Rollup merge of rust-lang#112662 - Vanille-N:symbol_unique, r=RalfJung

`#[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

Author: compiler-errors

core/src/ptr/unique.rs

@@ -32,6 +32,8 @@ use crate::ptr::NonNull;
 )]
 #[doc(hidden)]
 #[repr(transparent)]
+// Lang item used experimentally by Miri to define the semantics of `Unique`.
+#[cfg_attr(not(bootstrap), lang = "ptr_unique")]
 pub struct Unique<T: ?Sized> {
     pointer: NonNull<T>,
     // NOTE: this marker has no consequences for variance, but is necessary