Rollup merge of rust-lang#70733 - yoshuawuyts:arc-increment-refcount, r=Mark-Simulacrum

Add Arc::{incr,decr}_strong_count

This adds two `unsafe` methods to `Arc`: `incr_strong_count` and `decr_strong_count`. A suggestion to add methods to change the strong count in `Arc` came up in during review in rust-lang#68700 (comment), and from asking a few people this seemed like generally useful to have.

References:
- [Motivation from rust-lang#68700](rust-lang#68700 (comment))
- [Real world example in an executor](https://docs.rs/extreme/666.666.666666/src/extreme/lib.rs.html#13)

Author: Dylan-DPC

src/liballoc/sync.rs

@@ -776,6 +776,79 @@ impl<T: ?Sized> Arc<T> {
         this.inner().strong.load(SeqCst)
     }
 
+    /// Increments the strong reference count on the `Arc<T>` associated with the
+    /// provided pointer by one.
+    ///
+    /// # Safety
+    ///
+    /// The pointer must have been obtained through `Arc::into_raw`, and the
+    /// associated `Arc` instance must be valid (i.e. the strong count must be at
+    /// least 1) for the duration of this method.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(arc_mutate_strong_count)]
+    ///
+    /// use std::sync::Arc;
+    ///
+    /// let five = Arc::new(5);
+    ///
+    /// unsafe {
+    ///     let ptr = Arc::into_raw(five);
+    ///     Arc::incr_strong_count(ptr);
+    ///
+    ///     // This assertion is deterministic because we haven't shared
+    ///     // the `Arc` between threads.
+    ///     let five = Arc::from_raw(ptr);
+    ///     assert_eq!(2, Arc::strong_count(&five));
+    /// }
+    /// ```
+    #[inline]
+    #[unstable(feature = "arc_mutate_strong_count", issue = "71983")]
+    pub unsafe fn incr_strong_count(ptr: *const T) {
+        // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
+        let arc = mem::ManuallyDrop::new(Arc::<T>::from_raw(ptr));
+        // Now increase refcount, but don't drop new refcount either
+        let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
+    }
+
+    /// Decrements the strong reference count on the `Arc<T>` associated with the
+    /// provided pointer by one.
+    ///
+    /// # Safety
+    ///
+    /// The pointer must have been obtained through `Arc::into_raw`, and the
+    /// associated `Arc` instance must be valid (i.e. the strong count must be at
+    /// least 1) when invoking this method. This method can be used to release the final
+    /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
+    /// released.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(arc_mutate_strong_count)]
+    ///
+    /// use std::sync::Arc;
+    ///
+    /// let five = Arc::new(5);
+    ///
+    /// unsafe {
+    ///     let ptr = Arc::into_raw(five);
+    ///     Arc::decr_strong_count(ptr);
+    ///
+    ///     // This assertion is deterministic because we haven't shared
+    ///     // the `Arc` between threads.
+    ///     let five = Arc::from_raw(ptr);
+    ///     assert_eq!(0, Arc::strong_count(&five));
+    /// }
+    /// ```
+    #[inline]
+    #[unstable(feature = "arc_mutate_strong_count", issue = "71983")]
+    pub unsafe fn decr_strong_count(ptr: *const T) {
+        mem::drop(Arc::from_raw(ptr));
+    }
+
     #[inline]
     fn inner(&self) -> &ArcInner<T> {
         // This unsafety is ok because while this arc is alive we're guaranteed

src/liballoc/task.rs

@@ -1,6 +1,6 @@
 #![unstable(feature = "wake_trait", issue = "69912")]
 //! Types and Traits for working with asynchronous tasks.
-use core::mem::{self, ManuallyDrop};
+use core::mem::ManuallyDrop;
 use core::task::{RawWaker, RawWakerVTable, Waker};
 
 use crate::sync::Arc;
@@ -60,9 +60,11 @@ impl<W: Wake + Send + Sync + 'static> From<Arc<W>> for RawWaker {
 fn raw_waker<W: Wake + Send + Sync + 'static>(waker: Arc<W>) -> RawWaker {
     // Increment the reference count of the arc to clone it.
     unsafe fn clone_waker<W: Wake + Send + Sync + 'static>(waker: *const ()) -> RawWaker {
-        let waker: Arc<W> = Arc::from_raw(waker as *const W);
-        mem::forget(Arc::clone(&waker));
-        raw_waker(waker)
+        Arc::incr_strong_count(waker as *const W);
+        RawWaker::new(
+            waker as *const (),
+            &RawWakerVTable::new(clone_waker::<W>, wake::<W>, wake_by_ref::<W>, drop_waker::<W>),
+        )
     }
 
     // Wake by value, moving the Arc into the Wake::wake function
@@ -79,7 +81,7 @@ fn raw_waker<W: Wake + Send + Sync + 'static>(waker: Arc<W>) -> RawWaker {
 
     // Decrement the reference count of the Arc on drop
     unsafe fn drop_waker<W: Wake + Send + Sync + 'static>(waker: *const ()) {
-        mem::drop(Arc::from_raw(waker as *const W));
+        Arc::decr_strong_count(waker as *const W);
     }
 
     RawWaker::new(