Rollup merge of rust-lang#90666 - bdbai:arc_new_cyclic, r=m-ou-se

Stabilize arc_new_cyclic

This stabilizes feature `arc_new_cyclic` as the implementation has been merged for one year and there is no unresolved questions. The FCP is not started yet.

Closes rust-lang#75861 .

`@rustbot` label +T-libs-api

library/alloc/src/rc.rs

@@ -374,33 +374,51 @@ impl<T> Rc<T> {
         }
     }
 
-    /// Constructs a new `Rc<T>` using a weak reference to itself. Attempting
-    /// to upgrade the weak reference before this function returns will result
-    /// in a `None` value. However, the weak reference may be cloned freely and
-    /// stored for use at a later time.
+    /// Constructs a new `Rc<T>` using a closure `data_fn` that has access to a
+    /// weak reference to the constructing `Rc<T>`.
+    ///
+    /// Generally, a structure circularly referencing itself, either directly or
+    /// indirectly, should not hold a strong reference to prevent a memory leak.
+    /// In `data_fn`, initialization of `T` can make use of the weak reference
+    /// by cloning and storing it inside `T` for use at a later time.
+    ///
+    /// Since the new `Rc<T>` is not fully-constructed until `Rc<T>::new_cyclic`
+    /// returns, calling [`upgrade`] on the weak reference inside `data_fn` 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>`] is dropped normally.
     ///
     /// # Examples
     ///
     /// ```
-    /// #![feature(arc_new_cyclic)]
     /// #![allow(dead_code)]
     /// use std::rc::{Rc, Weak};
     ///
     /// struct Gadget {
-    ///     self_weak: Weak<Self>,
-    ///     // ... more fields
+    ///     me: Weak<Gadget>,
     /// }
+    ///
     /// impl Gadget {
-    ///     pub fn new() -> Rc<Self> {
-    ///         Rc::new_cyclic(|self_weak| {
-    ///             Gadget { self_weak: self_weak.clone(), /* ... */ }
-    ///         })
+    ///     /// Construct a reference counted Gadget.
+    ///     fn new() -> Rc<Self> {
+    ///         Rc::new_cyclic(|me| Gadget { me: me.clone() })
+    ///     }
+    ///
+    ///     /// Return a reference counted pointer to Self.
+    ///     fn me(&self) -> Rc<Self> {
+    ///         self.me.upgrade().unwrap()
     ///     }
     /// }
     /// ```
+    /// [`upgrade`]: Weak::upgrade
     #[cfg(not(no_global_oom_handling))]
-    #[unstable(feature = "arc_new_cyclic", issue = "75861")]
-    pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> Rc<T> {
+    #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
+    pub fn new_cyclic<F>(data_fn: F) -> Rc<T>
+    where
+        F: FnOnce(&Weak<T>) -> T,
+    {
         // Construct the inner in the "uninitialized" state with a single
         // weak reference.
         let uninit_ptr: NonNull<_> = Box::leak(box RcBox {

library/alloc/src/sync.rs

@@ -351,30 +351,51 @@ impl<T> Arc<T> {
         unsafe { Self::from_inner(Box::leak(x).into()) }
     }
 
-    /// Constructs a new `Arc<T>` using a weak reference to itself. Attempting
-    /// to upgrade the weak reference before this function returns will result
-    /// in a `None` value. However, the weak reference may be cloned freely and
-    /// stored for use at a later time.
+    /// Constructs a new `Arc<T>` using a closure `data_fn` that has access to
+    /// a weak reference to the constructing `Arc<T>`.
     ///
-    /// # Examples
+    /// Generally, a structure circularly referencing itself, either directly or
+    /// indirectly, should not hold a strong reference to prevent a memory leak.
+    /// In `data_fn`, initialization of `T` can make use of the weak reference
+    /// by cloning and storing it inside `T` for use at a later time.
+    ///
+    /// Since the new `Arc<T>` is not fully-constructed until
+    /// `Arc<T>::new_cyclic` returns, calling [`upgrade`] on the weak
+    /// reference inside `data_fn` 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>`] is dropped normally.
+    ///
+    /// # Example
     /// ```
-    /// #![feature(arc_new_cyclic)]
     /// #![allow(dead_code)]
-    ///
     /// use std::sync::{Arc, Weak};
     ///
-    /// struct Foo {
-    ///     me: Weak<Foo>,
+    /// struct Gadget {
+    ///     me: Weak<Gadget>,
     /// }
     ///
-    /// let foo = Arc::new_cyclic(|me| Foo {
-    ///     me: me.clone(),
-    /// });
+    /// impl Gadget {
+    ///     /// Construct a reference counted Gadget.
+    ///     fn new() -> Arc<Self> {
+    ///         Arc::new_cyclic(|me| Gadget { me: me.clone() })
+    ///     }
+    ///
+    ///     /// Return a reference counted pointer to Self.
+    ///     fn me(&self) -> Arc<Self> {
+    ///         self.me.upgrade().unwrap()
+    ///     }
+    /// }
     /// ```
+    /// [`upgrade`]: Weak::upgrade
     #[cfg(not(no_global_oom_handling))]
     #[inline]
-    #[unstable(feature = "arc_new_cyclic", issue = "75861")]
-    pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> Arc<T> {
+    #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
+    pub fn new_cyclic<F>(data_fn: F) -> Arc<T>
+    where
+        F: FnOnce(&Weak<T>) -> T,
+    {
         // Construct the inner in the "uninitialized" state with a single
         // weak reference.
         let uninit_ptr: NonNull<_> = Box::leak(box ArcInner {