Allow Weak::as_ptr and friends for unsized T

library/alloc/src/rc.rs

@@ -1721,7 +1721,21 @@ impl<T> Weak<T> {
     pub fn new() -> Weak<T> {
         Weak { ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0") }
     }
+}
 
+pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
+    let address = ptr.as_ptr() as *mut () as usize;
+    address == usize::MAX
+}
+
+/// Helper type to allow accessing the reference counts without
+/// making any assertions about the data field.
+struct WeakInner<'a> {
+    weak: &'a Cell<usize>,
+    strong: &'a Cell<usize>,
+}
+
+impl<T: ?Sized> Weak<T> {
     /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
     ///
     /// The pointer is valid only if there are some strong references. The pointer may be dangling,
@@ -1841,33 +1855,20 @@ impl<T> Weak<T> {
     /// [`new`]: Weak::new
     #[stable(feature = "weak_into_raw", since = "1.45.0")]
     pub unsafe fn from_raw(ptr: *const T) -> Self {
-        if ptr.is_null() {
-            Self::new()
-        } else {
-            // See Rc::from_raw for details
-            unsafe {
-                let offset = data_offset(ptr);
-                let fake_ptr = ptr as *mut RcBox<T>;
-                let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
-                Weak { ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw") }
-            }
-        }
-    }
-}
+        // SAFETY: data_offset is safe to call, because this pointer originates from a Weak.
+        // See Weak::as_ptr for context on how the input pointer is derived.
+        let offset = unsafe { data_offset(ptr) };
 
-pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
-    let address = ptr.as_ptr() as *mut () as usize;
-    address == usize::MAX
-}
+        // Reverse the offset to find the original RcBox.
+        // SAFETY: we use wrapping_offset here because the pointer may be dangling (but only if T: Sized).
+        let ptr = unsafe {
+            set_data_ptr(ptr as *mut RcBox<T>, (ptr as *mut u8).wrapping_offset(-offset))
+        };
 
-/// Helper type to allow accessing the reference counts without
-/// making any assertions about the data field.
-struct WeakInner<'a> {
-    weak: &'a Cell<usize>,
-    strong: &'a Cell<usize>,
-}
+        // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
+        Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
+    }
 
-impl<T: ?Sized> Weak<T> {
     /// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
     /// dropping of the inner value if successful.
     ///

library/alloc/src/rc/tests.rs

@@ -190,6 +190,48 @@ fn test_into_from_raw_unsized() {
     assert_eq!(rc2.to_string(), "123");
 }
 
+#[test]
+fn into_from_weak_raw() {
+    let x = Rc::new(box "hello");
+    let y = Rc::downgrade(&x);
+
+    let y_ptr = Weak::into_raw(y);
+    unsafe {
+        assert_eq!(**y_ptr, "hello");
+
+        let y = Weak::from_raw(y_ptr);
+        let y_up = Weak::upgrade(&y).unwrap();
+        assert_eq!(**y_up, "hello");
+        drop(y_up);
+
+        assert_eq!(Rc::try_unwrap(x).map(|x| *x), Ok("hello"));
+    }
+}
+
+#[test]
+fn test_into_from_weak_raw_unsized() {
+    use std::fmt::Display;
+    use std::string::ToString;
+
+    let arc: Rc<str> = Rc::from("foo");
+    let weak: Weak<str> = Rc::downgrade(&arc);
+
+    let ptr = Weak::into_raw(weak.clone());
+    let weak2 = unsafe { Weak::from_raw(ptr) };
+
+    assert_eq!(unsafe { &*ptr }, "foo");
+    assert!(weak.ptr_eq(&weak2));
+
+    let arc: Rc<dyn Display> = Rc::new(123);
+    let weak: Weak<dyn Display> = Rc::downgrade(&arc);
+
+    let ptr = Weak::into_raw(weak.clone());
+    let weak2 = unsafe { Weak::from_raw(ptr) };
+
+    assert_eq!(unsafe { &*ptr }.to_string(), "123");
+    assert!(weak.ptr_eq(&weak2));
+}
+
 #[test]
 fn get_mut() {
     let mut x = Rc::new(3);

library/alloc/src/sync.rs

@@ -1509,7 +1509,16 @@ impl<T> Weak<T> {
     pub fn new() -> Weak<T> {
         Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
     }
+}
+
+/// Helper type to allow accessing the reference counts without
+/// making any assertions about the data field.
+struct WeakInner<'a> {
+    weak: &'a atomic::AtomicUsize,
+    strong: &'a atomic::AtomicUsize,
+}
 
+impl<T: ?Sized> Weak<T> {
     /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
     ///
     /// The pointer is valid only if there are some strong references. The pointer may be dangling,
@@ -1629,28 +1638,20 @@ impl<T> Weak<T> {
     /// [`forget`]: std::mem::forget
     #[stable(feature = "weak_into_raw", since = "1.45.0")]
     pub unsafe fn from_raw(ptr: *const T) -> Self {
-        if ptr.is_null() {
-            Self::new()
-        } else {
-            // See Arc::from_raw for details
-            unsafe {
-                let offset = data_offset(ptr);
-                let fake_ptr = ptr as *mut ArcInner<T>;
-                let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
-                Weak { ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw") }
-            }
-        }
-    }
-}
+        // SAFETY: data_offset is safe to call, because this pointer originates from a Weak.
+        // See Weak::as_ptr for context on how the input pointer is derived.
+        let offset = unsafe { data_offset(ptr) };
+
+        // Reverse the offset to find the original ArcInner.
+        // SAFETY: we use wrapping_offset here because the pointer may be dangling (but only if T: Sized)
+        let ptr = unsafe {
+            set_data_ptr(ptr as *mut ArcInner<T>, (ptr as *mut u8).wrapping_offset(-offset))
+        };
 
-/// Helper type to allow accessing the reference counts without
-/// making any assertions about the data field.
-struct WeakInner<'a> {
-    weak: &'a atomic::AtomicUsize,
-    strong: &'a atomic::AtomicUsize,
-}
+        // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
+        unsafe { Weak { ptr: NonNull::new_unchecked(ptr) } }
+    }
 
-impl<T: ?Sized> Weak<T> {
     /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
     /// dropping of the inner value if successful.
     ///

library/alloc/src/sync/tests.rs

@@ -140,6 +140,48 @@ fn test_into_from_raw_unsized() {
     assert_eq!(arc2.to_string(), "123");
 }
 
+#[test]
+fn into_from_weak_raw() {
+    let x = Arc::new(box "hello");
+    let y = Arc::downgrade(&x);
+
+    let y_ptr = Weak::into_raw(y);
+    unsafe {
+        assert_eq!(**y_ptr, "hello");
+
+        let y = Weak::from_raw(y_ptr);
+        let y_up = Weak::upgrade(&y).unwrap();
+        assert_eq!(**y_up, "hello");
+        drop(y_up);
+
+        assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
+    }
+}
+
+#[test]
+fn test_into_from_weak_raw_unsized() {
+    use std::fmt::Display;
+    use std::string::ToString;
+
+    let arc: Arc<str> = Arc::from("foo");
+    let weak: Weak<str> = Arc::downgrade(&arc);
+
+    let ptr = Weak::into_raw(weak.clone());
+    let weak2 = unsafe { Weak::from_raw(ptr) };
+
+    assert_eq!(unsafe { &*ptr }, "foo");
+    assert!(weak.ptr_eq(&weak2));
+
+    let arc: Arc<dyn Display> = Arc::new(123);
+    let weak: Weak<dyn Display> = Arc::downgrade(&arc);
+
+    let ptr = Weak::into_raw(weak.clone());
+    let weak2 = unsafe { Weak::from_raw(ptr) };
+
+    assert_eq!(unsafe { &*ptr }.to_string(), "123");
+    assert!(weak.ptr_eq(&weak2));
+}
+
 #[test]
 fn test_cowarc_clone_make_mut() {
     let mut cow0 = Arc::new(75);