`#[lang_item]` for `core::ptr::Unique` Tree Borrows is about to introduce experimental special handling of `core::ptr::Unique` in Miri to give it a semantics. As of now there does not seem to be a clean way (i.e. other than `&format!("{adt:?}") == "std::ptr::Unique"`) to check if an `AdtDef` represents a `Unique`. r? `@RalfJung` Draft: making a lang item