error[E0658]: use of unstable library feature 'format_args_nl': `format_args_nl` is only for internal language use and is subject to change --> $DIR/feature-gate-format_args_nl.rs:2:5 | LL | format_args_nl!(""); | ^^^^^^^^^^^^^^ | = help: add `#![feature(format_args_nl)]` to the crate attributes to enable error: aborting due to previous error For more information about this error, try `rustc --explain E0658`.