While let lifetime problem

[yakongxing]

#[derive(Debug,Default)]
struct C<T>(T, Option<Box<C<T>>>);

impl<T: Default> C<T> {
    fn get(&mut self) -> Self {
        let mut cur = self;
        while let Some(x) = cur.1.as_mut() {
            cur = x.as_mut();
        }
        // Ok
        // while cur.1.is_some(){
        //     cur = cur.1.as_mut().unwrap().as_mut();
        // }

        std::mem::take(cur) // Error
    }
}

why cur.1.as_mut borrow continues after while loop, so that `std::mem::take(cur)` line can not compile

[Redglyph]

It's a limitation of the current borrow checker, unfortunately. The let Some(x) is not released with while, unlike if.

You can work around as you did, or with something like this (a bit ugly, but is optimized correctly by the compiler):

    while true {
        if cur.1.is_none() {
            break;
        }
        let x = cur.1.as_mut().unwrap();
        cur = x.as_mut();
    }

Or you can use the polonius-the-crab crate for more complex situations.