Models assaulting into, out of or through difficult terrain will be slowed down in the same manner as models moving through such terrain in the Movement phase. Roll two D6 and pick the highest to see how far they can move. If their move is insufficient to reach the target unit then the charge does not proceed, and no close combat ensues. The models are not moved, and it is assumed that they simply did not have time to scramble through the difficult terrain or thought better of it due to their poor progress.
Designer's note: Note that difficult terrain is handled differently to the Movement phase - this way we save lots of pointless Assault moves which won't result in combats occurring.