Fix AbiExample for Arc/Rc's Weaks

frozen-abi/src/abi_digester.rs

@@ -691,4 +691,12 @@ mod tests {
             Variant2(u8, u16, #[serde(skip)] u32),
         }
     }
+
+    #[frozen_abi(digest = "B1PcwZdUfGnxaRid9e6ZwkST3NZ2KUEYobA1DkxWrYLP")]
+    #[derive(Serialize, AbiExample)]
+    struct TestArcWeak(std::sync::Weak<u64>);
+
+    #[frozen_abi(digest = "4R8uCLR1BVU1aFgkSaNyKcFD1FeM6rGdsjbJBFpnqx4v")]
+    #[derive(Serialize, AbiExample)]
+    struct TestRcWeak(std::rc::Weak<u64>);
 }

frozen-abi/src/abi_example.rs

@@ -316,10 +316,20 @@ impl<T: AbiExample> AbiExample for std::sync::Arc<T> {
     }
 }
 
+// When T is weakly owned by the likes of `std::{sync, rc}::Weak`s, we need to uphold the ownership
+// of T in some way at least during abi digesting... However, there's no easy way. Stashing them
+// into static is confronted with Send/Sync issue. Stashing them into thread_local is confronted
+// with not enough (T + 'static) lifetime bound..  So, just leak the examples. This should be
+// tolerated, considering ::example() should ever be called inside tests, not in production code...
+fn leak_and_inhibit_drop<'a, T>(t: T) -> &'a mut T {
+    Box::leak(Box::new(t))
+}
+
 impl<T: AbiExample> AbiExample for std::sync::Weak<T> {
     fn example() -> Self {
-        info!("AbiExample for (Weak<T>): {}", type_name::<Self>());
-        std::sync::Arc::downgrade(&std::sync::Arc::new(T::example()))
+        info!("AbiExample for (Arc's Weak<T>): {}", type_name::<Self>());
+        // leaking is needed otherwise Arc::upgrade() will always return None...
+        std::sync::Arc::downgrade(leak_and_inhibit_drop(std::sync::Arc::new(T::example())))
     }
 }
 
@@ -330,6 +340,14 @@ impl<T: AbiExample> AbiExample for std::rc::Rc<T> {
     }
 }
 
+impl<T: AbiExample> AbiExample for std::rc::Weak<T> {
+    fn example() -> Self {
+        info!("AbiExample for (Rc's Weak<T>): {}", type_name::<Self>());
+        // leaking is needed otherwise Rc::upgrade() will always return None...
+        std::rc::Rc::downgrade(leak_and_inhibit_drop(std::rc::Rc::new(T::example())))
+    }
+}
+
 impl<T: AbiExample> AbiExample for std::sync::Mutex<T> {
     fn example() -> Self {
         info!("AbiExample for (Mutex<T>): {}", type_name::<Self>());