6 files changed, 108 insertions, 16 deletions
diff --git a/library/alloc/src/alloc.rs b/library/alloc/src/alloc.rs
index c9b98fa4e5a..76630a746dd 100644
--- a/library/alloc/src/alloc.rs
+++ b/library/alloc/src/alloc.rs
@@ -17,6 +17,7 @@ unsafe extern "Rust" {
     #[rustc_allocator]
     #[rustc_nounwind]
     #[rustc_std_internal_symbol]
+    #[rustc_allocator_zeroed_variant = "__rust_alloc_zeroed"]
     fn __rust_alloc(size: usize, align: usize) -> *mut u8;
     #[rustc_deallocator]
     #[rustc_nounwind]
diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index 3db37f1d16f..fa12d379c8c 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -1672,7 +1672,7 @@ unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Default> Default for Box<T> {
-    /// Creates a `Box<T>`, with the `Default` value for T.
+    /// Creates a `Box<T>`, with the `Default` value for `T`.
     #[inline]
     fn default() -> Self {
         let mut x: Box<mem::MaybeUninit<T>> = Box::new_uninit();
@@ -1695,6 +1695,7 @@ impl<T: Default> Default for Box<T> {
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Default for Box<[T]> {
+    /// Creates an empty `[T]` inside a `Box`.
     #[inline]
     fn default() -> Self {
         let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
@@ -1717,6 +1718,19 @@ impl Default for Box<str> {
 }
 
 #[cfg(not(no_global_oom_handling))]
+#[stable(feature = "pin_default_impls", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for Pin<Box<T>>
+where
+    T: ?Sized,
+    Box<T>: Default,
+{
+    #[inline]
+    fn default() -> Self {
+        Box::into_pin(Box::<T>::default())
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
     /// Returns a new box with a `clone()` of this box's contents.
diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index c091e496c50..639c5d4c930 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -102,6 +102,7 @@
 #![feature(async_iterator)]
 #![feature(bstr)]
 #![feature(bstr_internals)]
+#![feature(cast_maybe_uninit)]
 #![feature(char_internals)]
 #![feature(char_max_len)]
 #![feature(clone_to_uninit)]
diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index 5018ff4ad71..529b583cdd2 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -2357,7 +2357,7 @@ impl<T: Default> Default for Rc<T> {
     /// assert_eq!(*x, 0);
     /// ```
     #[inline]
-    fn default() -> Rc<T> {
+    fn default() -> Self {
         unsafe {
             Self::from_inner(
                 Box::leak(Box::write(
@@ -2373,7 +2373,7 @@ impl<T: Default> Default for Rc<T> {
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "more_rc_default_impls", since = "1.80.0")]
 impl Default for Rc<str> {
-    /// Creates an empty str inside an Rc
+    /// Creates an empty `str` inside an `Rc`.
     ///
     /// This may or may not share an allocation with other Rcs on the same thread.
     #[inline]
@@ -2387,7 +2387,7 @@ impl Default for Rc<str> {
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "more_rc_default_impls", since = "1.80.0")]
 impl<T> Default for Rc<[T]> {
-    /// Creates an empty `[T]` inside an Rc
+    /// Creates an empty `[T]` inside an `Rc`.
     ///
     /// This may or may not share an allocation with other Rcs on the same thread.
     #[inline]
@@ -2397,6 +2397,19 @@ impl<T> Default for Rc<[T]> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "pin_default_impls", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for Pin<Rc<T>>
+where
+    T: ?Sized,
+    Rc<T>: Default,
+{
+    #[inline]
+    fn default() -> Self {
+        unsafe { Pin::new_unchecked(Rc::<T>::default()) }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 trait RcEqIdent<T: ?Sized + PartialEq, A: Allocator> {
     fn eq(&self, other: &Rc<T, A>) -> bool;
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index b8925f4544f..29caa7bc539 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -3654,6 +3654,19 @@ impl<T> Default for Arc<[T]> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
+#[stable(feature = "pin_default_impls", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for Pin<Arc<T>>
+where
+    T: ?Sized,
+    Arc<T>: Default,
+{
+    #[inline]
+    fn default() -> Self {
+        unsafe { Pin::new_unchecked(Arc::<T>::default()) }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + Hash, A: Allocator> Hash for Arc<T, A> {
     fn hash<H: Hasher>(&self, state: &mut H) {
diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index 2321aab2c51..2e40227a058 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -49,7 +49,27 @@
 //! v[1] = v[1] + 5;
 //! ```
 //!
+//! # Memory layout
+//!
+//! When the type is non-zero-sized and the capacity is nonzero, [`Vec`] uses the [`Global`]
+//! allocator for its allocation. It is valid to convert both ways between such a [`Vec`] and a raw
+//! pointer allocated with the [`Global`] allocator, provided that the [`Layout`] used with the
+//! allocator is correct for a sequence of `capacity` elements of the type, and the first `len`
+//! values pointed to by the raw pointer are valid. More precisely, a `ptr: *mut T` that has been
+//! allocated with the [`Global`] allocator with [`Layout::array::<T>(capacity)`][Layout::array] may
+//! be converted into a vec using
+//! [`Vec::<T>::from_raw_parts(ptr, len, capacity)`](Vec::from_raw_parts). Conversely, the memory
+//! backing a `value: *mut T` obtained from [`Vec::<T>::as_mut_ptr`] may be deallocated using the
+//! [`Global`] allocator with the same layout.
+//!
+//! For zero-sized types (ZSTs), or when the capacity is zero, the `Vec` pointer must be non-null
+//! and sufficiently aligned. The recommended way to build a `Vec` of ZSTs if [`vec!`] cannot be
+//! used is to use [`ptr::NonNull::dangling`].
+//!
 //! [`push`]: Vec::push
+//! [`ptr::NonNull::dangling`]: NonNull::dangling
+//! [`Layout`]: crate::alloc::Layout
+//! [Layout::array]: crate::alloc::Layout::array
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
@@ -523,18 +543,23 @@ impl<T> Vec<T> {
     /// This is highly unsafe, due to the number of invariants that aren't
     /// checked:
     ///
-    /// * `ptr` must have been allocated using the global allocator, such as via
-    ///   the [`alloc::alloc`] function.
-    /// * `T` needs to have the same alignment as what `ptr` was allocated with.
+    /// * If `T` is not a zero-sized type and the capacity is nonzero, `ptr` must have
+    ///   been allocated using the global allocator, such as via the [`alloc::alloc`]
+    ///   function. If `T` is a zero-sized type or the capacity is zero, `ptr` need
+    ///   only be non-null and aligned.
+    /// * `T` needs to have the same alignment as what `ptr` was allocated with,
+    ///   if the pointer is required to be allocated.
     ///   (`T` having a less strict alignment is not sufficient, the alignment really
     ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
     ///   allocated and deallocated with the same layout.)
-    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
-    ///   to be the same size as the pointer was allocated with. (Because similar to
-    ///   alignment, [`dealloc`] must be called with the same layout `size`.)
+    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes), if
+    ///   nonzero, needs to be the same size as the pointer was allocated with.
+    ///   (Because similar to alignment, [`dealloc`] must be called with the same
+    ///   layout `size`.)
     /// * `length` needs to be less than or equal to `capacity`.
     /// * The first `length` values must be properly initialized values of type `T`.
-    /// * `capacity` needs to be the capacity that the pointer was allocated with.
+    /// * `capacity` needs to be the capacity that the pointer was allocated with,
+    ///   if the pointer is required to be allocated.
     /// * The allocated size in bytes must be no larger than `isize::MAX`.
     ///   See the safety documentation of [`pointer::offset`].
     ///
@@ -770,12 +795,16 @@ impl<T> Vec<T> {
     /// order as the arguments to [`from_raw_parts`].
     ///
     /// After calling this function, the caller is responsible for the
-    /// memory previously managed by the `Vec`. The only way to do
-    /// this is to convert the raw pointer, length, and capacity back
-    /// into a `Vec` with the [`from_raw_parts`] function, allowing
-    /// the destructor to perform the cleanup.
+    /// memory previously managed by the `Vec`. Most often, one does
+    /// this by converting the raw pointer, length, and capacity back
+    /// into a `Vec` with the [`from_raw_parts`] function; more generally,
+    /// if `T` is non-zero-sized and the capacity is nonzero, one may use
+    /// any method that calls [`dealloc`] with a layout of
+    /// `Layout::array::<T>(capacity)`; if `T` is zero-sized or the
+    /// capacity is zero, nothing needs to be done.
     ///
     /// [`from_raw_parts`]: Vec::from_raw_parts
+    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
     ///
     /// # Examples
     ///
@@ -1755,6 +1784,12 @@ impl<T, A: Allocator> Vec<T, A> {
     /// may still invalidate this pointer.
     /// See the second example below for how this guarantee can be used.
     ///
+    /// The method also guarantees that, as long as `T` is not zero-sized and the capacity is
+    /// nonzero, the pointer may be passed into [`dealloc`] with a layout of
+    /// `Layout::array::<T>(capacity)` in order to deallocate the backing memory. If this is done,
+    /// be careful not to run the destructor of the `Vec`, as dropping it will result in
+    /// double-frees. Wrapping the `Vec` in a [`ManuallyDrop`] is the typical way to achieve this.
+    ///
     /// # Examples
     ///
     /// ```
@@ -1787,9 +1822,24 @@ impl<T, A: Allocator> Vec<T, A> {
     /// }
     /// ```
     ///
+    /// Deallocating a vector using [`Box`] (which uses [`dealloc`] internally):
+    ///
+    /// ```
+    /// use std::mem::{ManuallyDrop, MaybeUninit};
+    ///
+    /// let mut v = ManuallyDrop::new(vec![0, 1, 2]);
+    /// let ptr = v.as_mut_ptr();
+    /// let capacity = v.capacity();
+    /// let slice_ptr: *mut [MaybeUninit<i32>] =
+    ///     std::ptr::slice_from_raw_parts_mut(ptr.cast(), capacity);
+    /// drop(unsafe { Box::from_raw(slice_ptr) });
+    /// ```
+    ///
     /// [`as_mut_ptr`]: Vec::as_mut_ptr
     /// [`as_ptr`]: Vec::as_ptr
     /// [`as_non_null`]: Vec::as_non_null
+    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
+    /// [`ManuallyDrop`]: core::mem::ManuallyDrop
     #[stable(feature = "vec_as_ptr", since = "1.37.0")]
     #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
     #[rustc_never_returns_null_ptr]
@@ -3126,7 +3176,7 @@ impl<T, A: Allocator> Vec<T, A> {
         // - but the allocation extends out to `self.buf.capacity()` elements, possibly
         // uninitialized
         let spare_ptr = unsafe { ptr.add(self.len) };
-        let spare_ptr = spare_ptr.cast::<MaybeUninit<T>>();
+        let spare_ptr = spare_ptr.cast_uninit();
         let spare_len = self.buf.capacity() - self.len;
 
         // SAFETY: