An unknown external lang item was used.

Erroneous code example:

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

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

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

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

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