Assigning to a for loop's element suggests replacing it with invalid code

[ginnyTheCat]

Code

fn main() {
    for x in [1, 2, 3] {
        x += 1;
        println!("{x}");
    }
}

Current output

error[E0384]: cannot assign twice to immutable variable `x`
 --> src/main.rs:3:9
  |
2 |     for x in [1, 2, 3] {
  |         - first assignment to `x`
3 |         x += 1;
  |         ^^^^^^ cannot assign twice to immutable variable
  |
help: consider making this binding mutable
  |
2 |     for (mut x) x in [1, 2, 3] {
  |         +++++++

For more information about this error, try `rustc --explain E0384`.
error: could not compile `tmp` (bin "tmp") due to 1 previous error

Desired output

error[E0384]: cannot assign twice to immutable variable `x`
 --> src/main.rs:3:9
  |
2 |     for x in [1, 2, 3] {
  |         - first assignment to `x`
3 |         x += 1;
  |         ^^^^^^ cannot assign twice to immutable variable
  |
help: consider making this binding mutable
  |
2 |     for mut x in [1, 2, 3] {
  |         ++++

For more information about this error, try `rustc --explain E0384`.
error: could not compile `tmp` (bin "tmp") due to 1 previous error

Rationale and extra context

I noticed this at first since I thought the parens are unneeded, but even in that case it should only add (mut before and ) after, not (mut x) before, since (mut x) x is syntactically incorrect.

Other cases

Rust Version

rustc 1.96.0-nightly (900485642 2026-04-08)
binary: rustc
commit-hash: 900485642855f4729d926ecf24680a791f9293cf
commit-date: 2026-04-08
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.2

Anything else?

No response

[lms0806]

@rustbot claim

[lms0806]

We confirmed that the same occurs in version 1.94.1.

[ginnyTheCat]

This seems to have regressed in #149101. I'm unsure whether the test case that was introduced there was just not reviewed well enough or whether this is even intended. In case it is, I would maybe suggest adding - below x to make it more clear and correct.

[lms0806]

This seems to have regressed in #149101. I'm unsure whether the test case that was introduced there was just not reviewed well enough or whether this is even intended. In case it is, I would maybe suggest adding - below x to make it more clear and correct.

The merge appears to have been successful, but there seems to be an issue with the parentheses in the part format!((mut {}) ", binding_name).

[lms0806]

Actually, if I use (mut x), a warning pops up.

• code

for (mut x) in [1, 2, 3] {
    x += 1;
    println!("{x}");
}

• result

warning: unnecessary parentheses around pattern
  --> src\main.rs:68:9
   |
68 |     for (mut x) in [1, 2, 3] {
   |         ^     ^
   |
   = note: `#[warn(unused_parens)]` (part of `#[warn(unused)]`) on by default
help: remove these parentheses
   |
68 -     for (mut x) in [1, 2, 3] {
68 +     for mut x  in [1, 2, 3] {
   |