Bad parse error when omitting struct fields in constructor expression

``` rust
struct Foo {
    f: uint
}

fn foo() -> Foo {
    Foo {
        123,
    }
}
```

```
foo.rs:6:8: 6:9 error: expected `;` or `}` after expression but found `{`
foo.rs:6     Foo {
                 ^
```

I would expect something much more... helpful. (Yes, I'm tired and the bug is obvious, but it's still a silly error message!)
