The lang attribute was used in an invalid context.
Erroneous code example:
#![feature(lang_items)]
#[lang = "cookie"]
fn cookie() -> ! { // error: definition of an unknown language item: `cookie`
loop {}
}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).