"Invalid cast" with vec::iter(fail)

```
fn main() {
    vec::iter(fail) {|i|
        log (error, i * 2);
    };
}
```

Breaks rustc with:

```
Assertion failed: (S->getType()->isPointerTy() && "Invalid cast"), function getPointerCast, file /Users/jruderman/code/rust/src/llvm/lib/VMCore/Constants.cpp, line 1405.
```
