An unknown external lang item was used. Erroneous code example:
```compile_fail,E0264
#![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();
}
```