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.

5 files changed, 55 insertions, 65 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])
     /// ```
diff --git a/library/std/src/ffi/c_str.rs b/library/std/src/ffi/c_str.rs
index d859bff1a45..66fee2fe548 100644
--- a/library/std/src/ffi/c_str.rs
+++ b/library/std/src/ffi/c_str.rs
@@ -973,7 +973,8 @@ impl<'a> From<&'a CString> for Cow<'a, CStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<CString> for Arc<CStr> {
-    /// Converts a [`CString`] into an <code>[Arc]<[CStr]></code> without copying or allocating.
+    /// Converts a [`CString`] into an <code>[Arc]<[CStr]></code> by moving the [`CString`]
+    /// data into a new [`Arc`] buffer.
     #[inline]
     fn from(s: CString) -> Arc<CStr> {
         let arc: Arc<[u8]> = Arc::from(s.into_inner());
@@ -992,7 +993,8 @@ impl From<&CStr> for Arc<CStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<CString> for Rc<CStr> {
-    /// Converts a [`CString`] into an <code>[Rc]<[CStr]></code> without copying or allocating.
+    /// Converts a [`CString`] into an <code>[Rc]<[CStr]></code> by moving the [`CString`]
+    /// data into a new [`Arc`] buffer.
     #[inline]
     fn from(s: CString) -> Rc<CStr> {
         let rc: Rc<[u8]> = Rc::from(s.into_inner());
diff --git a/library/std/src/ffi/os_str.rs b/library/std/src/ffi/os_str.rs
index 982ad189878..81f72e34d93 100644
--- a/library/std/src/ffi/os_str.rs
+++ b/library/std/src/ffi/os_str.rs
@@ -989,7 +989,8 @@ impl Clone for Box<OsStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<OsString> for Arc<OsStr> {
-    /// Converts an [`OsString`] into an <code>[Arc]<[OsStr]></code> without copying or allocating.
+    /// Converts an [`OsString`] into an <code>[Arc]<[OsStr]></code> by moving the [`OsString`]
+    /// data into a new [`Arc`] buffer.
     #[inline]
     fn from(s: OsString) -> Arc<OsStr> {
         let arc = s.inner.into_arc();
@@ -1008,7 +1009,8 @@ impl From<&OsStr> for Arc<OsStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<OsString> for Rc<OsStr> {
-    /// Converts an [`OsString`] into an <code>[Rc]<[OsStr]></code> without copying or allocating.
+    /// Converts an [`OsString`] into an <code>[Rc]<[OsStr]></code> by moving the [`OsString`]
+    /// data into a new [`Rc`] buffer.
     #[inline]
     fn from(s: OsString) -> Rc<OsStr> {
         let rc = s.inner.into_rc();
diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index f509403fe2e..558333518f1 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -1766,7 +1766,8 @@ impl<'a> From<Cow<'a, Path>> for PathBuf {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<PathBuf> for Arc<Path> {
-    /// Converts a [`PathBuf`] into an [`Arc`] by moving the [`PathBuf`] data into a new [`Arc`] buffer.
+    /// Converts a [`PathBuf`] into an <code>[Arc]<[Path]></code> by moving the [`PathBuf`] data
+    /// into a new [`Arc`] buffer.
     #[inline]
     fn from(s: PathBuf) -> Arc<Path> {
         let arc: Arc<OsStr> = Arc::from(s.into_os_string());
@@ -1786,7 +1787,8 @@ impl From<&Path> for Arc<Path> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<PathBuf> for Rc<Path> {
-    /// Converts a [`PathBuf`] into an [`Rc`] by moving the [`PathBuf`] data into a new `Rc` buffer.
+    /// Converts a [`PathBuf`] into an <code>[Rc]<[Path]></code> by moving the [`PathBuf`] data into
+    /// a new [`Rc`] buffer.
     #[inline]
     fn from(s: PathBuf) -> Rc<Path> {
         let rc: Rc<OsStr> = Rc::from(s.into_os_string());
@@ -1796,7 +1798,7 @@ impl From<PathBuf> for Rc<Path> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<&Path> for Rc<Path> {
-    /// Converts a [`Path`] into an [`Rc`] by copying the [`Path`] data into a new `Rc` buffer.
+    /// Converts a [`Path`] into an [`Rc`] by copying the [`Path`] data into a new [`Rc`] buffer.
     #[inline]
     fn from(s: &Path) -> Rc<Path> {
         let rc: Rc<OsStr> = Rc::from(s.as_os_str());