A lang item was redefined.
Erroneous code example:
#![feature(lang_items)] #[lang = "arc"] struct Foo; // error: duplicate lang item found: `arc`
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.