Introduce CoerceShared lang item and trait

compiler/rustc_hir/src/lang_items.rs

@@ -441,6 +441,7 @@ language_item_table! {
 
     // Reborrowing related lang-items
     Reborrow,                sym::reborrow,            reborrow,                   Target::Trait,          GenericRequirement::Exact(0);
+    CoerceShared,            sym::coerce_shared,       coerce_shared,              Target::Trait,          GenericRequirement::Exact(0);
 }
 
 /// The requirement imposed on the generics of a lang item

compiler/rustc_span/src/symbol.rs

@@ -679,6 +679,7 @@ symbols! {
         cmpxchg16b_target_feature,
         cmse_nonsecure_entry,
         coerce_pointee_validated,
+        coerce_shared,
         coerce_unsized,
         cold,
         cold_path,

library/core/src/marker.rs

@@ -1372,3 +1372,12 @@ pub trait CoercePointeeValidated {
 pub trait Reborrow {
     // Empty.
 }
+
+/// Allows reborrowable value to be reborrowed as shared, creating a copy of
+/// that disables the source for writes for the lifetime of the copy.
+#[lang = "coerce_shared"]
+#[unstable(feature = "reborrow", issue = "145612")]
+pub trait CoerceShared: Reborrow {
+    /// The type of this value when reborrowed as shared.
+    type Target: Copy;
+}