Error code E0543

The note value is missing in a stability attribute.

Erroneous code example:

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

fn main() {
#[stable(since = "0.1.0", feature = "_deprecated_fn")]
#[deprecated(
    since = "1.0.0"
)] // invalid
fn _deprecated_fn() {}
}

To fix this issue, you need to provide the note field. Example:

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

fn main() {
#[stable(since = "0.1.0", feature = "_deprecated_fn")]
#[deprecated(
    since = "1.0.0",
    note = "explanation for deprecation"
)] // ok!
fn _deprecated_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.