Fix explicit deref problems in closure capture

[HKalbasi]

fix the need-mut part of #14562

Perhaps surprisingly, it wasn't unique immutable borrow. The code still doesn't emit any of them, and I think those won't happen in edition 2021 (which is currently the only thing implemented), since we always capture &mut *x instead of &mut x. But I'm not very sure about it.

[HKalbasi]

I'm going to merge this since it is a problem anyway.

If someone find a case that we need unique immutable borrow in edition 2021, it would be highly appreciated.

@bors r+

[bors]

📌 Commit 7cb4318 has been approved by HKalbasi

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7cb4318 with merge b218009...

[bors]

☀️ Test successful - checks-actions
Approved by: HKalbasi
Pushing b218009 to master...

[Veykril]

&mut *x is not a capture, you can only capture by value or by reference. This is very much a by unique immutable reference capture here, since x is not marked mut a by mutable reference capture is incorrect as is outlined in the rust reference. The edition is not relevant here either. Though it might not be significant for our purposes whether this is modeled as such or not.

[HKalbasi]

Since edition 2021, each place can be captured, so a closure can capture x, ***x, x.foo, ... each with 4 capture mode (by ref, by mut, by shallow and by unique) but in older editions only x can be captured with those capture modes.

For example in this test

size_and_align_expr! {
    minicore: copy, deref_mut;
    stmts: [
        struct X(i32, i64);
        let x: &mut X = &mut X(2, 6);
    ]
    || {
        (*x).0 = x.1 as i64;
    }
}

In edition 2021, captures are (*x).0 by mutable borrow and (*x).1, by shared borrow, but in older editions the only capture is x by unique immutable borrow.