1 files changed, 50 insertions, 0 deletions
diff --git a/third_party/rust/objc/src/rc/weak.rs b/third_party/rust/objc/src/rc/weak.rs
new file mode 100644
index 0000000000..289bd8dedb
--- /dev/null
+++ b/third_party/rust/objc/src/rc/weak.rs
@@ -0,0 +1,50 @@
+use std::cell::UnsafeCell;
+use std::ptr;
+
+use runtime::{Object, self};
+use super::StrongPtr;
+
+// Our pointer must have the same address even if we are moved, so Box it.
+// Although loading the WeakPtr may modify the pointer, it is thread safe,
+// so we must use an UnsafeCell to get a *mut without self being mutable.
+
+/// A pointer that weakly references an object, allowing to safely check
+/// whether it has been deallocated.
+pub struct WeakPtr(Box<UnsafeCell<*mut Object>>);
+
+impl WeakPtr {
+    /// Constructs a `WeakPtr` to the given object.
+    /// Unsafe because the caller must ensure the given object pointer is valid.
+    pub unsafe fn new(obj: *mut Object) -> Self {
+        let ptr = Box::new(UnsafeCell::new(ptr::null_mut()));
+        runtime::objc_initWeak(ptr.get(), obj);
+        WeakPtr(ptr)
+    }
+
+    /// Loads the object self points to, returning a `StrongPtr`.
+    /// If the object has been deallocated, the returned pointer will be null.
+    pub fn load(&self) -> StrongPtr {
+        unsafe {
+            let ptr = runtime::objc_loadWeakRetained(self.0.get());
+            StrongPtr::new(ptr)
+        }
+    }
+}
+
+impl Drop for WeakPtr {
+    fn drop(&mut self) {
+        unsafe {
+            runtime::objc_destroyWeak(self.0.get());
+        }
+    }
+}
+
+impl Clone for WeakPtr {
+    fn clone(&self) -> Self {
+        let ptr = Box::new(UnsafeCell::new(ptr::null_mut()));
+        unsafe {
+            runtime::objc_copyWeak(ptr.get(), self.0.get());
+        }
+        WeakPtr(ptr)
+    }
+}