Cotto hasn't seen a lot of southpaws, but he's done well against them when he has.
Let's throw away Pacquiao, whom Trout doesn't resemble in the least bit stylistically.
Judah, Quintana, and Chop-Chop are all vaguely
close (at least relative to Pacquiao) to looks he'll get from Trout.
I think Trout has entered his prime but still isn't on par with a 2007 Judah. Hell, I'm not sure he's better than an unbeaten Quintana or that version of Corley.
In all of Trout's best performances, he's been able to frustrate and annoy slower opponents with his right jab. Guys like D-Rod, Destroyer, and Rigo lack the dimensionality to do anything but plod into it repeatedly and perhaps become hesitant as a result (and wary of the infrequent but very fast left cross). I see Cotto being able to bob and weave through it, and not be deterred if he eats a couple so long as the end result is him getting to rip a few left hooks to Trout's body.
Trout is a legit super welter, and fairly strong, but I don't see him keeping Cotto at bay all night. (and certainly not walking him down)
Cotto on the frontfoot, slipping or ignoring the southpaw jab, has a great chance of felling some timber - or at the very least scaring the daylights out of Trout and forcing him into a pattern too negative to merit awarding points.
Cotto UD or late TKO.