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+A lang item was redefined.
+
+Erroneous code example:
+
+```compile_fail,E0152
+#![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:
+
+```ignore (only-for-syntax-highlight)
+#![no_std]
+```
+
+See also the [unstable book][1].
+
+[1]: https://doc.rust-lang.org/unstable-book/language-features/lang-items.html#writing-an-executable-without-stdlib