Rc不支持内部更改RefCell，官方文档为什么这么写，我的rustc版本是1.7 `fn main() { let value = Rc::new(RefCell::new(5)); let a = Rc::new(Cons(Rc::clone(&value), Rc::new(Nil))); let b = Cons(Rc::new(RefCell::new(3)), Rc::clone(&a)); let c = Cons(Rc::new(RefCell::new(4)), Rc::clone(&a)); *value.borrow_mut() += 10; println!("a after = {:?}", a); println!("b after = {:?}", b); println!("c after = {:?}", c);

} `

error[E0368]: binary assignment operation += cannot be applied to type Rc<RefCell<i32>> --> .\main.rs:11:5 | 11 | *value.borrow_mut() += 10; | | | cannot use += on type Rc<RefCell<i32>>

error: aborting due to previous error