Relax allocator requirements on some Rc APIs.

* Remove A: Clone bound from Rc::assume_init, Rc::downcast, and Rc::downcast_unchecked.
* Make From<Rc<[T; N]>> for Rc<[T]> allocator-aware.

Internal changes:

* Made Arc::internal_into_inner_with_allocator method into Arc::into_inner_with_allocator associated fn.
* Add private Rc::into_inner_with_allocator (to match Arc), so other fns don't have to juggle ManuallyDrop.

library/alloc/src/rc.rs

@@ -365,6 +365,12 @@ impl<T: ?Sized, A: Allocator> Rc<T, A> {
         unsafe { self.ptr.as_ref() }
     }
 
+    #[inline]
+    fn into_inner_with_allocator(this: Self) -> (NonNull<RcBox<T>>, A) {
+        let this = mem::ManuallyDrop::new(this);
+        (this.ptr, unsafe { ptr::read(&this.alloc) })
+    }
+
     #[inline]
     unsafe fn from_inner_in(ptr: NonNull<RcBox<T>>, alloc: A) -> Self {
         Self { ptr, phantom: PhantomData, alloc }
@@ -1145,12 +1151,9 @@ impl<T, A: Allocator> Rc<mem::MaybeUninit<T>, A> {
     /// ```
     #[unstable(feature = "new_uninit", issue = "63291")]
     #[inline]
-    pub unsafe fn assume_init(self) -> Rc<T, A>
-    where
-        A: Clone,
-    {
-        let md_self = mem::ManuallyDrop::new(self);
-        unsafe { Rc::from_inner_in(md_self.ptr.cast(), md_self.alloc.clone()) }
+    pub unsafe fn assume_init(self) -> Rc<T, A> {
+        let (ptr, alloc) = Rc::into_inner_with_allocator(self);
+        unsafe { Rc::from_inner_in(ptr.cast(), alloc) }
     }
 }
 
@@ -1189,12 +1192,9 @@ impl<T, A: Allocator> Rc<[mem::MaybeUninit<T>], A> {
     /// ```
     #[unstable(feature = "new_uninit", issue = "63291")]
     #[inline]
-    pub unsafe fn assume_init(self) -> Rc<[T], A>
-    where
-        A: Clone,
-    {
-        let md_self = mem::ManuallyDrop::new(self);
-        unsafe { Rc::from_ptr_in(md_self.ptr.as_ptr() as _, md_self.alloc.clone()) }
+    pub unsafe fn assume_init(self) -> Rc<[T], A> {
+        let (ptr, alloc) = Rc::into_inner_with_allocator(self);
+        unsafe { Rc::from_ptr_in(ptr.as_ptr() as _, alloc) }
     }
 }
 
@@ -1845,7 +1845,7 @@ 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
@@ -1869,10 +1869,8 @@ impl<A: Allocator + Clone> Rc<dyn Any, A> {
     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 (ptr, alloc) = Rc::into_inner_with_allocator(self);
+                Ok(Rc::from_inner_in(ptr.cast(), alloc))
             }
         } else {
             Err(self)
@@ -1909,10 +1907,8 @@ impl<A: Allocator + Clone> Rc<dyn Any, A> {
     #[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 (ptr, alloc) = Rc::into_inner_with_allocator(self);
+            Rc::from_inner_in(ptr.cast(), alloc)
         }
     }
 }
@@ -2661,12 +2657,13 @@ 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_inner_with_allocator(boxed_slice);
+            Ok(unsafe { Rc::from_inner_in(ptr.cast(), alloc) })
         } else {
             Err(boxed_slice)
         }

library/alloc/src/sync.rs

@@ -280,8 +280,8 @@ impl<T: ?Sized> Arc<T> {
 
 impl<T: ?Sized, A: Allocator> Arc<T, A> {
     #[inline]
-    fn internal_into_inner_with_allocator(self) -> (NonNull<ArcInner<T>>, A) {
-        let this = mem::ManuallyDrop::new(self);
+    fn into_inner_with_allocator(this: Self) -> (NonNull<ArcInner<T>>, A) {
+        let this = mem::ManuallyDrop::new(this);
         (this.ptr, unsafe { ptr::read(&this.alloc) })
     }
 
@@ -1290,7 +1290,7 @@ impl<T, A: Allocator> Arc<mem::MaybeUninit<T>, A> {
     #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
     pub unsafe fn assume_init(self) -> Arc<T, A> {
-        let (ptr, alloc) = self.internal_into_inner_with_allocator();
+        let (ptr, alloc) = Arc::into_inner_with_allocator(self);
         unsafe { Arc::from_inner_in(ptr.cast(), alloc) }
     }
 }
@@ -1332,7 +1332,7 @@ impl<T, A: Allocator> Arc<[mem::MaybeUninit<T>], A> {
     #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
     pub unsafe fn assume_init(self) -> Arc<[T], A> {
-        let (ptr, alloc) = self.internal_into_inner_with_allocator();
+        let (ptr, alloc) = Arc::into_inner_with_allocator(self);
         unsafe { Arc::from_ptr_in(ptr.as_ptr() as _, alloc) }
     }
 }
@@ -2499,7 +2499,7 @@ impl<A: Allocator> Arc<dyn Any + Send + Sync, A> {
     {
         if (*self).is::<T>() {
             unsafe {
-                let (ptr, alloc) = self.internal_into_inner_with_allocator();
+                let (ptr, alloc) = Arc::into_inner_with_allocator(self);
                 Ok(Arc::from_inner_in(ptr.cast(), alloc))
             }
         } else {
@@ -2540,7 +2540,7 @@ impl<A: Allocator> Arc<dyn Any + Send + Sync, A> {
         T: Any + Send + Sync,
     {
         unsafe {
-            let (ptr, alloc) = self.internal_into_inner_with_allocator();
+            let (ptr, alloc) = Arc::into_inner_with_allocator(self);
             Arc::from_inner_in(ptr.cast(), alloc)
         }
     }
@@ -3506,7 +3506,7 @@ impl<T, A: Allocator, const N: usize> TryFrom<Arc<[T], A>> for Arc<[T; N], A> {
 
     fn try_from(boxed_slice: Arc<[T], A>) -> Result<Self, Self::Error> {
         if boxed_slice.len() == N {
-            let (ptr, alloc) = boxed_slice.internal_into_inner_with_allocator();
+            let (ptr, alloc) = Arc::into_inner_with_allocator(boxed_slice);
             Ok(unsafe { Arc::from_inner_in(ptr.cast(), alloc) })
         } else {
             Err(boxed_slice)