rust - 为什么我不能在索引到不可变的 Vec 后调用 borrow_mut()？

Question

让我们尝试编译这段代码:

use std::cell::RefCell;

struct Foo {
    v: Vec<RefCell<u8>>,
}

impl Foo {
    fn f(&self, i: usize) {
        let t = &mut *self.v[i].borrow_mut();
        //let t = &mut *{self.v[i].borrow_mut()}; //compiled ok
    }
}

fn main() {}

编译错误:

error[E0596]: cannot borrow field `self.v` of immutable binding as mutable
 --> src/main.rs:9:23
  |
8 |     fn f(&self, i: usize) {
  |          ----- use `&mut self` here to make mutable
9 |         let t = &mut *self.v[i].borrow_mut();
  |                       ^^^^^^ cannot mutably borrow field of immutable binding

为什么这段代码需要在函数签名中添加&mut self才能编译？

Answer 1

这是一个 known issue当实际应该使用 Index 时，有时会选择 IndexMut。

您使用 {} 的解决方法是合理的，但您也可以显式使用 Index:

use std::cell::RefCell;

fn f(v: Vec<RefCell<u8>>) {
    use std::ops::Index;
    let _t = &mut v.index(0).borrow_mut();
}

fn main() {}

另见:

• Why does a mutable borrow of a closure through DerefMut not work?
• How to use `BorrowMut` contained within `RefCell`?