Compound assignment (`+=`) on uninitialized variables is allowed

[ben0x539]

This compiles:

fn main() {
    let i: int;
    i += 2;
}

Now, I can't seem to get that anything reads the value again to compile, but it seems like bad times anyway.

Thought to try this because of #12452.

[flaper87]

cc @flaper87

[nikomatsakis]

cc me

[edwardw]

If this were to be fixed as use of possibly uninitialized variable, it will void the following two tests from compile-fail/liveness-unused.rs:

fn f3() {
    let mut x = 3;
    //~^ ERROR variable `x` is assigned to, but never used
    x += 4;
    //~^ ERROR value assigned to `x` is never read
}

fn f3b() {
    let mut z = 3;
    //~^ ERROR variable `z` is assigned to, but never used
    loop {
        z += 4;
    }
}

But it makes sense, doesn't it?

[nikomatsakis]

@edwardw actually, that analysis (that gives those warnings) is completely separate, and I remember it drew some subtle distinctions regarding assignment vs use etc

[edwardw]

Very subtle indeed. I think I find the right compromise.

[nikomatsakis]

No, this is the wrong approach. liveness.rs shouldn't be responsible for reporting errors anymore. I thought I had removed the span_err but I guess I was wrong about that! Anyway, I believe there should be changes to borrowck.

[nikomatsakis]

Well, let me review the code, but I think liveness should just have the job of warnings (in general I'd like to be able to rm liveness.rs without affecting correctness of the compiler).

[edwardw]

Whoops, the patch has been half way through bors.

[nikomatsakis]

@edwardw just to be clear, the patch you made is pretty reasonable, it's just that I've been moving responsibility for these sorts of checks to the borrow checker, which is probably not clear from the code. I think I didn't do a good job of finishing the job of simplifying liveness.

Anyway the problem there is that the borrow checker classifies all expressions into assignees or uses, but this is both, so we need to change the move data to have a 3-way classification.

It'd also be nice to remove all span_err's from liveness and make sure that my assertion that liveness is purely about warnings is actually true. Maybe there are other oversights in the borrow checker!

[edwardw]

I see. Let's see what I can do about that.