error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
  --> $DIR/rustc-private.rs:3:1
   |
LL | extern crate libc;
   | ^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #27812 <https://github.com/rust-lang/rust/issues/27812> for more information
   = help: add `#![feature(rustc_private)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0658`.