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)]
#![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)]
#![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.