Aliasing rules are broken for closures

[kvark]

This compiles and runs:

let mut v = vec![0i];
let f = |i: uint| v.get_mut(i);
let x = f(0);
let y = f(0);
(*x,*y)

x and y are not supposed to live at the same time. Results in an UB (in a more complex case)
Rust:

rustc 0.12.0-nightly (af3889f 2014-09-18 21:20:38 +0000)

[kvark]

Initially discovered in rust-voxel. CC @aepsil0n

[alexcrichton]

Nominating.

[thestinger]

Aren't the old closures going to be removed as part of implementing unboxed closures? Unless the problem exists for regular types / trait objects, I think it will solve itself.

[nikomatsakis]

cc me

[nikomatsakis]

I suspect this is related to how we handle lifetimes in closures, which is one of the areas where the code doesn't properly treat closures in the desugared way we'd like to. This is good because I've been looking for breakage that results from this, so I'm glad to have some concrete examples.

In particular I suspect the problem is that we currently consider the closure region bound to be its enclosing expression, which is patently wrong (#3696). There is a better algorithm for handling this described in a comment but I haven't gotten around to implementing it. I was waiting to look into this until the dust form @pcwalton's SEME region work settled, which seems to have happened. So now is the time, I guess. :)

[pnkfelix]

Assigning P-backcompat-lang, 1.0.