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/middle/weak_lang_items.rs`. Example:

```
#![feature(lang_items)]

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