Warn when a borrow expression is unused

[ecstatic-morse]

Borrowing an expression is side-effect free, so there should never be a reason to write the first instead of the second.

&expr;
// vs
expr;

If you really wanted to do this you could assign the result to an unused variable:

let _ = &expr;

Therefore, we should add a lint for unused borrows to the compiler. Why is this important? A misplaced semi-colon before an && operator in a multi-line boolean expression causes subsequent clauses to be silently ignored.

let trex = TyrannosaurusRex::new();
let is_a_dog = has_a_tail(trex)
  && has_bad_breath(trex)
  && is_a_carnivore(trex); // Misplaced semi-colon (perhaps due to reordering of lines)
  && is_adorable(trex);

if is_a_dog {
    give_hug(trex); // Ouch!
}

[RalfJung]

FWIW, the corresponding PR #76894 has been closed due to inactivity. However, it was basically ready, just needed tending to some conflicts and test failures, so if someone wants to pick it up that shouldn't be too hard.

[inashivb]

Hi @RalfJung ! I'd like to pick it up if its OK?

[RalfJung]

@inashivb sure, go ahead. :)

(I'm not necessarily the best reviewer for this PR, and I am pretty swamped currently, so I'd appreciate if someone else reading along could help with reviewing.)

[inashivb]

@rustbot claim