Bug: disjoint borrows in a slice

[Nadrieril]

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This is valid Rust code:

fn disjoint_borrows_in_slice() {
    struct Test {
        a: i32,
        b: i32,
    }

    let inputs: &mut [_] = &mut [Test { a: 0, b: 0 }];
    let a = &mut inputs[0].a;
    let b = &mut inputs[0].b;

    *a = 0;
    *b = 1;
}

Aeneas can't support this today because we pretend that slice indexing is done through a function call. Rust actually considers indexing slices/arrays to be a built-in projection and therefore considers these borrows disjoint.