Rollup merge of #93109 - JakobDegen:arc-docs, r=m-ou-se

Improve `Arc` and `Rc` documentation

This makes two changes (I can split the PR if necessary, but the changes are pretty small):
 1. A bunch of trait implementations claimed to be zero cost; however, they use the `Arc<T>: From<Box<T>>` impl which is definitely not free, especially for large dynamically sized `T`.
 2.  The code in deferred initialization examples unnecessarily used excessive amounts of `unsafe`. This has been reduced.

2 files changed, 42 insertions, 58 deletions

diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index 33bee4324fd..bf5fcfdcbf5 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -451,12 +451,10 @@ impl<T> Rc<T> {
     ///
     /// let mut five = Rc::<u32>::new_uninit();
     ///
-    /// let five = unsafe {
-    ///     // Deferred initialization:
-    ///     Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
+    /// // Deferred initialization:
+    /// Rc::get_mut(&mut five).unwrap().write(5);
     ///
-    ///     five.assume_init()
-    /// };
+    /// let five = unsafe { five.assume_init() };
     ///
     /// assert_eq!(*five, 5)
     /// ```
@@ -543,12 +541,10 @@ impl<T> Rc<T> {
     ///
     /// let mut five = Rc::<u32>::try_new_uninit()?;
     ///
-    /// let five = unsafe {
-    ///     // Deferred initialization:
-    ///     Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
+    /// // Deferred initialization:
+    /// Rc::get_mut(&mut five).unwrap().write(5);
     ///
-    ///     five.assume_init()
-    /// };
+    /// let five = unsafe { five.assume_init() };
     ///
     /// assert_eq!(*five, 5);
     /// # Ok::<(), std::alloc::AllocError>(())
@@ -660,14 +656,13 @@ impl<T> Rc<[T]> {
     ///
     /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
     ///
-    /// let values = unsafe {
-    ///     // Deferred initialization:
-    ///     Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
-    ///     Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
-    ///     Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
+    /// // Deferred initialization:
+    /// let data = Rc::get_mut(&mut values).unwrap();
+    /// data[0].write(1);
+    /// data[1].write(2);
+    /// data[2].write(3);
     ///
-    ///     values.assume_init()
-    /// };
+    /// let values = unsafe { values.assume_init() };
     ///
     /// assert_eq!(*values, [1, 2, 3])
     /// ```
@@ -738,12 +733,10 @@ impl<T> Rc<mem::MaybeUninit<T>> {
     ///
     /// let mut five = Rc::<u32>::new_uninit();
     ///
-    /// let five = unsafe {
-    ///     // Deferred initialization:
-    ///     Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
+    /// // Deferred initialization:
+    /// Rc::get_mut(&mut five).unwrap().write(5);
     ///
-    ///     five.assume_init()
-    /// };
+    /// let five = unsafe { five.assume_init() };
     ///
     /// assert_eq!(*five, 5)
     /// ```
@@ -777,14 +770,13 @@ impl<T> Rc<[mem::MaybeUninit<T>]> {
     ///
     /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
     ///
-    /// let values = unsafe {
-    ///     // Deferred initialization:
-    ///     Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
-    ///     Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
-    ///     Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
+    /// // Deferred initialization:
+    /// let data = Rc::get_mut(&mut values).unwrap();
+    /// data[0].write(1);
+    /// data[1].write(2);
+    /// data[2].write(3);
     ///
-    ///     values.assume_init()
-    /// };
+    /// let values = unsafe { values.assume_init() };
     ///
     /// assert_eq!(*values, [1, 2, 3])
     /// ```
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index 7c065f37d1f..ab8a44b2147 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -437,12 +437,10 @@ impl<T> Arc<T> {
     ///
     /// let mut five = Arc::<u32>::new_uninit();
     ///
-    /// let five = unsafe {
-    ///     // Deferred initialization:
-    ///     Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
+    /// // Deferred initialization:
+    /// Arc::get_mut(&mut five).unwrap().write(5);
     ///
-    ///     five.assume_init()
-    /// };
+    /// let five = unsafe { five.assume_init() };
     ///
     /// assert_eq!(*five, 5)
     /// ```
@@ -545,12 +543,10 @@ impl<T> Arc<T> {
     ///
     /// let mut five = Arc::<u32>::try_new_uninit()?;
     ///
-    /// let five = unsafe {
-    ///     // Deferred initialization:
-    ///     Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
+    /// // Deferred initialization:
+    /// Arc::get_mut(&mut five).unwrap().write(5);
     ///
-    ///     five.assume_init()
-    /// };
+    /// let five = unsafe { five.assume_init() };
     ///
     /// assert_eq!(*five, 5);
     /// # Ok::<(), std::alloc::AllocError>(())
@@ -652,14 +648,13 @@ impl<T> Arc<[T]> {
     ///
     /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
     ///
-    /// let values = unsafe {
-    ///     // Deferred initialization:
-    ///     Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
-    ///     Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
-    ///     Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
+    /// // Deferred initialization:
+    /// let data = Arc::get_mut(&mut values).unwrap();
+    /// data[0].write(1);
+    /// data[1].write(2);
+    /// data[2].write(3);
     ///
-    ///     values.assume_init()
-    /// };
+    /// let values = unsafe { values.assume_init() };
     ///
     /// assert_eq!(*values, [1, 2, 3])
     /// ```
@@ -730,12 +725,10 @@ impl<T> Arc<mem::MaybeUninit<T>> {
     ///
     /// let mut five = Arc::<u32>::new_uninit();
     ///
-    /// let five = unsafe {
-    ///     // Deferred initialization:
-    ///     Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
+    /// // Deferred initialization:
+    /// Arc::get_mut(&mut five).unwrap().write(5);
     ///
-    ///     five.assume_init()
-    /// };
+    /// let five = unsafe { five.assume_init() };
     ///
     /// assert_eq!(*five, 5)
     /// ```
@@ -770,14 +763,13 @@ impl<T> Arc<[mem::MaybeUninit<T>]> {
     ///
     /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
     ///
-    /// let values = unsafe {
-    ///     // Deferred initialization:
-    ///     Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
-    ///     Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
-    ///     Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
+    /// // Deferred initialization:
+    /// let data = Arc::get_mut(&mut values).unwrap();
+    /// data[0].write(1);
+    /// data[1].write(2);
+    /// data[2].write(3);
     ///
-    ///     values.assume_init()
-    /// };
+    /// let values = unsafe { values.assume_init() };
     ///
     /// assert_eq!(*values, [1, 2, 3])
     /// ```