How to assign to the variable used in match expression inside a match branch?

Question

I\'m trying to implement a general function join() that can work on any iterator of iterators. I have a problem with the borrow checker in a match expr

Answer 1

In these kind of situations, I find useful to write the code in two pieces: first collect the data, then update the mutable:

fn next(&mut self) -> Option {
    loop {
        //collect the change into a local variable
        let ii = match &mut self.inner_iter {
            Some(ref mut it) => {
                match it.next() {
                    Some(x) => { return Some(x); }
                    None => self.outer_iter.next().map(|it| it.into_iter())
                }
            }
            None => { return None; }
        };
        //self.inner_iter is no longer borrowed, update
        self.inner_iter = ii;
    }
}

The fact that all the branches that do not modify the inner_iter do a return makes the code easier.