Spurious E0492 with closure capture

[zirconium-n]

Code

const fn foo() {
    let i = 0;
    let _ = || {
        i = 1;
    };
}

Current output

error[E0492]: constant functions cannot refer to interior mutable data
 --> src/lib.rs:5:13
  |
5 |       let _ = || {
  |  _____________^
6 | |         i = 1;
7 | |     };
  | |_____^ this borrow of an interior mutable value may end up in the final value

error[E0594]: cannot assign to `i`, as it is not declared as mutable
 --> src/lib.rs:6:9
  |
4 |     let i = 0;
  |         - help: consider changing this to be mutable: `mut i`
5 |     let _ = || {
6 |         i = 1;
  |         ^^^^^ cannot assign

Desired output

error[E0658]: mutable references are not allowed in constant functions
 --> src/lib.rs:5:13
  |
5 |       let _ = || {
  |  _____________^
6 | |         i = 1;
7 | |     };
  | |_____^

error[E0594]: cannot assign to `i`, as it is not declared as mutable
 --> src/lib.rs:6:9
  |
4 |     let i = 0;
  |         - help: consider changing this to be mutable: `mut i`
5 |     let _ = || {
6 |         i = 1;
  |         ^^^^^ cannot assign

Rationale and extra context

Apparently there's no interior mutability involved. So we should just output we cannot mutable borrow i in const context.

Incidentally(?) fixed by #112119. Still needs test though.

@rustbot label E-needs-test

Other cases

No response

Anything else?

rustc version

rustc +stable --version
rustc 1.70.0 (90c541806 2023-05-31)

[zirconium-n]

@rustbot claim

[zirconium-n]

Actually I feel mentioning mutable reference is still pretty weird, since it's hidden under closure capture. At least it's better than non-existent interior mutability.