# Error code E0547

The `issue` value is missing in a stability attribute.

Erroneous code example:

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

    fn main() {
    #[unstable(feature = "_unstable_fn")] // invalid
    fn _unstable_fn() {}

    #[rustc_const_unstable(feature = "_unstable_const_fn")] // invalid
    const fn _unstable_const_fn() {}
    }

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

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

    fn main() {
    #[unstable(feature = "_unstable_fn", issue = "none")] // ok!
    fn _unstable_fn() {}

    #[rustc_const_unstable(
        feature = "_unstable_const_fn",
        issue = "none"
    )] // ok!
    const fn _unstable_const_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.