fix new_cyclic_in for rc

Author: Matthew Giordano

alloc/src/rc.rs

@@ -687,54 +687,6 @@ impl<T, A: Allocator> Rc<T, A> {
         }
     }
 
-    /// TODO: document
-    #[cfg(not(no_global_oom_handling))]
-    #[unstable(feature = "allocator_api", issue = "32838")]
-    pub fn new_cyclic_in<F>(data_fn: F, alloc: A) -> Rc<T, A>
-    where
-        F: FnOnce(&Weak<T, A>) -> T,
-    {
-        // Construct the inner in the "uninitialized" state with a single
-        // weak reference.
-        let uninit_ptr: NonNull<_> = Box::leak(
-            Box::new_in(RcBox {
-                strong: Cell::new(0),
-                weak: Cell::new(1),
-                value: mem::MaybeUninit::<T>::uninit(),
-            }),
-            alloc,
-        )
-        .into();
-
-        let init_ptr: NonNull<RcBox<T>> = uninit_ptr.cast();
-
-        let weak = Weak { ptr: init_ptr, alloc: Global };
-
-        // It's important we don't give up ownership of the weak pointer, or
-        // else the memory might be freed by the time `data_fn` returns. If
-        // we really wanted to pass ownership, we could create an additional
-        // weak pointer for ourselves, but this would result in additional
-        // updates to the weak reference count which might not be necessary
-        // otherwise.
-        let data = data_fn(&weak);
-
-        let strong = unsafe {
-            let inner = init_ptr.as_ptr();
-            ptr::write(ptr::addr_of_mut!((*inner).value), data);
-
-            let prev_value = (*inner).strong.get();
-            debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
-            (*inner).strong.set(1);
-
-            Rc::from_inner(init_ptr)
-        };
-
-        // Strong references should collectively own a shared weak reference,
-        // so don't run the destructor for our old weak reference.
-        mem::forget(weak);
-        strong
-    }
-
     /// Constructs a new `Rc` with uninitialized contents in the provided allocator.
     ///
     /// # Examples
@@ -2312,6 +2264,85 @@ impl<T: ?Sized, A: Allocator + Clone> Clone for Rc<T, A> {
     }
 }
 
+impl<T, A: Allocator + Clone> Rc<T, A> {
+    /// Constructs a new `Rc<T, A>` in the given allocator while giving you a `Weak<T, A>` to the allocation,
+    /// to allow you to construct a `T` which holds a weak pointer to itself.
+    ///
+    /// Generally, a structure circularly referencing itself, either directly or
+    /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
+    /// Using this function, you get access to the weak pointer during the
+    /// initialization of `T`, before the `Rc<T, A>` is created, such that you can
+    /// clone and store it inside the `T`.
+    ///
+    /// `new_cyclic` first allocates the managed allocation for the `Rc<T, A>`,
+    /// then calls your closure, giving it a `Weak<T, A>` to this allocation,
+    /// and only afterwards completes the construction of the `Rc<T, A>` by placing
+    /// the `T` returned from your closure into the allocation.
+    ///
+    /// Since the new `Rc<T, A>` is not fully-constructed until `Rc<T, A>::new_cyclic`
+    /// returns, calling [`upgrade`] on the weak reference inside your closure will
+    /// fail and result in a `None` value.
+    ///
+    /// # Panics
+    ///
+    /// If `data_fn` panics, the panic is propagated to the caller, and the
+    /// temporary [`Weak<T, A>`] is dropped normally.
+    ///
+    /// # Examples
+    /// See [`new_cyclic`].
+    ///
+    /// [`new_cyclic`]: Rc::new_cyclic
+    /// [`upgrade`]: Weak::upgrade
+    #[cfg(not(no_global_oom_handling))]
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    pub fn new_cyclic_in<F>(data_fn: F, alloc: A) -> Rc<T, A>
+    where
+        F: FnOnce(&Weak<T, A>) -> T,
+    {
+        // Note: comments and implementation are copied from Rc::new_cyclic.
+
+        // Construct the inner in the "uninitialized" state with a single
+        // weak reference.
+        let uninit_ptr: NonNull<_> = Box::leak(Box::new_in(
+            RcBox {
+                strong: Cell::new(0),
+                weak: Cell::new(1),
+                value: mem::MaybeUninit::<T>::uninit(),
+            },
+            alloc.clone(),
+        ))
+        .into();
+
+        let init_ptr: NonNull<RcBox<T>> = uninit_ptr.cast();
+
+        let weak = Weak { ptr: init_ptr, alloc: alloc.clone() };
+
+        // It's important we don't give up ownership of the weak pointer, or
+        // else the memory might be freed by the time `data_fn` returns. If
+        // we really wanted to pass ownership, we could create an additional
+        // weak pointer for ourselves, but this would result in additional
+        // updates to the weak reference count which might not be necessary
+        // otherwise.
+        let data = data_fn(&weak);
+
+        let strong = unsafe {
+            let inner = init_ptr.as_ptr();
+            ptr::write(ptr::addr_of_mut!((*inner).value), data);
+
+            let prev_value = (*inner).strong.get();
+            debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
+            (*inner).strong.set(1);
+
+            Rc::from_inner_in(init_ptr, alloc)
+        };
+
+        // Strong references should collectively own a shared weak reference,
+        // so don't run the destructor for our old weak reference.
+        mem::forget(weak);
+        strong
+    }
+}
+
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Default> Default for Rc<T> {