Description
Retrieving an element from a vector of mutable references works fine calling index_mut() directly
use std::ops::IndexMut;
fn explicit_trait_call(mut a: Vec<&mut i32>) {
let _i: &mut i32 = *a.index_mut(0);
}but fails when using supposedly equivalent indexing syntax:
fn indexing(mut a: Vec<&mut i32>) {
let _i: &mut i32 = a[0];
}The error message is rather surprising:
error[E0596]: cannot borrow data in an index of `std::vec::Vec<&mut i32>` as mutable
--> src/lib.rs:4:24
|
4 | let _i: &mut i32 = a[0];
| ^^^^ cannot borrow as mutable
|
= help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `std::vec::Vec<&mut i32>`
It's clearly not true that IndexMut is not implemented for Vec, as evidenced by the first implementation.
The explicit trait call version only works because the explicit type annotation for _i triggers an implicit reborrow. My gut feeling is that the compiler is selecting Index instead of IndexMut here since the assignment looks like a move when not taking into account the implicit reborrow. I feel that the rejected code should be accepted. In any case, the error message should be fixed.
This post was inspired by a question of StackOverflow today, and I discussed this on the user forum as well.