segfault: alt allows uniqueness violation

[paulstansifer]

We shouldn't be able to duplicate a reference to a unique value. But:

let x = some(~1);
alt x {
  some(y) {
    let a <- x;
    let b <- y;
    // disaster ensues, eventually, potentially, distantly
  }
}

The some() isn't even necessary.

[paulstansifer]

This is thematically related to #2329.

[nikomatsakis]

D'oh. This is my fault, liveness or borrowck should prevent moves from bindings---probably borrowck. In any case, this would go away if #2329 were implemented. But it'd be easy to fix in the meantime.

[nikomatsakis]

Actually, I'm a bit surprised that this passes... I should look into this.

[nikomatsakis]

Oh, right, the actual bug here is that borrowck doesn't understand let x <- v.

[nikomatsakis]

this should be fully fixed now.