extern_types associated functions aren't visible from other crates

[crumblingstatue]

crate foo:

#![feature(extern_types)]

extern "C" {
    pub type Foo;
}

impl Foo {
    pub fn new() -> FooBox {
        let raw = unimplemented!("Get through ffi");
        FooBox(raw)
    }
}

pub struct FooBox(*mut Foo);

impl Drop for FooBox {
    fn drop(&mut self) {
        unimplemented!("call ffi drop");
    }
}

crate foouser:

extern crate foo;

fn main() {
    let foo = foo::Foo::new();
    println!("{:p}", &foo);
}

Error:

error[E0599]: no function or associated item named `new` found for type `foo::Foo` in the current scope
 --> foouser.rs:4:15
  |
4 |     let foo = foo::Foo::new();
  |               ^^^^^^^^^^^^^ function or associated item not found in `foo::Foo`

This error does not occur if main is located within the same crate.

[crumblingstatue]

This breaks real life code: jeremyletang/rust-sfml@eb87055

It breaks the various use sites in tests, examples, etc.

And of course, it would break any user code that uses these inherent methods.

[petrochenkov]

Fixed in #77375.