I went the “only let introduces bindings” route, and I’m pretty happy so far:

if (left.next(), right.next())
... is (Some(let l), Some(let r)) { /* use l and r */ }
... is (Some(let l), None       ) { /* use l       */ }
... is (None,        Some(let r)) { /* use r       */ }
... is (None,        None       ) { /* use nothing */ }
}