If you're foot was trapped by a boulder, would you sever it to get free?
Hint: don't do that.
Even if you somehow were able to overpower your instincts of avoiding self harm and powered through the pain, you're probably not going to live for much longer. There are 2 arteries in your foot, which will lead you to bleed out within 30 minutes. The much safer bet is to just wait and hope for rescue.