How is baptism of desire compatible with Canon 5 of Session 7 of the Council of Trent?

Session 7 deals with the sacraments in general; its Canon 5 anathematizes anyone who says the sacraments were instituted only to nourish faith. If instead you mean the canon that baptism is necessary and not optional for salvation, the answer is that the Council is not denying baptism's necessity; it is condemning the idea that you can reach heaven without it. Baptism of desire is itself a doctrine taught by Trent, in chapter 4 of Session 6, so it cannot contradict the canons. How it is applied is where people disagree; some limit it to catechumens who desired baptism and died before receiving it, but Trent does not add that qualification, it simply says baptism of desire. Those arguments will not be settled, but you must hold that baptism of desire is real and a genuine doctrine, and leave its exact extent and application to the Church, which is the only authority competent to apply it.