A stability attribute has been used outside of the standard library.
Erroneous code examples:
#[rustc_deprecated(since = "b", reason = "text")] // invalid #[stable(feature = "a", since = "b")] // invalid #[unstable(feature = "b", issue = "0")] // invalid fn foo(){}
These attributes are meant to only be used by the standard library and are rejected in your own crates.