Borrow checker doesn't accept certain valid case when branch involves

[upsuper]

The compiler currently rejects the following code:

struct X {
    next: Option<Box<X>>,
}

fn main() {
    let mut b = Some(Box::new(X { next: None }));
    let mut p = &mut b;
    while let Some(now) = p {
        if true {
            p = &mut now.next;
        }
    }
}

It complains that now double mutably borrows p.0.

This explanation is not very convincing. If you rewrite it this way:

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

the compile would happily accept it, and compiler certainly also accepts

    while let Some(now) = p {
        // empty body
    }

so basically whatever the condition of if evaluates to, the original code should always be valid in terms of borrowing.

I'm not sure if I'm missing something, but it seems to me this is a bug of the borrow checker?

[upsuper]

FWIW, if the code inside if is changed to

p = &mut p.as_mut().unwrap().next;

then it compiles. But I see no reason why the original code should be rejected.

cc @iovxw