spurious "unused extern crate"

[RalfJung]

When compiling this code

#![feature(libc, extern_crate_item_prelude)]

extern crate libc;

use std::ffi::CString;

mod mlibc {
    use libc::{c_char, size_t};
    extern {
        #[link_name = "strlen"]
        pub fn my_strlen(str: *const c_char) -> size_t;
    }
}

fn strlen(str: String) -> usize {
    // C string is terminated with a zero
    let s = CString::new(str).unwrap();
    unsafe {
        mlibc::my_strlen(s.as_ptr()) as usize
    }
}

pub fn main() {
    let len = strlen("Rust".to_string());
    assert_eq!(len, 4);
}

with --edition 2018 -Wunused, I get an incorrect warning:

warning: unused extern crate
  --> tests/run-pass-fullmir/foreign-fn-linkname.rs:15:1
   |
15 | extern crate libc;
   | ^^^^^^^^^^^^^^^^^^ help: remove it
   |
   = note: `-W unused-extern-crates` implied by `-W unused`

The extern crate is used; removing it breaks the program.

[petrochenkov]

Have you checked this on nightly?
This looks like a duplicate of #56038 and it was fixed a couple of days ago.

[RalfJung]

Yes, this is with

rustc 1.32.0-nightly (5aff30734 2018-11-19)

[RalfJung]

Note that I need to add -Wunused. No warning is shown without that flag.

[Mark-Simulacrum]

I can reproduce on playground with #![warn(unused)] as well: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=f042e0d802065bb01c52be2d274f00b6

[RalfJung]

I cannot reproduce the warning any more, it seems to be fixed.

However, I am not sure if there is a testcase for that.

[kadiwa4]

@rustbot label: +E-needs-test
Test could be added to tests/ui/foreign/foreign-fn-linkname.rs (which is the example above).
The #[allow(unused_extern_crates)] in std can probably also be removed

[Suyashtnt]

@rustbot claim

[Noratrieb]

Unassigning because they told me that they don't have the time to do this.

[apekros]

@rustbot claim