Factor out from_gcbox helper

Signed-off-by: Anders Kaseorg <[email protected]>

gc/src/lib.rs

@@ -72,22 +72,30 @@ impl<T: Trace> Gc<T> {
     /// assert_eq!(*five, 5);
     /// ```
     pub fn new(value: T) -> Self {
-        assert!(mem::align_of::<GcBox<T>>() > 1);
+        unsafe { Gc::from_gcbox(GcBox::new(value)) }
+    }
+}
 
-        unsafe {
-            // Allocate the memory for the object
-            let ptr = GcBox::new(value);
+impl<T: Trace + ?Sized> Gc<T> {
+    /// Constructs a `Gc` that points to a new `GcBox`.
+    ///
+    /// # Safety
+    ///
+    /// `ptr` must point to a valid `GcBox` on the thread-local
+    /// `GcBox` chain.
+    #[inline]
+    unsafe fn from_gcbox(ptr: NonNull<GcBox<T>>) -> Gc<T> {
+        assert!(mem::align_of_val::<GcBox<T>>(ptr.as_ref()) > 1);
 
-            // When we create a Gc<T>, all pointers which have been moved to the
-            // heap no longer need to be rooted, so we unroot them.
-            ptr.as_ref().value().unroot();
-            let gc = Gc {
-                ptr_root: Cell::new(ptr),
-                marker: PhantomData,
-            };
-            gc.set_root();
-            gc
-        }
+        // When we create a Gc<T>, all pointers which have been moved to the
+        // heap no longer need to be rooted, so we unroot them.
+        ptr.as_ref().value().unroot();
+        let gc = Gc {
+            ptr_root: Cell::new(ptr),
+            marker: PhantomData,
+        };
+        gc.set_root();
+        gc
     }
 }