How to assign to the variable used in match expres

问题:

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 expression inside the next() method implementation. Here is a simplified version of my code:

pub struct Join<I>
where
    I: Iterator,
    I::Item: IntoIterator,
{
    outer_iter: I,
    inner_iter: Option<<I::Item as IntoIterator>::IntoIter>,
}

impl<I> Join<I>
where
    I: Iterator,
    I::Item: IntoIterator,
{
    pub fn new(mut iter: I) -> Join<I> {
        let inner_iter = iter.next().map(|it| it.into_iter());
        Join {
            outer_iter: iter,
            inner_iter,
        }
    }
}

impl<I> Iterator for Join<I>
where
    I: Iterator,
    I::Item: IntoIterator,
{
    type Item = <I::Item as IntoIterator>::Item;

    fn next(&mut self) -> Option<Self::Item> {
        loop {
            match &mut self.inner_iter {
                Some(ref mut it) => match it.next() {
                    Some(x) => {
                        return Some(x);
                    }
                    None => {
                        self.inner_iter = self.outer_iter.next().map(|it| it.into_iter());
                    }
                },
                None => {
                    return None;
                }
            }
        }
    }
}

pub trait MyItertools: Iterator {
    fn join(self) -> Join<Self>
    where
        Self: Sized,
        Self::Item: IntoIterator,
    {
        Join::new(self)
    }
}

impl<I> MyItertools for I where I: Iterator {}

#[cfg(test)]
mod test {
    use super::MyItertools;

    #[test]
    fn it_works() {
        let input = [[1], [2]];
        let expected = [&1, &2];
        assert_eq!(input.iter().join().collect::<Vec<_>>(), expected);
    }
}

Text of error:

error[E0506]: cannot assign to `self.inner_iter` because it is borrowed
  --> src/main.rs:39:25
   |
33 |             match &mut self.inner_iter {
   |                        --------------- borrow of `self.inner_iter` occurs here
...
39 |                         self.inner_iter = self.outer_iter.next().map(|it| it.into_iter());
   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ assignment to borrowed `self.inner_iter` occurs here

I understand why the borrow checker complains about my code, but I did not find a good solution, only an ugly workaround:

fn next(&mut self) -> Option<Self::Item> {
    loop {
        match self.inner_iter.take() {
            Some(mut it) => {
                match it.next() {
                    Some(x) => { self.inner_iter = Some(it); return Some(x); }
                    None => { self.inner_iter = self.outer_iter.next().map(|it| it.into_iter()); }
                }
            }
            None => { return None; }
        }
    }
}

I imagine that situations like this occur regularly; how can I rewrite my code to deal with them or avoid them?

回答1:

Here is a simpler reproduction of the problem:

fn main() {
    let mut a = (42, true);
    match a {
        (ref _i, true) => a = (99, false),
        (ref _i, false) => a = (42, true),
    }
    println!("{:?}", a);
}

error[E0506]: cannot assign to `a` because it is borrowed
 --> src/main.rs:4:27
  |
4 |         (ref _i, true) => a = (99, false),
  |          ------           ^^^^^^^^^^^^^^^ assignment to borrowed `a` occurs here
  |          |
  |          borrow of `a` occurs here

error[E0506]: cannot assign to `a` because it is borrowed
 --> src/main.rs:5:28
  |
5 |         (ref _i, false) => a = (42, true),
  |          ------            ^^^^^^^^^^^^^^ assignment to borrowed `a` occurs here
  |          |
  |          borrow of `a` occurs here

This is a weakness of the AST-based borrow checker. When non-lexical lifetimes are enabled, this works as-is. The enhanced MIR-based borrow checker can see that there's no borrow of the matched-on variable at the point at which you try to replace it.

---

For what it's worth, your join is just a flat_map:

input.iter().flat_map(|x| x)

Or a flatten:

input.iter().flatten()

You can see how these implement next for another idea:

fn next(&mut self) -> Option<Self::Item> {
    loop {
        if let Some(v) = self.inner_iter.as_mut().and_then(|i| i.next()) {
            return Some(v);
        }
        match self.outer_iter.next() {
            Some(x) => self.inner_iter = Some(x.into_iter()),
            None => return None,
        }
    }
}

This clearly delineates that the iterator value doesn't really borrow from inner_iter.

---

Without looking at flatten, I would have chosen to clearly indicate that there's no overlapping borrowing by taking the Option and restoring it if it is Some, as you've done:

match self.inner_iter.take() {
    Some(mut it) => match it.next() {
        Some(x) => {
            self.inner_iter = Some(it);
            return Some(x);
        }
        None => {
            self.inner_iter = self.outer_iter.next().map(|it| it.into_iter());
        }
    },
    None => {
        return None;
    }
}

回答2:

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<Self::Item> {
    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.