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# Error code E0549

A `deprecated` attribute wasn't paired with a `stable`/`unstable` attribute with `#![feature(staged_api)]` enabled.

Erroneous code example:

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

    fn main() {
    #[deprecated(
        since = "1.0.1",
        note = "explanation for deprecation"
    )] // invalid
    fn _deprecated_fn() {}
    }

To fix this issue, you need to add also an attribute `stable` or `unstable`. Example:

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

    fn main() {
    #[stable(since = "1.0.0", feature = "test")]
    #[deprecated(
        since = "1.0.1",
        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.