IndexMut chosen when Index would suffice

[alexcrichton]

This code incorrectly reports that vec needs to be listed as mutable:

use std::cell::RefCell;

fn stuff(a: &mut u32) {
}

pub fn main() {
    let vals: Vec<RefCell<u32>> = vec![];

    stuff(&mut *vals[0].borrow_mut());
}

<anon>:9:17: 9:21 error: cannot borrow immutable local variable `vals` as mutable
<anon>:9     stuff(&mut *vals[0].borrow_mut());
                         ^~~~
error: aborting due to previous error
playpen: application terminated with error code 101

If, however, you change the code slightly:

use std::cell::RefCell;

fn stuff(a: &mut u32) {
}

pub fn main() {
    let vals: Vec<RefCell<u32>> = vec![];

    // notice the parens + & around vals[0]
    stuff(&mut *(&vals[0]).borrow_mut());
}

Then this will compile successfully.

[carllerche]

I just hit this (replying to track issue).

[nagisa]

Duplicate of #28935

[alexcrichton]

Aha!