Add Share impl for Rc

library/alloc/src/lib.rs

@@ -144,6 +144,7 @@
 #![feature(ptr_metadata)]
 #![feature(rev_into_inner)]
 #![feature(set_ptr_value)]
+#![feature(share_trait)]
 #![feature(sized_type_properties)]
 #![feature(slice_from_ptr_range)]
 #![feature(slice_index_methods)]

library/alloc/src/rc.rs

@@ -245,7 +245,7 @@
 use core::cell::{Cell, CloneFromCell};
 #[cfg(not(no_global_oom_handling))]
 use core::clone::TrivialClone;
-use core::clone::{CloneToUninit, UseCloned};
+use core::clone::{CloneToUninit, Share, UseCloned};
 use core::cmp::Ordering;
 use core::hash::{Hash, Hasher};
 use core::intrinsics::abort;
@@ -2525,6 +2525,12 @@ fn clone(&self) -> Self {
 #[unstable(feature = "ergonomic_clones", issue = "132290")]
 impl<T: ?Sized, A: Allocator + Clone> UseCloned for Rc<T, A> {}
 
+// FIXME(share_trait): The initial `Share` impl set is still being confirmed in
+// rust-lang/rust#156756. This assumes cloning `Rc` creates a clone-as-alias
+// value because the new handle points to the same allocation.
+#[unstable(feature = "share_trait", issue = "156756")]
+impl<T: ?Sized, A: Allocator + Clone> Share for Rc<T, A> {}
+
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Default> Default for Rc<T> {

tests/ui/share-trait/share-trait-rc.rs

@@ -0,0 +1,32 @@
+//@ run-pass
+
+#![feature(share_trait)]
+
+use std::cell::Cell;
+use std::clone::Share;
+use std::rc::Rc;
+
+trait Value {
+    fn get(&self) -> i32;
+}
+
+impl Value for Cell<i32> {
+    fn get(&self) -> i32 {
+        Cell::get(self)
+    }
+}
+
+fn main() {
+    let value = Rc::new(Cell::new(1));
+    let shared = value.share();
+
+    assert!(Rc::ptr_eq(&value, &shared));
+    shared.set(2);
+    assert_eq!(value.get(), 2);
+
+    let dyn_value: Rc<dyn Value> = Rc::new(Cell::new(3));
+    let shared_dyn_value = dyn_value.share();
+
+    assert!(Rc::ptr_eq(&dyn_value, &shared_dyn_value));
+    assert_eq!(shared_dyn_value.get(), 3);
+}