Rollup merge of #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

compiler/rustc_hir/src/lang_items.rs

@@ -260,6 +260,8 @@ language_item_table! {
     EhCatchTypeinfo,         sym::eh_catch_typeinfo,   eh_catch_typeinfo,          Target::Static,         GenericRequirement::None;
 
     OwnedBox,                sym::owned_box,           owned_box,                  Target::Struct,         GenericRequirement::Minimum(1);
+    // Experimental language item for Miri
+    PtrUnique,               sym::ptr_unique,          ptr_unique,                 Target::Struct,         GenericRequirement::Exact(1);
 
     PhantomData,             sym::phantom_data,        phantom_data,               Target::Struct,         GenericRequirement::Exact(1);
 

compiler/rustc_span/src/symbol.rs

@@ -1156,6 +1156,7 @@ symbols! {
         ptr_null_mut,
         ptr_offset_from,
         ptr_offset_from_unsigned,
+        ptr_unique,
         pub_macro_rules,
         pub_restricted,
         public,

library/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