Borrow checker rejects a loop that pushes to a vec while reading its last element Help

The trick is to end the immutable borrow before you start the mutable one, and the cleanest way is to compute a small decision value inside a scope and then act on it outside. Something like let should_merge = matches!(events.last(), Some(e) if e.can_merge(&item)); then if should_merge { events.last_mut().unwrap().merge(item) } else { events.push(item) }. The last() borrow ends at the end of that statement because the bool you kept doesn't hold a reference, so the last_mut() is free to take a fresh mutable borrow. No clone, no unsafe, and it reads fine.

Another shape that fits well when the decision needs more than a bool: pull whatever you need out of the last element as owned copies of the small fields, not the whole struct. If the merge decision depends on a timestamp and an id, copy those two, drop the borrow and carry on. You said the struct is 200 bytes but the decision probably needs 16.

If your merge logic gets long, consider an enum for the decision, enum Action { Merge, Push }, computed in one function that takes &[Event] and returns the action. Keeps the borrow inside a function boundary where it obviously ends and makes the loop body two lines.

Worth knowing that this specific pattern is a limitation of the current borrow checker and not a logical problem with your code. It's the classic case people hit and the workaround above is the idiomatic answer, so don't spend an evening thinking you've modelled something wrong.

Just wrap the vec in a RefCell and the problem goes away.