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.