# 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.