From 8bf4651e2cbe04511c7e92279073e46e33d3b0fc Mon Sep 17 00:00:00 2001
From: Remi Rampin <remirampin@gmail.com>
Date: Wed, 1 Jul 2015 15:51:17 -0400
Subject: [PATCH 1/3] Implement CoerceUnsized for rc::Weak

Fixes #26704
---
 src/liballoc/rc.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 3dfafd0a378b..d461eeea0b7e 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -734,6 +734,8 @@ pub struct Weak<T: ?Sized> {
 impl<T: ?Sized> !marker::Send for Weak<T> {}
 impl<T: ?Sized> !marker::Sync for Weak<T> {}
 
+impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
+
 #[unstable(feature = "rc_weak",
            reason = "Weak pointers may not belong in this module.")]
 impl<T: ?Sized> Weak<T> {

From 50421827ae6bff8085e499eae1427e6c037d76d1 Mon Sep 17 00:00:00 2001
From: Remi Rampin <remirampin@gmail.com>
Date: Wed, 1 Jul 2015 17:54:54 -0400
Subject: [PATCH 2/3] Add test to dst-coerce-rc.rs

---
 src/test/run-pass/dst-coerce-rc.rs | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/test/run-pass/dst-coerce-rc.rs b/src/test/run-pass/dst-coerce-rc.rs
index 67dd4021cb5c..f147a2dca634 100644
--- a/src/test/run-pass/dst-coerce-rc.rs
+++ b/src/test/run-pass/dst-coerce-rc.rs
@@ -10,10 +10,10 @@
 
 // Test a very simple custom DST coercion.
 
-#![feature(core)]
+#![feature(core, rc_weak)]
 
 use std::cell::RefCell;
-use std::rc::Rc;
+use std::rc::{Rc, Weak};
 
 trait Baz {
     fn get(&self) -> i32;
@@ -36,9 +36,13 @@ fn main() {
     let b: Rc<Baz> = a.clone();
     assert_eq!(b.get(), 42);
 
+    let c: Weak<i32> = a.downgrade();
+    let d: Weak<Baz> = c.clone();
+
     let _c = b.clone();
 
     let a: Rc<RefCell<i32>> = Rc::new(RefCell::new(42));
     let b: Rc<RefCell<Baz>> = a.clone();
     assert_eq!(b.borrow().get(), 42);
+    let c: Weak<RefCell<Baz>> = a.downgrade();
 }

From 3278e793b2d0332c6659b732178892aab11654ee Mon Sep 17 00:00:00 2001
From: Remi Rampin <remirampin@gmail.com>
Date: Wed, 1 Jul 2015 17:58:05 -0400
Subject: [PATCH 3/3] Implement CoerceUnsized for arc::Weak

---
 src/liballoc/arc.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/liballoc/arc.rs b/src/liballoc/arc.rs
index dd9c1d1fd188..c84649f92e7f 100644
--- a/src/liballoc/arc.rs
+++ b/src/liballoc/arc.rs
@@ -145,6 +145,8 @@ pub struct Weak<T: ?Sized> {
 unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> { }
 unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> { }
 
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {