Improve borrow checking

[heckad]

I tried this code:

fn main() {
    let mut v = vec![1, 2, 3];

    let head = {
        std::mem::replace(&mut v, v.split_off(1))
    };
}

I expected to see this happen: all good.

Instead, this happened: error: cannot borrow v as mutable more than once at a time

If I rewrite this code onto

fn main() {
    let mut v = vec![1, 2, 3];

    let head = {
        let t = v.split_off(1);
        std::mem::replace(&mut v, t)
    };
}

then all be nice.

[jonas-schievink]

This is covered by the work on generalized two-phase borrows, which is tracked in #49434. Possibly an RFC amendment would be required to make this case work, since I'm not sure if that is covered by the existing design work.