd3f7de61
Pull-request: https://github.com/SerenityOS/serenity/pull/10327
Reviewed-by: https://github.com/IdanHo ✅

Userland/Libraries/LibJS/Runtime/Completion.h

@@ -26,6 +26,19 @@ namespace JS {
         _temporary_result.release_value();     \
     })
 
+// MUST() is to the spec's `!` what TRY() is to `?`.
+// https://tc39.es/ecma262/#sec-returnifabrupt-shorthands
+#define MUST(expression)                                           \
+    ({                                                             \
+        auto _temporary_result = (expression);                     \
+        VERIFY(!_temporary_result.is_error());                     \
+        /* The return value of "! Something()" is commonly      */ \
+        /* ignored, so we assign to a temporary variable here   */ \
+        /* to avoid having to (void) all the things.            */ \
+        auto _temporary_value = _temporary_result.release_value(); \
+        move(_temporary_value);                                    \
+    })
+
 // 6.2.3 The Completion Record Specification Type, https://tc39.es/ecma262/#sec-completion-record-specification-type
 class [[nodiscard]] Completion {
 public: