Implementing faster StringBuilder that only works with ASCII strings
This is very, very buggy. This is the most reliable way I could find to reproduce the bug.

static void Main(string[] args)
{
    var sb = new FastStringBuilder();
    var chars = new[] { 'a', 'b', 'e', 'd' };
    sb.Append(new string(chars));
    Console.WriteLine(sb.ToString());
}


Note how args is not used. Now run the program with different arguments:

$ ./Test
dfhj
$ ./Test abc
OZ?'
$ ./Test abcdef
"-~s