Auto merge of #116113 - kpreid:arcmut, r=dtolnay

 Generalize `{Rc,Arc}::make_mut()` to unsized types.

* `{Rc,Arc}::make_mut()` now accept any type implementing the new unstable trait `core::clone::CloneToUninit`.
* `CloneToUninit` is implemented for `T: Clone` and for `[T] where T: Clone`.
* `CloneToUninit` is a generalization of the existing internal trait `alloc::alloc::WriteCloneIntoRaw`.
* New feature gate: `clone_to_uninit`

This allows performing `make_mut()` on `Rc<[T]>` and `Arc<[T]>`, which was not previously possible.

---

Previous PR description, now obsolete:

>  Add `{Rc, Arc}::make_mut_slice()`
>
> These functions behave identically to `make_mut()`, but operate on `Arc<[T]>` instead of `Arc<T>`.
>
> This allows performing the operation on slices, which was not previously possible because `make_mut()` requires `T: Clone` (and slices, being `!Sized`, do not and currently cannot implement `Clone`).
>
> Feature gate: `make_mut_slice`

try-job: test-various

10 files changed, 492 insertions, 58 deletions

diff --git a/library/alloc/src/alloc.rs b/library/alloc/src/alloc.rs
index 6677534eafc..1833a7f477f 100644
--- a/library/alloc/src/alloc.rs
+++ b/library/alloc/src/alloc.rs
@@ -424,29 +424,3 @@ pub mod __alloc_error_handler {
         }
     }
 }
-
-#[cfg(not(no_global_oom_handling))]
-/// Specialize clones into pre-allocated, uninitialized memory.
-/// Used by `Box::clone` and `Rc`/`Arc::make_mut`.
-pub(crate) trait WriteCloneIntoRaw: Sized {
-    unsafe fn write_clone_into_raw(&self, target: *mut Self);
-}
-
-#[cfg(not(no_global_oom_handling))]
-impl<T: Clone> WriteCloneIntoRaw for T {
-    #[inline]
-    default unsafe fn write_clone_into_raw(&self, target: *mut Self) {
-        // Having allocated *first* may allow the optimizer to create
-        // the cloned value in-place, skipping the local and move.
-        unsafe { target.write(self.clone()) };
-    }
-}
-
-#[cfg(not(no_global_oom_handling))]
-impl<T: Copy> WriteCloneIntoRaw for T {
-    #[inline]
-    unsafe fn write_clone_into_raw(&self, target: *mut Self) {
-        // We can always copy in-place, without ever involving a local value.
-        unsafe { target.copy_from_nonoverlapping(self, 1) };
-    }
-}
diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index 01a954ed75b..1ec095a46f7 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -188,6 +188,8 @@
 use core::any::Any;
 use core::async_iter::AsyncIterator;
 use core::borrow;
+#[cfg(not(no_global_oom_handling))]
+use core::clone::CloneToUninit;
 use core::cmp::Ordering;
 use core::error::Error;
 use core::fmt;
@@ -207,7 +209,7 @@ use core::slice;
 use core::task::{Context, Poll};
 
 #[cfg(not(no_global_oom_handling))]
-use crate::alloc::{handle_alloc_error, WriteCloneIntoRaw};
+use crate::alloc::handle_alloc_error;
 use crate::alloc::{AllocError, Allocator, Global, Layout};
 #[cfg(not(no_global_oom_handling))]
 use crate::borrow::Cow;
@@ -1346,7 +1348,7 @@ impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
         // Pre-allocate memory to allow writing the cloned value directly.
         let mut boxed = Self::new_uninit_in(self.1.clone());
         unsafe {
-            (**self).write_clone_into_raw(boxed.as_mut_ptr());
+            (**self).clone_to_uninit(boxed.as_mut_ptr());
             boxed.assume_init()
         }
     }
diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index 022a14b931a..0f759d64f66 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -103,6 +103,7 @@
 #![feature(assert_matches)]
 #![feature(async_fn_traits)]
 #![feature(async_iterator)]
+#![feature(clone_to_uninit)]
 #![feature(coerce_unsized)]
 #![feature(const_align_of_val)]
 #![feature(const_box)]
diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index 2b7ab2f6e25..3745ecb48c1 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -249,6 +249,8 @@ use std::boxed::Box;
 use core::any::Any;
 use core::borrow;
 use core::cell::Cell;
+#[cfg(not(no_global_oom_handling))]
+use core::clone::CloneToUninit;
 use core::cmp::Ordering;
 use core::fmt;
 use core::hash::{Hash, Hasher};
@@ -268,8 +270,6 @@ use core::slice::from_raw_parts_mut;
 
 #[cfg(not(no_global_oom_handling))]
 use crate::alloc::handle_alloc_error;
-#[cfg(not(no_global_oom_handling))]
-use crate::alloc::WriteCloneIntoRaw;
 use crate::alloc::{AllocError, Allocator, Global, Layout};
 use crate::borrow::{Cow, ToOwned};
 #[cfg(not(no_global_oom_handling))]
@@ -1749,7 +1749,8 @@ impl<T: ?Sized, A: Allocator> Rc<T, A> {
     }
 }
 
-impl<T: Clone, A: Allocator + Clone> Rc<T, A> {
+#[cfg(not(no_global_oom_handling))]
+impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Rc<T, A> {
     /// Makes a mutable reference into the given `Rc`.
     ///
     /// If there are other `Rc` pointers to the same allocation, then `make_mut` will
@@ -1800,31 +1801,52 @@ impl<T: Clone, A: Allocator + Clone> Rc<T, A> {
     /// assert!(76 == *data);
     /// assert!(weak.upgrade().is_none());
     /// ```
-    #[cfg(not(no_global_oom_handling))]
     #[inline]
     #[stable(feature = "rc_unique", since = "1.4.0")]
     pub fn make_mut(this: &mut Self) -> &mut T {
+        let size_of_val = size_of_val::<T>(&**this);
+
         if Rc::strong_count(this) != 1 {
             // Gotta clone the data, there are other Rcs.
-            // Pre-allocate memory to allow writing the cloned value directly.
-            let mut rc = Self::new_uninit_in(this.alloc.clone());
-            unsafe {
-                let data = Rc::get_mut_unchecked(&mut rc);
-                (**this).write_clone_into_raw(data.as_mut_ptr());
-                *this = rc.assume_init();
-            }
+
+            let this_data_ref: &T = &**this;
+            // `in_progress` drops the allocation if we panic before finishing initializing it.
+            let mut in_progress: UniqueRcUninit<T, A> =
+                UniqueRcUninit::new(this_data_ref, this.alloc.clone());
+
+            // Initialize with clone of this.
+            let initialized_clone = unsafe {
+                // Clone. If the clone panics, `in_progress` will be dropped and clean up.
+                this_data_ref.clone_to_uninit(in_progress.data_ptr());
+                // Cast type of pointer, now that it is initialized.
+                in_progress.into_rc()
+            };
+
+            // Replace `this` with newly constructed Rc.
+            *this = initialized_clone;
         } else if Rc::weak_count(this) != 0 {
             // Can just steal the data, all that's left is Weaks
-            let mut rc = Self::new_uninit_in(this.alloc.clone());
+
+            // We don't need panic-protection like the above branch does, but we might as well
+            // use the same mechanism.
+            let mut in_progress: UniqueRcUninit<T, A> =
+                UniqueRcUninit::new(&**this, this.alloc.clone());
             unsafe {
-                let data = Rc::get_mut_unchecked(&mut rc);
-                data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
+                // Initialize `in_progress` with move of **this.
+                // We have to express this in terms of bytes because `T: ?Sized`; there is no
+                // operation that just copies a value based on its `size_of_val()`.
+                ptr::copy_nonoverlapping(
+                    ptr::from_ref(&**this).cast::<u8>(),
+                    in_progress.data_ptr().cast::<u8>(),
+                    size_of_val,
+                );
 
                 this.inner().dec_strong();
                 // Remove implicit strong-weak ref (no need to craft a fake
                 // Weak here -- we know other Weaks can clean up for us)
                 this.inner().dec_weak();
-                ptr::write(this, rc.assume_init());
+                // Replace `this` with newly constructed Rc that has the moved data.
+                ptr::write(this, in_progress.into_rc());
             }
         }
         // This unsafety is ok because we're guaranteed that the pointer
@@ -3686,3 +3708,67 @@ unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for UniqueRc<T, A> {
         }
     }
 }
+
+/// A unique owning pointer to a [`RcBox`] **that does not imply the contents are initialized,**
+/// but will deallocate it (without dropping the value) when dropped.
+///
+/// This is a helper for [`Rc::make_mut()`] to ensure correct cleanup on panic.
+/// It is nearly a duplicate of `UniqueRc<MaybeUninit<T>, A>` except that it allows `T: !Sized`,
+/// which `MaybeUninit` does not.
+#[cfg(not(no_global_oom_handling))]
+struct UniqueRcUninit<T: ?Sized, A: Allocator> {
+    ptr: NonNull<RcBox<T>>,
+    layout_for_value: Layout,
+    alloc: Option<A>,
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl<T: ?Sized, A: Allocator> UniqueRcUninit<T, A> {
+    /// Allocate a RcBox with layout suitable to contain `for_value` or a clone of it.
+    fn new(for_value: &T, alloc: A) -> UniqueRcUninit<T, A> {
+        let layout = Layout::for_value(for_value);
+        let ptr = unsafe {
+            Rc::allocate_for_layout(
+                layout,
+                |layout_for_rcbox| alloc.allocate(layout_for_rcbox),
+                |mem| mem.with_metadata_of(ptr::from_ref(for_value) as *const RcBox<T>),
+            )
+        };
+        Self { ptr: NonNull::new(ptr).unwrap(), layout_for_value: layout, alloc: Some(alloc) }
+    }
+
+    /// Returns the pointer to be written into to initialize the [`Rc`].
+    fn data_ptr(&mut self) -> *mut T {
+        let offset = data_offset_align(self.layout_for_value.align());
+        unsafe { self.ptr.as_ptr().byte_add(offset) as *mut T }
+    }
+
+    /// Upgrade this into a normal [`Rc`].
+    ///
+    /// # Safety
+    ///
+    /// The data must have been initialized (by writing to [`Self::data_ptr()`]).
+    unsafe fn into_rc(mut self) -> Rc<T, A> {
+        let ptr = self.ptr;
+        let alloc = self.alloc.take().unwrap();
+        mem::forget(self);
+        // SAFETY: The pointer is valid as per `UniqueRcUninit::new`, and the caller is responsible
+        // for having initialized the data.
+        unsafe { Rc::from_ptr_in(ptr.as_ptr(), alloc) }
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl<T: ?Sized, A: Allocator> Drop for UniqueRcUninit<T, A> {
+    fn drop(&mut self) {
+        // SAFETY:
+        // * new() produced a pointer safe to deallocate.
+        // * We own the pointer unless into_rc() was called, which forgets us.
+        unsafe {
+            self.alloc
+                .take()
+                .unwrap()
+                .deallocate(self.ptr.cast(), rcbox_layout_for_value_layout(self.layout_for_value));
+        }
+    }
+}
diff --git a/library/alloc/src/rc/tests.rs b/library/alloc/src/rc/tests.rs
index 0f09be7721f..5e2e4beb94a 100644
--- a/library/alloc/src/rc/tests.rs
+++ b/library/alloc/src/rc/tests.rs
@@ -316,6 +316,24 @@ fn test_cowrc_clone_weak() {
     assert!(cow1_weak.upgrade().is_none());
 }
 
+/// This is similar to the doc-test for `Rc::make_mut()`, but on an unsized type (slice).
+#[test]
+fn test_cowrc_unsized() {
+    use std::rc::Rc;
+
+    let mut data: Rc<[i32]> = Rc::new([10, 20, 30]);
+
+    Rc::make_mut(&mut data)[0] += 1; // Won't clone anything
+    let mut other_data = Rc::clone(&data); // Won't clone inner data
+    Rc::make_mut(&mut data)[1] += 1; // Clones inner data
+    Rc::make_mut(&mut data)[2] += 1; // Won't clone anything
+    Rc::make_mut(&mut other_data)[0] *= 10; // Won't clone anything
+
+    // Now `data` and `other_data` point to different allocations.
+    assert_eq!(*data, [11, 21, 31]);
+    assert_eq!(*other_data, [110, 20, 30]);
+}
+
 #[test]
 fn test_show() {
     let foo = Rc::new(75);
diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
index f9d884e0ea5..90672164cb9 100644
--- a/library/alloc/src/sync.rs
+++ b/library/alloc/src/sync.rs
@@ -10,6 +10,8 @@
 
 use core::any::Any;
 use core::borrow;
+#[cfg(not(no_global_oom_handling))]
+use core::clone::CloneToUninit;
 use core::cmp::Ordering;
 use core::fmt;
 use core::hash::{Hash, Hasher};
@@ -30,8 +32,6 @@ use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
 
 #[cfg(not(no_global_oom_handling))]
 use crate::alloc::handle_alloc_error;
-#[cfg(not(no_global_oom_handling))]
-use crate::alloc::WriteCloneIntoRaw;
 use crate::alloc::{AllocError, Allocator, Global, Layout};
 use crate::borrow::{Cow, ToOwned};
 use crate::boxed::Box;
@@ -2150,7 +2150,8 @@ unsafe impl<T: ?Sized, A: Allocator> DerefPure for Arc<T, A> {}
 #[unstable(feature = "receiver_trait", issue = "none")]
 impl<T: ?Sized> Receiver for Arc<T> {}
 
-impl<T: Clone, A: Allocator + Clone> Arc<T, A> {
+#[cfg(not(no_global_oom_handling))]
+impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Arc<T, A> {
     /// Makes a mutable reference into the given `Arc`.
     ///
     /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
@@ -2201,10 +2202,11 @@ impl<T: Clone, A: Allocator + Clone> Arc<T, A> {
     /// assert!(76 == *data);
     /// assert!(weak.upgrade().is_none());
     /// ```
-    #[cfg(not(no_global_oom_handling))]
     #[inline]
     #[stable(feature = "arc_unique", since = "1.4.0")]
     pub fn make_mut(this: &mut Self) -> &mut T {
+        let size_of_val = mem::size_of_val::<T>(&**this);
+
         // Note that we hold both a strong reference and a weak reference.
         // Thus, releasing our strong reference only will not, by itself, cause
         // the memory to be deallocated.
@@ -2215,13 +2217,19 @@ impl<T: Clone, A: Allocator + Clone> Arc<T, A> {
         // deallocated.
         if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
             // Another strong pointer exists, so we must clone.
-            // Pre-allocate memory to allow writing the cloned value directly.
-            let mut arc = Self::new_uninit_in(this.alloc.clone());
-            unsafe {
-                let data = Arc::get_mut_unchecked(&mut arc);
-                (**this).write_clone_into_raw(data.as_mut_ptr());
-                *this = arc.assume_init();
-            }
+
+            let this_data_ref: &T = &**this;
+            // `in_progress` drops the allocation if we panic before finishing initializing it.
+            let mut in_progress: UniqueArcUninit<T, A> =
+                UniqueArcUninit::new(this_data_ref, this.alloc.clone());
+
+            let initialized_clone = unsafe {
+                // Clone. If the clone panics, `in_progress` will be dropped and clean up.
+                this_data_ref.clone_to_uninit(in_progress.data_ptr());
+                // Cast type of pointer, now that it is initialized.
+                in_progress.into_arc()
+            };
+            *this = initialized_clone;
         } else if this.inner().weak.load(Relaxed) != 1 {
             // Relaxed suffices in the above because this is fundamentally an
             // optimization: we are always racing with weak pointers being
@@ -2240,11 +2248,22 @@ impl<T: Clone, A: Allocator + Clone> Arc<T, A> {
             let _weak = Weak { ptr: this.ptr, alloc: this.alloc.clone() };
 
             // Can just steal the data, all that's left is Weaks
-            let mut arc = Self::new_uninit_in(this.alloc.clone());
+            //
+            // We don't need panic-protection like the above branch does, but we might as well
+            // use the same mechanism.
+            let mut in_progress: UniqueArcUninit<T, A> =
+                UniqueArcUninit::new(&**this, this.alloc.clone());
             unsafe {
-                let data = Arc::get_mut_unchecked(&mut arc);
-                data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
-                ptr::write(this, arc.assume_init());
+                // Initialize `in_progress` with move of **this.
+                // We have to express this in terms of bytes because `T: ?Sized`; there is no
+                // operation that just copies a value based on its `size_of_val()`.
+                ptr::copy_nonoverlapping(
+                    ptr::from_ref(&**this).cast::<u8>(),
+                    in_progress.data_ptr().cast::<u8>(),
+                    size_of_val,
+                );
+
+                ptr::write(this, in_progress.into_arc());
             }
         } else {
             // We were the sole reference of either kind; bump back up the
@@ -3809,6 +3828,68 @@ fn data_offset_align(align: usize) -> usize {
     layout.size() + layout.padding_needed_for(align)
 }
 
+/// A unique owning pointer to a [`ArcInner`] **that does not imply the contents are initialized,**
+/// but will deallocate it (without dropping the value) when dropped.
+///
+/// This is a helper for [`Arc::make_mut()`] to ensure correct cleanup on panic.
+#[cfg(not(no_global_oom_handling))]
+struct UniqueArcUninit<T: ?Sized, A: Allocator> {
+    ptr: NonNull<ArcInner<T>>,
+    layout_for_value: Layout,
+    alloc: Option<A>,
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl<T: ?Sized, A: Allocator> UniqueArcUninit<T, A> {
+    /// Allocate a ArcInner with layout suitable to contain `for_value` or a clone of it.
+    fn new(for_value: &T, alloc: A) -> UniqueArcUninit<T, A> {
+        let layout = Layout::for_value(for_value);
+        let ptr = unsafe {
+            Arc::allocate_for_layout(
+                layout,
+                |layout_for_arcinner| alloc.allocate(layout_for_arcinner),
+                |mem| mem.with_metadata_of(ptr::from_ref(for_value) as *const ArcInner<T>),
+            )
+        };
+        Self { ptr: NonNull::new(ptr).unwrap(), layout_for_value: layout, alloc: Some(alloc) }
+    }
+
+    /// Returns the pointer to be written into to initialize the [`Arc`].
+    fn data_ptr(&mut self) -> *mut T {
+        let offset = data_offset_align(self.layout_for_value.align());
+        unsafe { self.ptr.as_ptr().byte_add(offset) as *mut T }
+    }
+
+    /// Upgrade this into a normal [`Arc`].
+    ///
+    /// # Safety
+    ///
+    /// The data must have been initialized (by writing to [`Self::data_ptr()`]).
+    unsafe fn into_arc(mut self) -> Arc<T, A> {
+        let ptr = self.ptr;
+        let alloc = self.alloc.take().unwrap();
+        mem::forget(self);
+        // SAFETY: The pointer is valid as per `UniqueArcUninit::new`, and the caller is responsible
+        // for having initialized the data.
+        unsafe { Arc::from_ptr_in(ptr.as_ptr(), alloc) }
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl<T: ?Sized, A: Allocator> Drop for UniqueArcUninit<T, A> {
+    fn drop(&mut self) {
+        // SAFETY:
+        // * new() produced a pointer safe to deallocate.
+        // * We own the pointer unless into_arc() was called, which forgets us.
+        unsafe {
+            self.alloc.take().unwrap().deallocate(
+                self.ptr.cast(),
+                arcinner_layout_for_value_layout(self.layout_for_value),
+            );
+        }
+    }
+}
+
 #[stable(feature = "arc_error", since = "1.52.0")]
 impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
     #[allow(deprecated, deprecated_in_future)]
diff --git a/library/alloc/tests/arc.rs b/library/alloc/tests/arc.rs
index d564a30b103..c37a80dca95 100644
--- a/library/alloc/tests/arc.rs
+++ b/library/alloc/tests/arc.rs
@@ -209,3 +209,21 @@ fn weak_may_dangle() {
     // `val` dropped here while still borrowed
     // borrow might be used here, when `val` is dropped and runs the `Drop` code for type `std::sync::Weak`
 }
+
+/// This is similar to the doc-test for `Arc::make_mut()`, but on an unsized type (slice).
+#[test]
+fn make_mut_unsized() {
+    use alloc::sync::Arc;
+
+    let mut data: Arc<[i32]> = Arc::new([10, 20, 30]);
+
+    Arc::make_mut(&mut data)[0] += 1; // Won't clone anything
+    let mut other_data = Arc::clone(&data); // Won't clone inner data
+    Arc::make_mut(&mut data)[1] += 1; // Clones inner data
+    Arc::make_mut(&mut data)[2] += 1; // Won't clone anything
+    Arc::make_mut(&mut other_data)[0] *= 10; // Won't clone anything
+
+    // Now `data` and `other_data` point to different allocations.
+    assert_eq!(*data, [11, 21, 31]);
+    assert_eq!(*other_data, [110, 20, 30]);
+}
diff --git a/library/core/src/clone.rs b/library/core/src/clone.rs
index d448c5338fc..d7ce65f6c53 100644
--- a/library/core/src/clone.rs
+++ b/library/core/src/clone.rs
@@ -36,6 +36,9 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use crate::mem::{self, MaybeUninit};
+use crate::ptr;
+
 /// A common trait for the ability to explicitly duplicate an object.
 ///
 /// Differs from [`Copy`] in that [`Copy`] is implicit and an inexpensive bit-wise copy, while
@@ -204,6 +207,189 @@ pub struct AssertParamIsCopy<T: Copy + ?Sized> {
     _field: crate::marker::PhantomData<T>,
 }
 
+/// A generalization of [`Clone`] to dynamically-sized types stored in arbitrary containers.
+///
+/// This trait is implemented for all types implementing [`Clone`], and also [slices](slice) of all
+/// such types. You may also implement this trait to enable cloning trait objects and custom DSTs
+/// (structures containing dynamically-sized fields).
+///
+/// # Safety
+///
+/// Implementations must ensure that when `.clone_to_uninit(dst)` returns normally rather than
+/// panicking, it always leaves `*dst` initialized as a valid value of type `Self`.
+///
+/// # See also
+///
+/// * [`Clone::clone_from`] is a safe function which may be used instead when `Self` is a [`Sized`]
+///   and the destination is already initialized; it may be able to reuse allocations owned by
+///   the destination.
+/// * [`ToOwned`], which allocates a new destination container.
+///
+/// [`ToOwned`]: ../../std/borrow/trait.ToOwned.html
+#[unstable(feature = "clone_to_uninit", issue = "126799")]
+pub unsafe trait CloneToUninit {
+    /// Performs copy-assignment from `self` to `dst`.
+    ///
+    /// This is analogous to to `std::ptr::write(dst, self.clone())`,
+    /// except that `self` may be a dynamically-sized type ([`!Sized`](Sized)).
+    ///
+    /// Before this function is called, `dst` may point to uninitialized memory.
+    /// After this function is called, `dst` will point to initialized memory; it will be
+    /// sound to create a `&Self` reference from the pointer.
+    ///
+    /// # Safety
+    ///
+    /// Behavior is undefined if any of the following conditions are violated:
+    ///
+    /// * `dst` must be [valid] for writes.
+    /// * `dst` must be properly aligned.
+    /// * `dst` must have the same [pointer metadata] (slice length or `dyn` vtable) as `self`.
+    ///
+    /// [valid]: ptr#safety
+    /// [pointer metadata]: crate::ptr::metadata()
+    ///
+    /// # Panics
+    ///
+    /// This function may panic. (For example, it might panic if memory allocation for a clone
+    /// of a value owned by `self` fails.)
+    /// If the call panics, then `*dst` should be treated as uninitialized memory; it must not be
+    /// read or dropped, because even if it was previously valid, it may have been partially
+    /// overwritten.
+    ///
+    /// The caller may also need to take care to deallocate the allocation pointed to by `dst`,
+    /// if applicable, to avoid a memory leak, and may need to take other precautions to ensure
+    /// soundness in the presence of unwinding.
+    ///
+    /// Implementors should avoid leaking values by, upon unwinding, dropping all component values
+    /// that might have already been created. (For example, if a `[Foo]` of length 3 is being
+    /// cloned, and the second of the three calls to `Foo::clone()` unwinds, then the first `Foo`
+    /// cloned should be dropped.)
+    unsafe fn clone_to_uninit(&self, dst: *mut Self);
+}
+
+#[unstable(feature = "clone_to_uninit", issue = "126799")]
+unsafe impl<T: Clone> CloneToUninit for T {
+    default unsafe fn clone_to_uninit(&self, dst: *mut Self) {
+        // SAFETY: The safety conditions of clone_to_uninit() are a superset of those of
+        // ptr::write().
+        unsafe {
+            // We hope the optimizer will figure out to create the cloned value in-place,
+            // skipping ever storing it on the stack and the copy to the destination.
+            ptr::write(dst, self.clone());
+        }
+    }
+}
+
+// Specialized implementation for types that are [`Copy`], not just [`Clone`],
+// and can therefore be copied bitwise.
+#[unstable(feature = "clone_to_uninit", issue = "126799")]
+unsafe impl<T: Copy> CloneToUninit for T {
+    unsafe fn clone_to_uninit(&self, dst: *mut Self) {
+        // SAFETY: The safety conditions of clone_to_uninit() are a superset of those of
+        // ptr::copy_nonoverlapping().
+        unsafe {
+            ptr::copy_nonoverlapping(self, dst, 1);
+        }
+    }
+}
+
+#[unstable(feature = "clone_to_uninit", issue = "126799")]
+unsafe impl<T: Clone> CloneToUninit for [T] {
+    #[cfg_attr(debug_assertions, track_caller)]
+    default unsafe fn clone_to_uninit(&self, dst: *mut Self) {
+        let len = self.len();
+        // This is the most likely mistake to make, so check it as a debug assertion.
+        debug_assert_eq!(
+            len,
+            dst.len(),
+            "clone_to_uninit() source and destination must have equal lengths",
+        );
+
+        // SAFETY: The produced `&mut` is valid because:
+        // * The caller is obligated to provide a pointer which is valid for writes.
+        // * All bytes pointed to are in MaybeUninit, so we don't care about the memory's
+        //   initialization status.
+        let uninit_ref = unsafe { &mut *(dst as *mut [MaybeUninit<T>]) };
+
+        // Copy the elements
+        let mut initializing = InitializingSlice::from_fully_uninit(uninit_ref);
+        for element_ref in self.iter() {
+            // If the clone() panics, `initializing` will take care of the cleanup.
+            initializing.push(element_ref.clone());
+        }
+        // If we reach here, then the entire slice is initialized, and we've satisfied our
+        // responsibilities to the caller. Disarm the cleanup guard by forgetting it.
+        mem::forget(initializing);
+    }
+}
+
+#[unstable(feature = "clone_to_uninit", issue = "126799")]
+unsafe impl<T: Copy> CloneToUninit for [T] {
+    #[cfg_attr(debug_assertions, track_caller)]
+    unsafe fn clone_to_uninit(&self, dst: *mut Self) {
+        let len = self.len();
+        // This is the most likely mistake to make, so check it as a debug assertion.
+        debug_assert_eq!(
+            len,
+            dst.len(),
+            "clone_to_uninit() source and destination must have equal lengths",
+        );
+
+        // SAFETY: The safety conditions of clone_to_uninit() are a superset of those of
+        // ptr::copy_nonoverlapping().
+        unsafe {
+            ptr::copy_nonoverlapping(self.as_ptr(), dst.as_mut_ptr(), len);
+        }
+    }
+}
+
+/// Ownership of a collection of values stored in a non-owned `[MaybeUninit<T>]`, some of which
+/// are not yet initialized. This is sort of like a `Vec` that doesn't own its allocation.
+/// Its responsibility is to provide cleanup on unwind by dropping the values that *are*
+/// initialized, unless disarmed by forgetting.
+///
+/// This is a helper for `impl<T: Clone> CloneToUninit for [T]`.
+struct InitializingSlice<'a, T> {
+    data: &'a mut [MaybeUninit<T>],
+    /// Number of elements of `*self.data` that are initialized.
+    initialized_len: usize,
+}
+
+impl<'a, T> InitializingSlice<'a, T> {
+    #[inline]
+    fn from_fully_uninit(data: &'a mut [MaybeUninit<T>]) -> Self {
+        Self { data, initialized_len: 0 }
+    }
+
+    /// Push a value onto the end of the initialized part of the slice.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the slice is already fully initialized.
+    #[inline]
+    fn push(&mut self, value: T) {
+        MaybeUninit::write(&mut self.data[self.initialized_len], value);
+        self.initialized_len += 1;
+    }
+}
+
+impl<'a, T> Drop for InitializingSlice<'a, T> {
+    #[cold] // will only be invoked on unwind
+    fn drop(&mut self) {
+        let initialized_slice = ptr::slice_from_raw_parts_mut(
+            MaybeUninit::slice_as_mut_ptr(self.data),
+            self.initialized_len,
+        );
+        // SAFETY:
+        // * the pointer is valid because it was made from a mutable reference
+        // * `initialized_len` counts the initialized elements as an invariant of this type,
+        //   so each of the pointed-to elements is initialized and may be dropped.
+        unsafe {
+            ptr::drop_in_place::<[T]>(initialized_slice);
+        }
+    }
+}
+
 /// Implementations of `Clone` for primitive types.
 ///
 /// Implementations that cannot be described in Rust
diff --git a/library/core/tests/clone.rs b/library/core/tests/clone.rs
index 64193e11558..23efab2f1b5 100644
--- a/library/core/tests/clone.rs
+++ b/library/core/tests/clone.rs
@@ -1,3 +1,6 @@
+use core::clone::CloneToUninit;
+use core::mem::MaybeUninit;
+
 #[test]
 #[allow(suspicious_double_ref_op)]
 fn test_borrowed_clone() {
@@ -14,3 +17,66 @@ fn test_clone_from() {
     b.clone_from(&a);
     assert_eq!(*b, 5);
 }
+
+#[test]
+fn test_clone_to_uninit_slice_success() {
+    // Using `String`s to exercise allocation and Drop of the individual elements;
+    // if something is aliased or double-freed, at least Miri will catch that.
+    let a: [String; 3] = ["a", "b", "c"].map(String::from);
+
+    let mut storage: MaybeUninit<[String; 3]> = MaybeUninit::uninit();
+    let b: [String; 3] = unsafe {
+        a[..].clone_to_uninit(storage.as_mut_ptr() as *mut [String]);
+        storage.assume_init()
+    };
+
+    assert_eq!(a, b);
+}
+
+#[test]
+#[cfg(panic = "unwind")]
+fn test_clone_to_uninit_slice_drops_on_panic() {
+    use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};
+
+    /// A static counter is OK to use as long as _this one test_ isn't run several times in
+    /// multiple threads.
+    static COUNTER: AtomicUsize = AtomicUsize::new(0);
+    /// Counts how many instances are live, and panics if a fifth one is created
+    struct CountsDropsAndPanics {}
+    impl CountsDropsAndPanics {
+        fn new() -> Self {
+            COUNTER.fetch_add(1, Relaxed);
+            Self {}
+        }
+    }
+    impl Clone for CountsDropsAndPanics {
+        fn clone(&self) -> Self {
+            if COUNTER.load(Relaxed) == 4 { panic!("intentional panic") } else { Self::new() }
+        }
+    }
+    impl Drop for CountsDropsAndPanics {
+        fn drop(&mut self) {
+            COUNTER.fetch_sub(1, Relaxed);
+        }
+    }
+
+    let a: [CountsDropsAndPanics; 3] = core::array::from_fn(|_| CountsDropsAndPanics::new());
+    assert_eq!(COUNTER.load(Relaxed), 3);
+
+    let panic_payload = std::panic::catch_unwind(|| {
+        let mut storage: MaybeUninit<[CountsDropsAndPanics; 3]> = MaybeUninit::uninit();
+        // This should panic halfway through
+        unsafe {
+            a[..].clone_to_uninit(storage.as_mut_ptr() as *mut [CountsDropsAndPanics]);
+        }
+    })
+    .unwrap_err();
+    assert_eq!(panic_payload.downcast().unwrap(), Box::new("intentional panic"));
+
+    // Check for lack of leak, which is what this test is looking for
+    assert_eq!(COUNTER.load(Relaxed), 3, "leaked during clone!");
+
+    // Might as well exercise the rest of the drops
+    drop(a);
+    assert_eq!(COUNTER.load(Relaxed), 0);
+}
diff --git a/library/core/tests/lib.rs b/library/core/tests/lib.rs
index 6845a630dff..3a2c98db0d5 100644
--- a/library/core/tests/lib.rs
+++ b/library/core/tests/lib.rs
@@ -8,6 +8,7 @@
 #![feature(async_iterator)]
 #![feature(bigint_helper_methods)]
 #![feature(cell_update)]
+#![feature(clone_to_uninit)]
 #![feature(const_align_offset)]
 #![feature(const_align_of_val_raw)]
 #![feature(const_black_box)]
@@ -53,6 +54,7 @@
 #![feature(slice_split_once)]
 #![feature(split_as_slice)]
 #![feature(maybe_uninit_fill)]
+#![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(maybe_uninit_write_slice)]
 #![feature(maybe_uninit_uninit_array_transpose)]