An unknown external lang item was used. Erroneous code example:

#![feature(lang_items)]

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/weak_lang_items.rs. Example:

#![feature(lang_items)]

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