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

Erroneous code example:

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

#[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:

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

#[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.