Conflict with mutable/immutable borrows about scopeguard HOT 3 OPEN

defer is using a closure, you can use a macro less version of that to see it better - but it has the borrow checker errors as any closure would have. The version of the scope guard that takes and owns a value exists to solve this problem (see examples in docs).

from scopeguard.

Hey, thanks for the response. Based on the example in the docs, I made it to here:

fn find_number(preamble_len: usize, numbers: &[u64]) -> Option<u64> {
    let mut iter = numbers.iter();
    let mut preamble: VecDeque<&u64> = iter.by_ref().take(preamble_len).collect();
    for value in iter {
        let mut preamble2 = scopeguard::guard(preamble, |mut preamble| {
            preamble.pop_front();
            preamble.push_back(value);
        });
        if preamble2.iter().copied().combinations(2)
            .any(|c| c.iter().copied().sum::<u64>() == *value) {
            continue;
        }
        return Some(*value);
    }
    None
}

However, I think this situation is more complex than what you show in the doc, because the value being mutated is outside the scope that I want to exit on, so I'm getting a bunch of value moved on previous iteration type issues.

It seems pretty clear that the complexity required here isn't worth it, but as a purely academic exercise, I'd be delighted to figure out how to make this work.

Here's a playground link if you're interested to experiment: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=c4407108eabe72ab21604d7c09d43159

from scopeguard.

The academic solution is scopeguard::guard(&mut preamble

Innovations welcome, in this crate or elsewhere 🙂

from scopeguard.