A lang item was redefined.
Erroneous code example:
#![feature(lang_items)]
#[lang = "owned_box"]
struct Foo<T>(T); // error: duplicate lang item found: `owned_box`Lang items are already implemented in the standard library. Unless you are writing a free-standing application (e.g., a kernel), you do not need to provide them yourself.
You can build a free-standing crate by adding #![no_std] to the crate
attributes:
#![no_std]See also the unstable book.