error[E0658]: use of unstable library feature 'unicode_internals' --> $DIR/accidental-stable-in-unstable.rs:6:5 | LL | use core::unicode::UNICODE_VERSION; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | = help: add `#![feature(unicode_internals)]` to the crate attributes to enable error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0658`.