sync: arc: Implement write() and assume_init() for UniqueArc

Implementation of write() and assume_init() method for UniqueArc,
write() is for proper initialization for UniqueArc, assume_init()
can check whether the instance is properly initialized or not,
UB should be triggered immediately if it isn't, however, for the
current implementation, the compiler won't complain. Fix it in
the future iteration.

Signed-off-by: I Hsin Cheng <richard120310@gmail.com>

Author: vax-r

sync/src/arc/arc.rs

@@ -39,6 +39,30 @@ impl<T> UniqueArc<T> {
     }
 }
 
+impl<T> UniqueArc<MaybeUninit<T>> {
+    /// Converts a `UniqueArc<MaybeUninit<T>>` into a `UniqueArc<T>` by writing a value into it.
+    #[allow(dead_code)]
+    pub fn write(mut self, value: T) -> UniqueArc<T> {
+        Arc::get_mut(&mut self.inner).unwrap().write(value);
+        // SAFETY: We just wrote the value to be initialised.
+        unsafe { self.assume_init() }
+    }
+
+    /// Unsafely assume that `self` is initialized.
+    ///
+    /// # Safety
+    ///
+    /// The caller guarantees that the value behind this pointer has been initialized. It is
+    /// *immediate* UB to call this when the value is not initialized.
+    #[allow(dead_code)]
+    pub unsafe fn assume_init(self) -> UniqueArc<T> {
+        let inner = unsafe { self.inner.assume_init() };
+        UniqueArc {
+            inner: Arc::from_raw(Arc::into_raw(inner) as *const T),
+        }
+    }
+}
+
 impl<T: ?Sized> Deref for UniqueArc<T> {
     type Target = T;
 
@@ -119,6 +143,20 @@ mod tests {
         let x = UniqueArc::<MockData>::new_uninit().unwrap();
         assert_eq!(Arc::strong_count(&x.inner), 1);
     }
+
+    // FIXME : assume_init() doesn't triggered the UB even if the value isn't initialized
+    // explicitly.
+    #[test]
+    fn test_unique_arc_write() {
+        struct MockData {
+            a: u32,
+        }
+
+        let x = unsafe { UniqueArc::<MockData>::new_uninit().unwrap() };
+        let y = x.write(MockData{a: 10});
+
+        assert_eq!(y.a, 10);
+    }
 }