It is never at rest. If one allows light to expend momentum (and energy)
by, say, bouncing it off moving mirrors it will never slow down, but it
will lose momentum. Extrapolating to zero momentum and calling that a
state of rest we could say that if light were to be brought to rest it
would be insubstantial, that is to say, massless. But that would simply
be manipulating language by changing definitions; it wouldn't be physics.