extern crate foo with use self::foo doesn't work inside a function

EDIT (@steveklabnik) see https://github.com/rust-lang/rust/issues/27626#issuecomment-263955318

This code fails and rustc doesn't tell you why:

``` rust
fn main() {
    extern crate rand;
    use self::rand::Rng;
}
```

([playpen](http://is.gd/xYyadr))

I think it should at least warn, assuming there is some reason why it can't do what it looks like it does.
