Help understanding the lifetime error

#1
pub struct Stack<T> {
    head: Link<T>,
}

type Link<T> = Option<Box<Node<T>>>;

struct Node<T> {
    elem: T,
    next: Link<T>,
}

pub struct Iter<'a, T>(&'a Link<T>);

impl<'a, T> Iterator for Iter<'a, T> {
    type Item = &'a T;

    fn next(&mut self) -> Option<Self::Item> {
        self.0.as_ref().map(|node| {
            self.0 = &node.next;
            &node.elem
        })
    }
}

pub struct IterMut<'a, T>(&'a mut Link<T>);

impl<'a, T> Iterator for IterMut<'a, T> {
    type Item = &'a mut T;

    fn next(&mut self) -> Option<Self::Item> {
        self.0.as_mut().map(|node| {
            self.0 = &mut node.next;
            &mut node.elem
        })
    }
}

(Playground)

Why is there a lifetime issue when using &mut (line 31) but not with & (line 18)? Is there a way to make this work? One way is to change IterMut itself: http://cglab.ca/~abeinges/blah/too-many-lists/book/second-final.html. But I want to know how the &mut lifetimes are working different from the & lifetimes for exactly similar code.