Relax allocator bounds on more Arc/Rc methods/impls.

Remove `A: Clone` bound from:
* `downcast`, `downcast_unchecked`, `unwrap_or_clone` for both.
* `From<Vec<T, A>> for Arc<[T], A>` (Rc's already didn't require A: Clone)
* `TryFrom<Arc<[T], A>> for Arc<[T; N], A>`

Make existing `TryFrom<Rc<[T]>> for Rc<[T; N]>` allocator-aware.

library/alloc/src/rc.rs

@@ -1760,7 +1760,9 @@ impl<T: Clone, A: Allocator + Clone> Rc<T, A> {
         // reference to the allocation.
         unsafe { &mut this.ptr.as_mut().value }
     }
+}
 
+impl<T: Clone, A: Allocator> Rc<T, A> {
     /// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
     /// clone.
     ///
@@ -1796,7 +1798,7 @@ impl<T: Clone, A: Allocator + Clone> Rc<T, A> {
     }
 }
 
-impl<A: Allocator + Clone> Rc<dyn Any, A> {
+impl<A: Allocator> Rc<dyn Any, A> {
     /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
     ///
     /// # Examples
@@ -1819,12 +1821,11 @@ impl<A: Allocator + Clone> Rc<dyn Any, A> {
     #[stable(feature = "rc_downcast", since = "1.29.0")]
     pub fn downcast<T: Any>(self) -> Result<Rc<T, A>, Self> {
         if (*self).is::<T>() {
-            unsafe {
-                let ptr = self.ptr.cast::<RcBox<T>>();
-                let alloc = self.alloc.clone();
-                forget(self);
-                Ok(Rc::from_inner_in(ptr, alloc))
-            }
+            let this = mem::ManuallyDrop::new(self);
+            let ptr = this.ptr.cast::<RcBox<T>>();
+            // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
+            let alloc = unsafe { ptr::read(&this.alloc) };
+            unsafe { Ok(Rc::from_inner_in(ptr, alloc)) }
         } else {
             Err(self)
         }
@@ -1859,12 +1860,11 @@ impl<A: Allocator + Clone> Rc<dyn Any, A> {
     #[inline]
     #[unstable(feature = "downcast_unchecked", issue = "90850")]
     pub unsafe fn downcast_unchecked<T: Any>(self) -> Rc<T, A> {
-        unsafe {
-            let ptr = self.ptr.cast::<RcBox<T>>();
-            let alloc = self.alloc.clone();
-            mem::forget(self);
-            Rc::from_inner_in(ptr, alloc)
-        }
+        let this = mem::ManuallyDrop::new(self);
+        let ptr = this.ptr.cast::<RcBox<T>>();
+        // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
+        let alloc = unsafe { ptr::read(&this.alloc) };
+        unsafe { Rc::from_inner_in(ptr, alloc) }
     }
 }
 
@@ -2604,12 +2604,13 @@ impl From<Rc<str>> for Rc<[u8]> {
 }
 
 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
-impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]> {
-    type Error = Rc<[T]>;
+impl<T, A: Allocator, const N: usize> TryFrom<Rc<[T], A>> for Rc<[T; N], A> {
+    type Error = Rc<[T], A>;
 
-    fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error> {
+    fn try_from(boxed_slice: Rc<[T], A>) -> Result<Self, Self::Error> {
         if boxed_slice.len() == N {
-            Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) })
+            let (ptr, alloc) = Rc::into_raw_with_allocator(boxed_slice);
+            Ok(unsafe { Rc::from_raw_in(ptr as *mut [T; N], alloc) })
         } else {
             Err(boxed_slice)
         }

library/alloc/src/sync.rs

@@ -2187,7 +2187,9 @@ impl<T: Clone, A: Allocator + Clone> Arc<T, A> {
         // either unique to begin with, or became one upon cloning the contents.
         unsafe { Self::get_mut_unchecked(this) }
     }
+}
 
+impl<T: Clone, A: Allocator> Arc<T, A> {
     /// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
     /// clone.
     ///
@@ -2432,7 +2434,7 @@ unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Arc<T, A> {
     }
 }
 
-impl<A: Allocator + Clone> Arc<dyn Any + Send + Sync, A> {
+impl<A: Allocator> Arc<dyn Any + Send + Sync, A> {
     /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
     ///
     /// # Examples
@@ -2458,12 +2460,11 @@ impl<A: Allocator + Clone> Arc<dyn Any + Send + Sync, A> {
         T: Any + Send + Sync,
     {
         if (*self).is::<T>() {
-            unsafe {
-                let ptr = self.ptr.cast::<ArcInner<T>>();
-                let alloc = self.alloc.clone();
-                mem::forget(self);
-                Ok(Arc::from_inner_in(ptr, alloc))
-            }
+            let this = mem::ManuallyDrop::new(self);
+            let ptr = this.ptr.cast::<ArcInner<T>>();
+            // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
+            let alloc = unsafe { ptr::read(&this.alloc) };
+            unsafe { Ok(Arc::from_inner_in(ptr, alloc)) }
         } else {
             Err(self)
         }
@@ -2501,12 +2502,11 @@ impl<A: Allocator + Clone> Arc<dyn Any + Send + Sync, A> {
     where
         T: Any + Send + Sync,
     {
-        unsafe {
-            let ptr = self.ptr.cast::<ArcInner<T>>();
-            let alloc = self.alloc.clone();
-            mem::forget(self);
-            Arc::from_inner_in(ptr, alloc)
-        }
+        let this = mem::ManuallyDrop::new(self);
+        let ptr = this.ptr.cast::<ArcInner<T>>();
+        // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
+        let alloc = unsafe { ptr::read(&this.alloc) };
+        unsafe { Arc::from_inner_in(ptr, alloc) }
     }
 }
 
@@ -3432,7 +3432,7 @@ impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Arc<T, A> {
 
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "shared_from_slice", since = "1.21.0")]
-impl<T, A: Allocator + Clone> From<Vec<T, A>> for Arc<[T], A> {
+impl<T, A: Allocator> From<Vec<T, A>> for Arc<[T], A> {
     /// Allocate a reference-counted slice and move `v`'s items into it.
     ///
     /// # Example
@@ -3507,13 +3507,13 @@ impl From<Arc<str>> for Arc<[u8]> {
 }
 
 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
-impl<T, A: Allocator + Clone, const N: usize> TryFrom<Arc<[T], A>> for Arc<[T; N], A> {
+impl<T, A: Allocator, const N: usize> TryFrom<Arc<[T], A>> for Arc<[T; N], A> {
     type Error = Arc<[T], A>;
 
     fn try_from(boxed_slice: Arc<[T], A>) -> Result<Self, Self::Error> {
         if boxed_slice.len() == N {
-            let alloc = boxed_slice.alloc.clone();
-            Ok(unsafe { Arc::from_raw_in(Arc::into_raw(boxed_slice) as *mut [T; N], alloc) })
+            let (ptr, alloc) = Arc::into_raw_with_allocator(boxed_slice);
+            Ok(unsafe { Arc::from_raw_in(ptr as *mut [T; N], alloc) })
         } else {
             Err(boxed_slice)
         }