The since value is missing in a stability attribute.

Erroneous code example:

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

#[stable(feature = "_stable_fn")] // invalid
fn _stable_fn() {}

#[rustc_const_stable(feature = "_stable_const_fn")] // invalid
const fn _stable_const_fn() {}

#[stable(feature = "_deprecated_fn", since = "0.1.0")]
#[deprecated(
    note = "explanation for deprecation"
)] // invalid
fn _deprecated_fn() {}

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

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

#[stable(feature = "_stable_fn", since = "1.0.0")] // ok!
fn _stable_fn() {}

#[stable(feature = "_stable_const_fn", since = "1.0.0")]
#[rustc_const_stable(feature = "_stable_const_fn", since = "1.0.0")] // ok!
const fn _stable_const_fn() {}

#[stable(feature = "_deprecated_fn", since = "0.1.0")]
#[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.