Rollup merge of #41266 - projektir:weak_docs_rc, r=alexcrichton

Updating docs for std::rc::Rc

The same changes as PR [#41240 ](https://github.com/rust-lang/rust/pull/41240), but for [`std::rc::Weak`](https://doc.rust-lang.org/std/rc/struct.Weak.html). At least, as far as I am aware, the Weak pointer is the same for both, and they're basically the same, just one is thread-safe and the other is not.

r? @alexcrichton

src/liballoc/rc.rs

@@ -922,18 +922,29 @@ impl<T> From<T> for Rc<T> {
     }
 }
 
-/// A weak version of [`Rc`][rc].
+/// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
+/// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
+/// pointer, which returns an [`Option`]`<`[`Rc`]`<T>>`.
 ///
-/// `Weak` pointers do not count towards determining if the inner value
+/// Since a `Weak` reference does not count towards ownership, it will not
-/// should be dropped.
+/// prevent the inner value from being dropped, and `Weak` itself makes no
+/// guarantees about the value still being present and may return [`None`]
+/// when [`upgrade`]d.
 ///
-/// The typical way to obtain a `Weak` pointer is to call
+/// A `Weak` pointer is useful for keeping a temporary reference to the value
-/// [`Rc::downgrade`][downgrade].
+/// within [`Rc`] without extending its lifetime. It is also used to prevent
+/// circular references between [`Rc`] pointers, since mutual owning references
+/// would never allow either [`Arc`] to be dropped. For example, a tree could
+/// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
+/// pointers from children back to their parents.
 ///
-/// See the [module-level documentation](./index.html) for more details.
+/// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
 ///
-/// [rc]: struct.Rc.html
+/// [`Rc`]: struct.Rc.html
-/// [downgrade]: struct.Rc.html#method.downgrade
+/// [`Rc::downgrade`]: struct.Rc.html#method.downgrade
+/// [`upgrade`]: struct.Weak.html#method.upgrade
+/// [`Option`]: ../../std/option/enum.Option.html
+/// [`None`]: ../../std/option/enum.Option.html#variant.None
 #[stable(feature = "rc_weak", since = "1.4.0")]
 pub struct Weak<T: ?Sized> {
     ptr: Shared<RcBox<T>>,
@@ -948,14 +959,11 @@ impl<T: ?Sized> !marker::Sync for Weak<T> {}
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
 
 impl<T> Weak<T> {
-    /// Constructs a new `Weak<T>`, without an accompanying instance of `T`.
+    /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
+    /// it. Calling [`upgrade`] on the return value always gives [`None`].
     ///
-    /// This allocates memory for `T`, but does not initialize it. Calling
+    /// [`upgrade`]: struct.Weak.html#method.upgrade
-    /// [`upgrade`][upgrade] on the return value always gives
+    /// [`None`]: ../../std/option/enum.Option.html
-    /// [`None`][option].
-    ///
-    /// [upgrade]: struct.Weak.html#method.upgrade
-    /// [option]: ../../std/option/enum.Option.html
     ///
     /// # Examples
     ///
@@ -980,13 +988,13 @@ impl<T> Weak<T> {
 }
 
 impl<T: ?Sized> Weak<T> {
-    /// Upgrades the `Weak` pointer to an [`Rc`][rc], if possible.
+    /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
+    /// the lifetime of the value if successful.
     ///
-    /// Returns [`None`][option] if the strong count has reached zero and the
+    /// Returns [`None`] if the value has since been dropped.
-    /// inner value was destroyed.
     ///
-    /// [rc]: struct.Rc.html
+    /// [`Rc`]: struct.Rc.html
-    /// [option]: ../../std/option/enum.Option.html
+    /// [`None`]: ../../std/option/enum.Option.html
     ///
     /// # Examples
     ///
@@ -1021,8 +1029,6 @@ impl<T: ?Sized> Weak<T> {
 impl<T: ?Sized> Drop for Weak<T> {
     /// Drops the `Weak` pointer.
     ///
-    /// This will decrement the weak reference count.
-    ///
     /// # Examples
     ///
     /// ```
@@ -1061,10 +1067,7 @@ impl<T: ?Sized> Drop for Weak<T> {
 
 #[stable(feature = "rc_weak", since = "1.4.0")]
 impl<T: ?Sized> Clone for Weak<T> {
-    /// Makes a clone of the `Weak` pointer.
+    /// Makes a clone of the `Weak` pointer that points to the same value.
-    ///
-    /// This creates another pointer to the same inner value, increasing the
-    /// weak reference count.
     ///
     /// # Examples
     ///
@@ -1091,14 +1094,11 @@ impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
 
 #[stable(feature = "downgraded_weak", since = "1.10.0")]
 impl<T> Default for Weak<T> {
-    /// Constructs a new `Weak<T>`, without an accompanying instance of `T`.
+    /// Constructs a new `Weak<T>`, allocating memory for `T` without initializing
+    /// it. Calling [`upgrade`] on the return value always gives [`None`].
     ///
-    /// This allocates memory for `T`, but does not initialize it. Calling
+    /// [`upgrade`]: struct.Weak.html#method.upgrade
-    /// [`upgrade`][upgrade] on the return value always gives
+    /// [`None`]: ../../std/option/enum.Option.html
-    /// [`None`][option].
-    ///
-    /// [upgrade]: struct.Weak.html#method.upgrade
-    /// [option]: ../../std/option/enum.Option.html
     ///
     /// # Examples
     ///