---

yaml
---
r: 151725
b: refs/heads/try2
c: 9c35ac5
h: refs/heads/master
i:
  151723: d514957
v: v3

Author: Keegan McAllister

[refs]

@@ -5,7 +5,7 @@ refs/heads/snap-stage3: 78a7676898d9f80ab540c6df5d4c9ce35bb50463
 refs/heads/try: 519addf6277dbafccbb4159db4b710c37eaa2ec5
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105
 refs/heads/ndm: f3868061cd7988080c30d6d5bf352a5a5fe2460b
-refs/heads/try2: a0594ebb8baa0544ed0f818c494be20b3c9957a5
+refs/heads/try2: 9c35ac5666b62697bdd197028864fa66127a5415
 refs/heads/dist-snap: ba4081a5a8573875fed17545846f6f6902c8ba8d
 refs/tags/release-0.2: c870d2dffb391e14efb05aa27898f1f6333a9596
 refs/tags/release-0.3: b5f0d0f648d9a6153664837026ba1be43d3e2503

branches/try2/src/libcore/cell.rs

@@ -186,6 +186,25 @@ impl<'b, T> Deref<T> for Ref<'b, T> {
     }
 }
 
+/// Copy a `Ref`.
+///
+/// The `RefCell` is already immutably borrowed, so this cannot fail.
+///
+/// A `Clone` implementation would interfere with the widespread
+/// use of `r.borrow().clone()` to clone the contents of a `RefCell`.
+#[experimental]
+pub fn clone_ref<'b, T>(orig: &Ref<'b, T>) -> Ref<'b, T> {
+    // Since this Ref exists, we know the borrow flag
+    // is not set to WRITING.
+    let borrow = orig.parent.borrow.get();
+    debug_assert!(borrow != WRITING && borrow != UNUSED);
+    orig.parent.borrow.set(borrow + 1);
+
+    Ref {
+        parent: orig.parent,
+    }
+}
+
 /// Wraps a mutable borrowed reference to a value in a `RefCell` box.
 pub struct RefMut<'b, T> {
     parent: &'b RefCell<T>
@@ -307,4 +326,19 @@ mod test {
         let _ = _b;
         let _b = x.borrow_mut();
     }
+
+    #[test]
+    fn clone_ref_updates_flag() {
+        let x = RefCell::new(0);
+        {
+            let b1 = x.borrow();
+            assert!(x.try_borrow_mut().is_none());
+            {
+                let _b2 = clone_ref(&b1);
+                assert!(x.try_borrow_mut().is_none());
+            }
+            assert!(x.try_borrow_mut().is_none());
+        }
+        assert!(x.try_borrow_mut().is_some());
+    }
 }