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 for more information = help: add `#![feature(rustc_private)]` to the crate attributes to enable error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0658`.