Error code E0264

An unknown external lang item was used.

Erroneous code example:

#![allow(unused)]
#![feature(lang_items)]

fn main() {
extern "C" {
    #[lang = "cake"] // error: unknown external lang item: `cake`
    fn cake();
}
}

A list of available external lang items is available in src/librustc_middle/middle/weak_lang_items.rs. Example:

#![allow(unused)]
#![feature(lang_items)]

fn main() {
extern "C" {
    #[lang = "panic_impl"] // ok!
    fn cake();
}
}