Can't 'use' from an 'extern crate' defined in a function

[brson]

I never realized this before:

fn main() {
    extern crate core;

    // Can't write this because 'self' refers to the outer module.
    use self::core::str;
}

Seems pretty strange.

[shepmaster]

I believe this to be the root cause of #27301.

A doc test with

/// ```
/// extern crate bignum;
/// use bignum::inits::Zero;
///
/// let a = bignum::BigNum::new(Zero::zero());
/// ```

becomes

fn main() {
    extern crate bignum;
    use bignum::inits::Zero;

    let a = bignum::BigNum::new(Zero::zero());
}

Which then suggests to use self::bignum::..., but that also fails to work due to this issue. It then suggests to use self::self::bignum::..., which is crazier.

[steveklabnik]

Triage: this is rust-lang/rfcs#959, and we'd need an RFC to change this stuff, so I'm going to give it a close.

[shepmaster]

@steveklabnik do you want to close #27301 as well then?

[steveklabnik]

Let's leave it open for now; I believe we could fix this by changing where the fn main is inserted, no?