Why does this fail to compile?

I'm trying to get a reference to the tail of a linked list, and here's the code I wrote:

pub struct ListNode {
    pub next: Option<Box<ListNode>>,
}

fn tail(mut head: &mut Option<Box<ListNode>>) -> &mut Option<Box<ListNode>> {
    while let Some(x) = head.as_mut() {
        head = &mut x.next;
    }
    head
}

This fails to compile because it thinks I'm borrowing head as mutable twice. I was able to get to compile by changing it to this:

fn tail(mut head: &mut Option<Box<ListNode>>) -> &mut Option<Box<ListNode>> {
    while head.is_some() {
        head = &mut head.as_mut().unwrap().next;
    }
    head
}

It appears to me that these two functions do the exact same thing, but the first one looks a lot cleaner. Why doesn't it compile? Is there a better way of doing this than what I changed it to?

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