Rollup merge of #89741 - sdroege:arc-rc-from-inner-unsafe, r=Mark-Simulacrum

Mark `Arc::from_inner` / `Rc::from_inner` as unsafe

While it's an internal function, it is easy to create invalid Arc/Rcs to
a dangling pointer with it.

Fixes https://github.com/rust-lang/rust/issues/89740

library/alloc/src/rc.rs

@@ -341,12 +341,12 @@ impl<T: ?Sized> Rc<T> {
         unsafe { self.ptr.as_ref() }
     }
 
-    fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
+    unsafe fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
         Self { ptr, phantom: PhantomData }
     }
 
     unsafe fn from_ptr(ptr: *mut RcBox<T>) -> Self {
-        Self::from_inner(unsafe { NonNull::new_unchecked(ptr) })
+        unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
     }
 }
 
@@ -367,9 +367,11 @@ impl<T> Rc<T> {
         // pointers, which ensures that the weak destructor never frees
         // the allocation while the strong destructor is running, even
         // if the weak pointer is stored inside the strong one.
-        Self::from_inner(
-            Box::leak(box RcBox { strong: Cell::new(1), weak: Cell::new(1), value }).into(),
-        )
+        unsafe {
+            Self::from_inner(
+                Box::leak(box RcBox { strong: Cell::new(1), weak: Cell::new(1), value }).into(),
+            )
+        }
     }
 
     /// Constructs a new `Rc<T>` using a weak reference to itself. Attempting
@@ -420,16 +422,16 @@ impl<T> Rc<T> {
         // otherwise.
         let data = data_fn(&weak);
 
-        unsafe {
+        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);
-        }
 
-        let strong = Rc::from_inner(init_ptr);
+            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.
@@ -521,10 +523,12 @@ impl<T> Rc<T> {
         // pointers, which ensures that the weak destructor never frees
         // the allocation while the strong destructor is running, even
         // if the weak pointer is stored inside the strong one.
-        Ok(Self::from_inner(
-            Box::leak(Box::try_new(RcBox { strong: Cell::new(1), weak: Cell::new(1), value })?)
-                .into(),
-        ))
+        unsafe {
+            Ok(Self::from_inner(
+                Box::leak(Box::try_new(RcBox { strong: Cell::new(1), weak: Cell::new(1), value })?)
+                    .into(),
+            ))
+        }
     }
 
     /// Constructs a new `Rc` with uninitialized contents, returning an error if the allocation fails
@@ -746,7 +750,7 @@ impl<T> Rc<mem::MaybeUninit<T>> {
     #[unstable(feature = "new_uninit", issue = "63291")]
     #[inline]
     pub unsafe fn assume_init(self) -> Rc<T> {
-        Rc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
+        unsafe { Rc::from_inner(mem::ManuallyDrop::new(self).ptr.cast()) }
     }
 }
 
@@ -1214,9 +1218,11 @@ impl Rc<dyn Any> {
     /// ```
     pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
         if (*self).is::<T>() {
-            let ptr = self.ptr.cast::<RcBox<T>>();
-            forget(self);
-            Ok(Rc::from_inner(ptr))
+            unsafe {
+                let ptr = self.ptr.cast::<RcBox<T>>();
+                forget(self);
+                Ok(Rc::from_inner(ptr))
+            }
         } else {
             Err(self)
         }
@@ -1489,8 +1495,10 @@ impl<T: ?Sized> Clone for Rc<T> {
     /// ```
     #[inline]
     fn clone(&self) -> Rc<T> {
-        self.inner().inc_strong();
-        Self::from_inner(self.ptr)
+        unsafe {
+            self.inner().inc_strong();
+            Self::from_inner(self.ptr)
+        }
     }
 }
 
@@ -2245,11 +2253,14 @@ impl<T: ?Sized> Weak<T> {
     #[stable(feature = "rc_weak", since = "1.4.0")]
     pub fn upgrade(&self) -> Option<Rc<T>> {
         let inner = self.inner()?;
+
         if inner.strong() == 0 {
             None
         } else {
-            inner.inc_strong();
-            Some(Rc::from_inner(self.ptr))
+            unsafe {
+                inner.inc_strong();
+                Some(Rc::from_inner(self.ptr))
+            }
         }
     }