Error code E0544

Multiple stability attributes were declared on the same item.

Erroneous code example:

#![allow(unused)]
#![feature(staged_api)]
#![stable(since = "1.0.0", feature = "rust1")]

fn main() {
#[stable(feature = "rust1", since = "1.0.0")]
#[stable(feature = "test", since = "2.0.0")] // invalid
fn foo() {}
}

To fix this issue, ensure that each item has at most one stability attribute.

#![allow(unused)]
#![feature(staged_api)]
#![stable(since = "1.0.0", feature = "rust1")]

fn main() {
#[stable(feature = "test", since = "2.0.0")] // ok!
fn foo() {}
}

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.