fixed behavior when dst is nullptr and removed magic number

Author: Sriya Pratipati

libc/src/__support/wchar/mbsrtowcs.cpp

@@ -22,12 +22,13 @@ namespace internal {
 ErrorOr<size_t> mbsrtowcs(wchar_t *__restrict dst, const char **__restrict src,
                           size_t len, mbstate *__restrict ps) {
   size_t i = 0;
+  constexpr size_t MAX_UTF8_LENGTH = 4;
   // Converting characters until we reach error or null terminator
   for (; i < len; ++i, ++dst) {
-    auto check = mbrtowc(dst, *src, 4, ps);
+    auto check = mbrtowc(dst, *src, MAX_UTF8_LENGTH, ps);
     // Encoding error/invalid mbstate
     if (!check.has_value())
-      return Error(check.error());
+      return check;
     // Successfully encoded, check for null terminator
     if (*dst == L'\0') {
       *src = nullptr;

libc/src/wchar/mbsrtowcs.cpp

@@ -27,7 +27,7 @@ LLVM_LIBC_FUNCTION(size_t, mbsrtowcs,
   static internal::mbstate internal_mbstate;
   wchar_t temp[len];
   auto ret = internal::mbsrtowcs(
-      dst == nullptr ? temp : dst, src, len,
+      dst == nullptr ? temp : dst, src, dst == nullptr ? SIZE_MAX : len,
       ps == nullptr ? &internal_mbstate
                     : reinterpret_cast<internal::mbstate *>(ps));
   if (!ret.has_value()) {

libc/test/src/wchar/mbsrtowcs_test.cpp

@@ -102,9 +102,9 @@ TEST_F(LlvmLibcMBSRToWCSTest, NullDestination) {
   // Four laughing cat emojis "😹😹😹😹"
   const char *src =
       "\xf0\x9f\x98\xb9\xf0\x9f\x98\xb9\xf0\x9f\x98\xb9\xf0\x9f\x98\xb9";
-  size_t n = LIBC_NAMESPACE::mbsrtowcs(nullptr, &src, 5, nullptr);
+  size_t n = LIBC_NAMESPACE::mbsrtowcs(nullptr, &src, 2, nullptr);
   ASSERT_ERRNO_SUCCESS();
-  // Null destination should still return correct number of read chars
+  // Null destination should ignore len and read till end of string
   ASSERT_EQ(static_cast<int>(n), 4);
 }