Ito's lemma applied to functions involving stopping times

Question

Recently, I come across an exercise in my book that asks us to apply Ito's formula to $$Y_t = e^{rt} \mathbf{1}_{ \{ \tau \leq t \} },$$ where $\tau$ is a stopping time. However, this is an inherent issue about computing $d \mathbf{1}_{ \{ \tau \leq t \} }$, as paths are non-differentiable. Can anyone explain to me the expression for $dY_t$?