Introduce Reborrow lang item and trait

Author: aapoalas

compiler/rustc_feature/src/unstable.rs

@@ -614,7 +614,7 @@ declare_features! (
     (unstable, proc_macro_hygiene, "1.30.0", Some(54727)),
     /// Allows the use of raw-dylibs on ELF platforms
     (incomplete, raw_dylib_elf, "1.87.0", Some(135694)),
-    (unstable, reborrow, "CURRENT_RUSTC_VERSION", None),
+    (unstable, reborrow, "CURRENT_RUSTC_VERSION", Some(145612)),
     /// Makes `&` and `&mut` patterns eat only one layer of references in Rust 2024.
     (incomplete, ref_pat_eat_one_layer_2024, "1.79.0", Some(123076)),
     /// Makes `&` and `&mut` patterns eat only one layer of references in Rust 2024—structural variant

compiler/rustc_hir/src/lang_items.rs

@@ -437,6 +437,9 @@ language_item_table! {
     DefaultTrait1,           sym::default_trait1,      default_trait1_trait,       Target::Trait,          GenericRequirement::None;
 
     ContractCheckEnsures,     sym::contract_check_ensures,      contract_check_ensures_fn,      Target::Fn, GenericRequirement::None;
+
+    // Reborrowing related lang-items
+    Reborrow,                sym::reborrow,            reborrow,                   Target::Trait,          GenericRequirement::Exact(0);
 }
 
 /// The requirement imposed on the generics of a lang item

compiler/rustc_span/src/symbol.rs

@@ -1754,6 +1754,7 @@ symbols! {
         readonly,
         realloc,
         reason,
+        reborrow,
         receiver,
         receiver_target,
         recursion_limit,

library/core/src/lib.rs

@@ -172,6 +172,7 @@
 #![feature(no_core)]
 #![feature(optimize_attribute)]
 #![feature(prelude_import)]
+#![feature(reborrow)]
 #![feature(repr_simd)]
 #![feature(rustc_allow_const_fn_unstable)]
 #![feature(rustc_attrs)]

library/core/src/marker.rs

@@ -1365,3 +1365,11 @@ pub macro CoercePointee($item:item) {
 pub trait CoercePointeeValidated {
     /* compiler built-in */
 }
+
+/// Allows value to be reborrowed as exclusive, creating a copy of the value
+/// that disables the source for reads and writes for the lifetime of the copy.
+#[lang = "reborrow"]
+#[unstable(feature = "reborrow", issue = "145612")]
+pub trait Reborrow {
+    // Empty.
+}