From 0bc9c729b3a3ef7e87f0828c79f32065fc63cab5 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Mon, 7 May 2018 10:40:00 -0700
Subject: [PATCH] Add impls for Weak

---
 serde/src/de/impls.rs  | 38 ++++++++++++++++++++++++++++++++++++++
 serde/src/lib.rs       |  8 ++++----
 serde/src/ser/impls.rs | 34 ++++++++++++++++++++++++++++++++++
 3 files changed, 76 insertions(+), 4 deletions(-)

diff --git a/serde/src/de/impls.rs b/serde/src/de/impls.rs
index 5a53a418..aa4ede6c 100644
--- a/serde/src/de/impls.rs
+++ b/serde/src/de/impls.rs
@@ -1474,6 +1474,44 @@ where
 
 ////////////////////////////////////////////////////////////////////////////////
 
+/// This impl requires the [`"rc"`] Cargo feature of Serde. The resulting
+/// `Weak<T>` has a reference count of 0 and cannot be upgraded.
+///
+/// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+#[cfg(all(feature = "rc", any(feature = "std", feature = "alloc")))]
+impl<'de, T: ?Sized> Deserialize<'de> for RcWeak<T>
+where
+    T: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        try!(Option::<T>::deserialize(deserializer));
+        Ok(RcWeak::new())
+    }
+}
+
+/// This impl requires the [`"rc"`] Cargo feature of Serde. The resulting
+/// `Weak<T>` has a reference count of 0 and cannot be upgraded.
+///
+/// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+#[cfg(all(feature = "rc", any(feature = "std", feature = "alloc")))]
+impl<'de, T: ?Sized> Deserialize<'de> for ArcWeak<T>
+where
+    T: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        try!(Option::<T>::deserialize(deserializer));
+        Ok(ArcWeak::new())
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
 #[cfg(all(feature = "unstable", feature = "rc", any(feature = "std", feature = "alloc")))]
 macro_rules! box_forwarded_impl {
     (
diff --git a/serde/src/lib.rs b/serde/src/lib.rs
index 923b7596..ce0c3326 100644
--- a/serde/src/lib.rs
+++ b/serde/src/lib.rs
@@ -179,14 +179,14 @@ mod lib {
     pub use std::boxed::Box;
 
     #[cfg(all(feature = "rc", feature = "alloc", not(feature = "std")))]
-    pub use alloc::rc::Rc;
+    pub use alloc::rc::{Rc, Weak as RcWeak};
     #[cfg(all(feature = "rc", feature = "std"))]
-    pub use std::rc::Rc;
+    pub use std::rc::{Rc, Weak as RcWeak};
 
     #[cfg(all(feature = "rc", feature = "alloc", not(feature = "std")))]
-    pub use alloc::arc::Arc;
+    pub use alloc::arc::{Arc, Weak as ArcWeak};
     #[cfg(all(feature = "rc", feature = "std"))]
-    pub use std::sync::Arc;
+    pub use std::sync::{Arc, Weak as ArcWeak};
 
     #[cfg(all(feature = "alloc", not(feature = "std")))]
     pub use alloc::{BTreeMap, BTreeSet, BinaryHeap, LinkedList, VecDeque};
diff --git a/serde/src/ser/impls.rs b/serde/src/ser/impls.rs
index f2de2822..fbe057cd 100644
--- a/serde/src/ser/impls.rs
+++ b/serde/src/ser/impls.rs
@@ -374,6 +374,40 @@ deref_impl!(<'a, T: ?Sized> Serialize for Cow<'a, T> where T: Serialize + ToOwne
 
 ////////////////////////////////////////////////////////////////////////////////
 
+/// This impl requires the [`"rc"`] Cargo feature of Serde.
+///
+/// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+#[cfg(all(feature = "rc", any(feature = "std", feature = "alloc")))]
+impl<T: ?Sized> Serialize for RcWeak<T>
+where
+    T: Serialize,
+{
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        self.upgrade().serialize(serializer)
+    }
+}
+
+/// This impl requires the [`"rc"`] Cargo feature of Serde.
+///
+/// [`"rc"`]: https://serde.rs/feature-flags.html#-features-rc
+#[cfg(all(feature = "rc", any(feature = "std", feature = "alloc")))]
+impl<T: ?Sized> Serialize for ArcWeak<T>
+where
+    T: Serialize,
+{
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        self.upgrade().serialize(serializer)
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
 #[cfg(feature = "unstable")]
 #[allow(deprecated)]
 impl<T> Serialize for NonZero<T>