The feature value is missing in a stability attribute.
Erroneous code example:
#![feature(staged_api)]
#![allow(internal_features)]
#![stable(since = "1.0.0", feature = "test")]
#[unstable(issue = "none")] // invalid
fn unstable_fn() {}
#[stable(since = "1.0.0")] // invalid
fn stable_fn() {}
To fix this issue, you need to provide the feature field. Example:
#![feature(staged_api)]
#![allow(internal_features)]
#![stable(since = "1.0.0", feature = "test")]
#[unstable(feature = "unstable_fn", issue = "none")] // ok!
fn unstable_fn() {}
#[stable(feature = "stable_fn", since = "1.0.0")] // ok!
fn stable_fn() {}
See the How Rust is Made and “Nightly Rust” appendix of the Book and the Stability attributes section of the Rustc Dev Guide for more details.