The pasuk in Devarim 22:25 says
But if a man finds the betrothed girl in the field, and the man overpowers her and lies with her, then only the man who lay with her shall die.
Now, the man can only be executed if there were witnesses and warning. But from 22:27
Because he found her in the field. The betrothed girl had cried out, but there was no one to save her.
it doesn't seem like there were witnesses, since there was no one to save her. So how can he be executed?
Even if we say there were witnesses, but they saw this from afar and we unable to help, still there was no warning. Unless we say that they were shouting the warning from afar as well. But that seems a stretch, since the intent of the pasuk seems to be that there were no people around in the field, and that it is that factor which makes the situation different from if it happened in the city.