Rust 如何在结构中使用选项类型获得对QCell的可变访问

我用的是板条箱。我有一个

struct MyStruct

，其中有一个

选项

。现在我想获得对内部

QCell

的可变访问权。代码如下：

#[derive(Default)]
struct MyStruct {
    pub val: i32,
    pub item: Option<Rc<QCell<MyStruct>>>
}

fn main() {
    let mut owner = QCellOwner::new();
    let struct1 = Rc::new(QCell::new(&owner, MyStruct { ..Default::default() }));
    let struct2 = Rc::new(QCell::new(&owner, MyStruct { ..Default::default() }));

    // Point struct2 from struct1.item
    owner.rw(struct1.borrow()).item = Some(Rc::clone(&struct2));

    // Get a mutable access to struct2 from struct1.item
    if let Some(v) = &owner.rw(struct1.borrow()).item {
        owner.rw(v.borrow()).val = 123;
    }
}

如何从

struct1.item

设置

struct2

的

val

error[E0499]: cannot borrow `owner` as mutable more than once at a time
  --> src/main.rs:xx:x
   |
xx |     if let Some(v) = &owner.rw(struct1.borrow()).item {
   |                       ----- first mutable borrow occurs here
xx |         owner.rw(v.borrow()).val = 123;
   |         ^^^^^    - first borrow later used here
   |         |
   |         second mutable borrow occurs here