我尝试从RefCell中借用：

use std::cell::RefCell;
use std::borrow::Borrow;
use std::rc::Rc;

let k = Some(Rc::new(RefCell::new(5)));
let what = k.as_ref().unwrap().clone();
let kk = what.borrow_mut();

它起作用了，但这个不行

let k = Some(Rc::new(RefCell::new(5)));
let what = k.as_ref().unwrap().clone();
let kk = what.borrow();

错误信息也无济于事：

error[E0283]: type annotations needed for `&Borrowed`
   --> src/bin/a.rs:404:19
    |
404 |     let kk = what.borrow();
    |         --        ^^^^^^ cannot infer type for type parameter `Borrowed` declared on the trait `Borrow`
    |         |
    |         consider giving `kk` the explicit type `&Borrowed`, where the type parameter `Borrowed` is specified
    |
    = note: multiple `impl`s satisfying `Rc<RefCell<i32>>: Borrow<_>` found in the following crates: `alloc`, `core`:
            - impl<T> Borrow<T> for Rc<T>
              where T: ?Sized;
            - impl<T> Borrow<T> for T
              where T: ?Sized;

我不明白为什么当我创建一个带有Borrow trait的本地绑定时，使用borrow()不能像borrow_mut()一样正常工作。应该做些什么来不可改变地借用？