mut in slice pattern: `[_, mut @ ..] = &[...]` is not needed?

Might be related to https://github.com/rust-lang/rust/issues/123076

### Code

```Rust
fn foo(s: &[usize]) {
    let [a, b @ ..] = s else {panic!()};
    let [a, mut b @ ..] = s else {panic!()};
}
```


### Current output

```Shell
error[E0277]: the size for values of type `[usize]` cannot be known at compilation time
 --> src/lib.rs:3:13
  |
3 |     let [a, mut b @ ..] = s else {panic!()};
  |             ^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `[usize]`
  = note: all local variables must have a statically known size
  = help: unsized locals are gated as an unstable feature

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (lib) due to 1 previous error
```


### Desired output

```Shell
3 |     let [a, mut b @ ..] = s else {panic!()};
  |             ^^^^^^^^^^ doesn't have a size known at compile-time
                ^^^ help: remove this `mut` because ...
```


### Rationale and extra context

I use `mut b @ ..` because I would like to change b later.
But removing `mut` makes it compile. So my thoughts are misleading.

### Other cases

_No response_

### Rust Version

```Shell
Nightly version: 1.81.0-nightly

(2024-06-18 8fcd4dd08e2ba3e922d9)
```


### Anything else?

_No response_

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mut in slice pattern: `[_, mut @ ..] = &[...]` is not needed? #126676

faptc
openedon Jun 19, 2024

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mut in slice pattern: [_, mut @ ..] = &[...] is not needed? #126676

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faptcopenedon Jun 19, 2024