Error code E0734

A stability attribute has been used outside of the standard library.

Erroneous code example:

#![allow(unused)]
fn main() {
#[stable(feature = "a", since = "b")] // invalid
#[unstable(feature = "b", issue = "none")] // invalid
fn foo(){}
}

These attributes are meant to only be used by the standard library and are rejected in your own crates.