IndexMut

[ghost]

This gives an error:

fn main() {
    let mut vec = vec![1; 3];
    vec[vec.len() - 1] = 2;
}

error[E0502]: cannot borrow `vec` as immutable because it is also borrowed as mutable
   |
   |     vec[vec.len() - 1] = 2;
   |     ----^^^-----------
   |     |   |
   |     |   immutable borrow occurs here
   |     mutable borrow occurs here
   |     mutable borrow later used here

But this gives no error:

use std::ops::IndexMut;
fn main() {
    let mut vec = vec![1; 3];
    *vec.index_mut(vec.len() - 1) = 2;
}

[Elinvynia]

Related docs: https://doc.rust-lang.org/std/ops/trait.IndexMut.html

Assuming the docs are not the issue here, this should work in my opinion.

[RustyYato]

This is because two-phased borrows only work with method syntax, maybe we should also allow it for index sugar?

[jonas-schievink]

Duplicate of #58419