The lang attribute is intended for marking special items that are built-in to
Rust itself. This includes special traits (like `Copy` and `Sized`) that affect
how the compiler behaves, as well as special functions that may be automatically
invoked (such as the handler for out-of-bounds accesses when indexing a slice).
Erroneous code example:

```compile_fail,E0522
#![feature(lang_items)]

#[lang = "cookie"]
fn cookie() -> ! { // error: definition of an unknown language item: `cookie`
    loop {}
}
```
