parseNoGC can work with code points directly.

source/dyaml/nogcutil.d

@@ -33,10 +33,15 @@ body
     Target v = 0;
     size_t atStart = true;
 
-    for (; !s.empty; s.popFront())
+    // We can safely foreach over individual code points.
+    // Even with UTF-8 any digit is ASCII and anything not ASCII (such as the start of
+    // a UTF-8 sequence) is not a digit.
+    foreach(i; 0 .. s.length)
     {
-        uint c = s.front;
-        if (c < '0')
+        dchar c = s[i];
+        // We can just take a char instead of decoding because anything non-ASCII is not
+        // going to be a decodable digit, i.e. we will end at such a byte.
+        if (c < '0' || c >= 0x80)
             break;
         if (radix < 10)
         {