Replace WriteCloneIntoRaw with CloneToUninit.

Author: kpreid

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) };
-    }
-}

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()
         }
     }

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)]

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))]
@@ -1810,7 +1810,7 @@ impl<T: Clone, A: Allocator + Clone> Rc<T, A> {
             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).clone_to_uninit(data.as_mut_ptr());
                 *this = rc.assume_init();
             }
         } else if Rc::weak_count(this) != 0 {

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;
@@ -2219,7 +2219,7 @@ impl<T: Clone, A: Allocator + Clone> Arc<T, A> {
             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).clone_to_uninit(data.as_mut_ptr());
                 *this = arc.assume_init();
             }
         } else if this.inner().weak.load(Relaxed) != 1 {