Arithmetic makes borrow checker overly restrictive

[dotdash]

UPDATE: This was fixed by #47167 but still needs a regression test to be added.

---

fn main() {
    let mut i = [1,2,3];
    i[i[0]] = 0; // Works
    i[i[0] - 1] = 0; // Is rejected
}

I had expect the - 1 not to make any difference, but the borrow checker complains with the following error:

<anon>:4:7: 4:11 error: cannot use `i[..]` because it was mutably borrowed
<anon>:4     i[i[0] - 1] = 0;
               ^~~~
<anon>:4:5: 4:6 note: borrow of `i` occurs here
<anon>:4     i[i[0] - 1] = 0;

[steveklabnik]

@rust-lang/lang, is this behavior expected? I wonder if there's something weird going on here

[nikomatsakis]

Does seem odd. I suspect that the borrow checker is (incorrectly) still locked into the days when indices were passed by reference, rather than by value.

[nikomatsakis]

triage: P-medium

[nikomatsakis]

Likely to be fixed by MIR, though.

[sanxiyn]

Still reproduces.

[nikomatsakis]

(Note: would be fixed by a MIR-based borrowck is what I meant, which is not yet done.)