锈“借用......”编译错误

问：

我有一个节点列表

struct ListNode {
  val: i32,
  next: Option<Box<ListNode>>
}

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

然后我有一个遍历函数（此处的代码仅作为示例进行简化）。

fn traverse(mut list: Option<Box<ListNode>>) {
    let mut ptr = &mut list;
    let mut counter = 0;

    while counter < 5 {

        match ptr {
            None => {
                break;
            },
            Some(ptr_node) => { 
                if true {
                    ptr = &mut ptr_node.next;
                }
            }
        }

        counter += 1;
    }
}

当我编译此代码时，出现此编译错误

error[E0503]: cannot use `*ptr` because it was mutably borrowed
   |
   |         match ptr {
   |               ^^^
   |               |
   |               use of borrowed `ptr.0`
   |               borrow later used here
...
   |             Some(ptr_node) => { 
   |                  -------- borrow of `ptr.0` occurs here

error[E0499]: cannot borrow `ptr.0` as mutable more than once at a time
   |
   |             Some(ptr_node) => { 
   |                  ^^^^^^^^ `ptr.0` was mutably borrowed here in the previous iteration of the loop

error: aborting due to 2 previous errors; 2 warnings emitted

但是如果我将我的遍历函数更改为这个

fn traverse(mut list: Option<Box<ListNode>>) {
    let mut ptr = &mut list;
    let mut counter = 0;

    while counter < 5 {

        match ptr {
            None => {
                break;
            },
            Some(ptr_node) => { 
                ptr = &mut ptr_node.next;
            }
        }

        counter += 1;
    }
}

基本上，我从代码中删除了“if”语句。 这将成功编译。

谁能解释为什么“if”语句会导致编译错误？