Nothing. You don't get any special treatment or consideration on your taxes for unpaid rent. You don't get to deduct unpaid rent, since you can't deduct income you never received. You can't deduct from your taxable income, that which you never received or paid taxes on in the first place.
Ok, then, am I to assume that all expenses and depreciation will be accepted as legitimate losses for the entire period of no rental income? I can lose money for awhile (lucky me) but when does the loss of rent become a presumed income and thus taxable? I’m guessing that any forbearance of rent will require a 1099 for barter at the $600 dollar level and become taxable income.