Slightly incorrect error on mutable borrow of immutable captured variable

[bkoropoff]

Code:

fn set(x: &mut usize) { *x = 1; }

fn main() {
    let x = 0;
    || set(&mut x);
}

Output:

<anon>:7:5: 7:19 error: closure cannot assign to immutable local variable `x`
<anon>:7     || set(&mut x);
             ^~~~~~~~~~~~~~

This should actually state that it cannot borrow (rather than assign) the variable. This has been around since 0.10 at least.

[steveklabnik]

Triage: no change today.

[cyplo]

Hello !
The message seems to be different now, is it what you would expect, @bkoropoff ?

error: closure cannot assign to immutable local variable `x`
 --> test.rs:5:5
  |
4 |     let x = 0;
  |         - consider changing this to `mut x`
5 |     || set(&mut x);
  |     ^^ cannot borrow mutably

rustc 1.18.0-nightly (28a742997 2017-04-13)

[jakubadamw]

rustc 1.22.0-nightly (981ce7d8d 2017-09-03)

Input:

fn f(x: Option<()>) {
	(|| x.take())();
}
fn main() {}

Output:

error[E0595]: closure cannot assign to immutable argument `x`
 --> test.rs:2:3
  |
1 | fn f(x: Option<()>) {
  |      - consider changing this to `mut x`
2 |     (|| x.take())();
  |   ^^ cannot borrow mutably

error: aborting due to previous error

Still not quite. f() doesn't really reassign the argument, it just calls a function that borrows mutably. It's really just a matter of a slight wording change to the error message to make it more general. Much like in the PR #29670, which ended up not being merged.

[estebank]

From #45622:

The output should point out at the use of x in some way, as well as changing the wording slightly (it is not an assignment):

error[E0595]: closure cannot modify immutable argument `x`
 --> src/main.rs:4:6
  |
1 | fn foo(x: &mut i32) {
  |        - consider changing this to `mut x`
...
4 |     (|| {
  |      ^^ cannot borrow mutably
5 |        process(&mut x);
  |                     - `x` used here