[JSC] Avoid using snprintf in DatePrototype.cpp

https://bugs.webkit.org/show_bug.cgi?id=271490
rdar://125258271

Reviewed by Justin Michaud and Keith Miller.

Clang is wrongly computing snprintf's potential size (llvm/llvm-project#71320), and generating wrong error.
But given that we already have efficient WTF String's mechanism, we should just use it instead of snprintf, slow and buggy.
This patch converts DatePrototype.cpp's snprintf usage to our own makeString.

* Source/JavaScriptCore/runtime/DatePrototype.cpp:
(JSC::JSC_DEFINE_HOST_FUNCTION):

Canonical link: https://commits.webkit.org/276582@main

Source/JavaScriptCore/runtime/DatePrototype.cpp

@@ -324,27 +324,30 @@ JSC_DEFINE_HOST_FUNCTION(dateProtoFuncToISOString, (JSGlobalObject* globalObject
     const GregorianDateTime* gregorianDateTime = thisDateObj->gregorianDateTimeUTC(vm.dateCache);
     if (!gregorianDateTime)
         return JSValue::encode(jsNontrivialString(vm, String("Invalid Date"_s)));
-    // Maximum amount of space we need in buffer: 7 (max. digits in year) + 2 * 5 (2 characters each for month, day, hour, minute, second) + 4 (. + 3 digits for milliseconds)
-    // 6 for formatting and one for null termination = 28. We add one extra character to allow us to force null termination.
-    char buffer[28];
+
+    // https://tc39.es/ecma262/#sec-date-time-string-format
+
     // If the year is outside the bounds of 0 and 9999 inclusive we want to use the extended year format (ES 15.9.1.15.1).
     int ms = static_cast<int>(fmod(thisDateObj->internalNumber(), msPerSecond));
     if (ms < 0)
         ms += msPerSecond;
 
-    int charactersWritten;
-    if (gregorianDateTime->year() > 9999 || gregorianDateTime->year() < 0) {
-        IGNORE_CLANG_WARNINGS_BEGIN("format-truncation"); // for Clang 18
-        charactersWritten = snprintf(buffer, sizeof(buffer), "%+07d-%02d-%02dT%02d:%02d:%02d.%03dZ", gregorianDateTime->year(), gregorianDateTime->month() + 1, gregorianDateTime->monthDay(), gregorianDateTime->hour(), gregorianDateTime->minute(), gregorianDateTime->second(), ms);
-        IGNORE_CLANG_WARNINGS_END;
-    } else
-        charactersWritten = snprintf(buffer, sizeof(buffer), "%04d-%02d-%02dT%02d:%02d:%02d.%03dZ", gregorianDateTime->year(), gregorianDateTime->month() + 1, gregorianDateTime->monthDay(), gregorianDateTime->hour(), gregorianDateTime->minute(), gregorianDateTime->second(), ms);
-
-    ASSERT(charactersWritten > 0 && static_cast<unsigned>(charactersWritten) < sizeof(buffer));
-    if (static_cast<unsigned>(charactersWritten) >= sizeof(buffer))
-        return JSValue::encode(jsEmptyString(vm));
+    int year = gregorianDateTime->year();
+    int month = gregorianDateTime->month() + 1;
+    int day = gregorianDateTime->monthDay();
+    int hour = gregorianDateTime->hour();
+    int minute = gregorianDateTime->minute();
+    int second = gregorianDateTime->second();
+
+    String prefix;
+    auto yearDigits = 4;
+    if (year < 0 || year > 9999) {
+        prefix = year < 0 ? "-"_s : "+"_s;
+        yearDigits = 6;
+        year = std::abs(year);
+    }
 
-    return JSValue::encode(jsNontrivialString(vm, String(buffer, charactersWritten)));
+    return JSValue::encode(jsNontrivialString(vm, makeString(prefix, pad('0', yearDigits, year), '-', pad('0', 2, month), '-', pad('0', 2, day), 'T', pad('0', 2, hour), ':', pad('0', 2, minute), ':', pad('0', 2, second), '.', pad('0', 3, ms), 'Z')));
 }
 
 JSC_DEFINE_HOST_FUNCTION(dateProtoFuncToDateString, (JSGlobalObject* globalObject, CallFrame* callFrame))