I don't know, borrow checker error messages are already pretty good at pointing out issues of this sort already. With a minor addition I think it would remain

I don't know, borrow checker error messages are already pretty good at pointing out issues of this sort already. With a minor addition I think it would remain very comprehensible: "you borrowed a here ^^^ ... gave a away here ^^^^, then used it's borrow again here ^^^^ after having given it away."