Using crates.io libc triggers unstable libc warning

[SSheldon]

With the crates.io distribution of libc (#20606) in my Cargo.toml:

[dependencies]
libc = "0.1"

I still get a warning on extern crate libc:

lib.rs:6:1: 6:19 warning: use of unstable library feature 'libc'
lib.rs:6 extern crate libc;
         ^~~~~~~~~~~~~~~~~~
lib.rs:6:1: 6:19 help: add #![feature(libc)] to the crate attributes to silence this warning
lib.rs:6 extern crate libc;
         ^~~~~~~~~~~~~~~~~~

Am I linking to the wrong crate? How do you choose the version from cargo over the version included in rustc?

[nagisa]

A rule of thumb is to always use the version shipped with the compiler unless you know you need the version from crates.io (e.g. crossbuilding).

The warning is expected since libc in crate is the same thing that is shipped with the rust compiler.

[SSheldon]

@nagisa, #19260 says of libc:

These crates will all be distributed, but participate in #![staged_api]. This means they will not be accessible in the stable channel's standard distribution, but they will all be available through crates.io.

So that suggests to me that the version shipped with the compiler shouldn't be used.

[SSheldon]

cc @alexcrichton, who has worked on the libc for crates.io

[alexcrichton]

Working on a fix, thanks for the report! I've also seen this recently and I've been baffled.

[tbu-]

@nagisa You shouldn't work with the explicitely-deprecated compiler-shipped libraries.

[stevedonovan]

I note that this is still the case for 1.12 stable, which seems alarming...

[steveklabnik]

@stevedonovan can you share your exact code? I can't reproduce.

[stevedonovan]

Sorry, I forgot to actually add libc to my crate that imported a crate needing libc, hence the issue! That makes sense, so I'm happy.