`extern crate` is always treated as `pub` within the crate, but is not visible outside it; `pub extern crate` makes no difference

[brson]

Two examples:

#![feature(core)]
#![crate_type = "lib"]
pub extern crate core;

That is accepted but core cannot be accessed from outside the crate.

#![feature(core)]
#![crate_type = "lib"]

mod foo {
    extern crate core;
    // behaves identically
    // pub extern crate core;
}

pub use foo::core;

That is accepted and reexports core.

In other words, extern crate is always public to other modules, but can never be made public to other crates.

There are no tests for pub extern crate so I suspect this is not intentional.

[eddyb]

Also see #21757.

[nikomatsakis]

triage: P-high

We should nail this down. I'm assigning this to @nrc since iirc he last touched the privacy code. ;)

[pnkfelix]

(any reason we can't just ... make pub extern crate a parse error ... ?)

Update: (with a warning cycle in the release series, of course)

Update 2: Hmm, I guess I missed the point that it can also be reexported, so making pub extern crate illegal is probably not the right call. (that plus RFC 385 explicitly said it was making pub extern crate legal.)

[brson]

@pnkfelix Making it illegal might still be a reasonable stop-gap if it looks like we won't fix the problem for a while.