Error code E0546

The feature value is missing in a stability attribute.

Erroneous code example:

#![allow(unused)]
#![feature(staged_api)]
#![stable(since = "1.0.0", feature = "test")]

fn main() {
#[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:

#![allow(unused)]
#![feature(staged_api)]
#![stable(since = "1.0.0", feature = "test")]

fn main() {
#[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.