cannot assign to `p.next` because it is borrowed

Question

I'm trying to solve problem 61 on leetcode using rust. And I implemented the code as below and it works fine

#[derive(PartialEq, Eq, Clone, Debug)]
pub struct ListNode {
    pub val: i32,
    pub next: Option<Box<ListNode>>,
}

impl ListNode {
    #[inline]
    fn new(val: i32) -> Self {
        ListNode { next: None, val }
    }
}

pub fn rotate_right(head: Option<Box<ListNode>>, k: i32) -> Option<Box<ListNode>> {
    match head {
        Some(mut head) => {
            let mut p = &mut head;

            let mut n = 1;
            while p.next.is_some() {
                p = p.next.as_mut().unwrap();
                n += 1;
            }

            let k = k % n;
            p = &mut head;
            for _ in 0..(n - k - 1) {
                p = p.next.as_mut().unwrap();
            }

            if p.next.is_none() {
                return Some(head);
            }

            let mut q = p.next.take().unwrap();
            p = &mut q;

            while p.next.is_some() {
                p = p.next.as_mut().unwrap();
            }
            p.next = Some(head);

            // while let Some(next) = &mut p.next {
            //     p = next;
            // }
            // p.next = Some(head);
            // `p.next`: cannot assign to `p.next` because it is borrowed

            Some(q)
        }
        None => None,
    }
}

I know it's ok to take ownership, like this

while p.next.is_some() {
    p = p.next.as_mut().unwrap();
}
p.next = Some(head);

But why is the &mut not released here?

while let Some(next) = &mut p.next {
    p = next;
}
p.next = Some(head);

The &mut of p.next was taken by next in Some(next). But seems it to be released?

@ChayimFriedman This does not seem to be the same type of problem with the duplicate. In the duplicate, the value returned after mutably borrowing, however, the lifetimes has ended in this example, it does not return. — tony, Aug 11 '23 at 06:17
Trust me, it is (because of the way `while let` is desugared). The key observations are the same anyway: this needs Polonius, and you've already found the workaround. — Chayim Friedman, Aug 11 '23 at 06:18
You can compile your code with Polonius (`cargo +nightly rustc -- -Zpolonius`) and see that it compiles successfully. — Chayim Friedman, Aug 11 '23 at 06:19
@ChayimFriedman you are right, I run it successfully with polonius. Thank you very much. — tony, Aug 11 '23 at 06:27

cannot assign to `p.next` because it is borrowed

0 Answers0