Bob gains his farmland back form the trash.
...except that Bob doesn't gain the Farmland. It just teleports to his discard pile magically.
To be even more nitpicky, Bob's farmland is never trashed. It is instead set aside, then discarded at the end of the Possessed turn.
This is not true. Bob's Farmland is trashed (and Bob could discard a Market Square for Alice to gain a Gold), and then it's set aside, and then it's not discarded, but rather it magically teleports itself to the discard pile (which arguably matters if someone trashes a Tunnel during a Possessed turn).
Whoah. That leads to another set of weird edge cases:
Graverobber and Rogue. If Alice possesses Bob and plays an upgrade, trashing his duchy to gain a gold. Can Alice then play Bob's Rogue to gain the Duchy from the trash (causing possession to "lose track." Or is a trashed card still "trashed" but then immediately in this mystical "set-aside" stack? (never in the trash pile)
Assuming that last mechanic is correct, a trashed card is "set aside" for the remainder of the turn. What happens if a possessed player trashes a fortress? Does it go to back to hand (text on the fortress card) or into this "set aside" pile (text on possession).
And what about Forager? If the trash is empty, and I play forager on a possessed turn to trash a copper, is the forager worth 1$, or 0$?