Inner function cannot be tested

I really would like to be able to this:

``` rust
fn outer(x: i32) -> i32 {
    fn inner(a: i32, b: i32) -> i32 {
        a + b
    }
    #[test]
    fn test_inner() {
        assert_eq!(inner(3, 5), 8);
    }
    inner(x, 5)
}

#[test]
fn test_outer() {
    assert_eq!(outer(3), 8);
}
```
