Warn against relying on ?Sized being last

[rust.git] / src / liballoc / rc.rs

diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 2254cde7f49cca749837f767ec46f1fc77256e98..00c4b5462be64d0953644ac5cd981b02bec21bcc 100644 (file)
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -1648,10 +1648,8 @@ pub fn new() -> Weak<T> {
 
     /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
     ///
-    /// It is up to the caller to ensure that the object is still alive when accessing it through
-    /// the pointer.
-    ///
-    /// The pointer may be [`null`] or be dangling in case the object has already been destroyed.
+    /// The pointer is valid only if there are some strong references. The pointer may be dangling
+    /// or even [`null`] otherwise.
     ///
     /// # Examples
     ///
@@ -1731,14 +1729,18 @@ pub fn into_raw(self) -> *const T {
     /// This can be used to safely get a strong reference (by calling [`upgrade`]
     /// later) or to deallocate the weak count by dropping the `Weak<T>`.
     ///
-    /// It takes ownership of one weak count. In case a [`null`] is passed, a dangling [`Weak`] is
-    /// returned.
+    /// It takes ownership of one weak count (with the exception of pointers created by [`new`],
+    /// as these don't have any corresponding weak count).
     ///
     /// # Safety
     ///
-    /// The pointer must represent one valid weak count. In other words, it must point to `T` which
-    /// is or *was* managed by an [`Rc`] and the weak count of that [`Rc`] must not have reached
-    /// 0. It is allowed for the strong count to be 0.
+    /// The pointer must have originated from the [`into_raw`] (or [`as_raw`], provided there was
+    /// a corresponding [`forget`] on the `Weak<T>`) and must still own its potential weak reference
+    /// count.
+    ///
+    /// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
+    /// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
+    /// by [`new`]).
     ///
     /// # Examples
     ///
@@ -1763,11 +1765,13 @@ pub fn into_raw(self) -> *const T {
     /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
     /// ```
     ///
-    /// [`null`]: ../../std/ptr/fn.null.html
     /// [`into_raw`]: struct.Weak.html#method.into_raw
     /// [`upgrade`]: struct.Weak.html#method.upgrade
     /// [`Rc`]: struct.Rc.html
     /// [`Weak`]: struct.Weak.html
+    /// [`as_raw`]: struct.Weak.html#method.as_raw
+    /// [`new`]: struct.Weak.html#method.new
+    /// [`forget`]: ../../std/mem/fn.forget.html
     #[unstable(feature = "weak_into_raw", issue = "60728")]
     pub unsafe fn from_raw(ptr: *const T) -> Self {
         if ptr.is_null() {
@@ -1832,7 +1836,7 @@ pub fn upgrade(&self) -> Option<Rc<T>> {
     /// If `self` was created using [`Weak::new`], this will return 0.
     ///
     /// [`Weak::new`]: #method.new
-    #[unstable(feature = "weak_counts", issue = "57977")]
+    #[stable(feature = "weak_counts", since = "1.41.0")]
     pub fn strong_count(&self) -> usize {
         if let Some(inner) = self.inner() {
             inner.strong()
@@ -1843,20 +1847,16 @@ pub fn strong_count(&self) -> usize {
 
     /// Gets the number of `Weak` pointers pointing to this allocation.
     ///
-    /// If `self` was created using [`Weak::new`], this will return `None`. If
-    /// not, the returned value is at least 1, since `self` still points to the
-    /// allocation.
-    ///
-    /// [`Weak::new`]: #method.new
-    #[unstable(feature = "weak_counts", issue = "57977")]
-    pub fn weak_count(&self) -> Option<usize> {
+    /// If no strong pointers remain, this will return zero.
+    #[stable(feature = "weak_counts", since = "1.41.0")]
+    pub fn weak_count(&self) -> usize {
         self.inner().map(|inner| {
             if inner.strong() > 0 {
                 inner.weak() - 1  // subtract the implicit weak ptr
             } else {
-                inner.weak()
+                0
             }
-        })
+        }).unwrap_or(0)
     }
 
     /// Returns `None` when the pointer is dangling and there is no allocated `RcBox`
@@ -2106,6 +2106,8 @@ impl<T: ?Sized> Unpin for Rc<T> { }
 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
     // Align the unsized value to the end of the `RcBox`.
     // Because it is ?Sized, it will always be the last field in memory.
+    // Note: This is a detail of the current implementation of the compiler,
+    // and is not a guaranteed language detail. Do not rely on it outside of std.
     data_offset_align(align_of_val(&*ptr))
 }