Why can I not call borrow_mut() after indexing into an immutable Vec?

Question

Let\'s try to compile this code:

use std::cell::RefCell;

struct Foo {
    v: Vec>,
}

impl Foo {
    fn f(&self, i: usize) {
            


        

Answer 1

This is a known issue where IndexMut is sometimes selected when Index should actually be used.

Your workaround of using {} is reasonable, but you can also use Index explicitly:

use std::cell::RefCell;

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

fn main() {}

See also:

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

Answer 2

Another workaround is to explicitly call RefCell::borrow_mut(&v[0]).