Try to avoid locals when cloning into Box/Rc/Arc

library/alloc/src/alloc.rs

@@ -397,3 +397,26 @@ pub mod __alloc_error_handler {
         unsafe { oom_impl(layout) }
     }
 }
+
+/// 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);
+}
+
+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()) };
+    }
+}
+
+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

@@ -151,7 +151,7 @@ use core::pin::Pin;
 use core::ptr::{self, Unique};
 use core::task::{Context, Poll};
 
-use crate::alloc::{handle_alloc_error, AllocError, Allocator, Global, Layout};
+use crate::alloc::{handle_alloc_error, AllocError, Allocator, Global, Layout, WriteCloneIntoRaw};
 use crate::borrow::Cow;
 use crate::raw_vec::RawVec;
 use crate::str::from_boxed_utf8_unchecked;
@@ -1014,10 +1014,14 @@ impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
     /// // But they are unique objects
     /// assert_ne!(&*x as *const i32, &*y as *const i32);
     /// ```
-    #[rustfmt::skip]
     #[inline]
     fn clone(&self) -> Self {
-        Self::new_in((**self).clone(), self.1.clone())
+        // 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());
+            boxed.assume_init()
+        }
     }
 
     /// Copies `source`'s contents into `self` without creating a new allocation.

library/alloc/src/rc.rs

@@ -263,7 +263,9 @@ use core::pin::Pin;
 use core::ptr::{self, NonNull};
 use core::slice::from_raw_parts_mut;
 
-use crate::alloc::{box_free, handle_alloc_error, AllocError, Allocator, Global, Layout};
+use crate::alloc::{
+    box_free, handle_alloc_error, AllocError, Allocator, Global, Layout, WriteCloneIntoRaw,
+};
 use crate::borrow::{Cow, ToOwned};
 use crate::string::String;
 use crate::vec::Vec;
@@ -1037,18 +1039,26 @@ impl<T: Clone> Rc<T> {
     #[stable(feature = "rc_unique", since = "1.4.0")]
     pub fn make_mut(this: &mut Self) -> &mut T {
         if Rc::strong_count(this) != 1 {
-            // Gotta clone the data, there are other Rcs
-            *this = Rc::new((**this).clone())
+            // Gotta clone the data, there are other Rcs.
+            // Pre-allocate memory to allow writing the cloned value directly.
+            let mut rc = Self::new_uninit();
+            unsafe {
+                let data = Rc::get_mut_unchecked(&mut rc);
+                (**this).write_clone_into_raw(data.as_mut_ptr());
+                *this = rc.assume_init();
+            }
         } else if Rc::weak_count(this) != 0 {
             // Can just steal the data, all that's left is Weaks
+            let mut rc = Self::new_uninit();
             unsafe {
-                let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value));
-                mem::swap(this, &mut swap);
-                swap.inner().dec_strong();
+                let data = Rc::get_mut_unchecked(&mut rc);
+                data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
+
+                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)
-                swap.inner().dec_weak();
-                forget(swap);
+                this.inner().dec_weak();
+                ptr::write(this, rc.assume_init());
             }
         }
         // This unsafety is ok because we're guaranteed that the pointer

library/alloc/src/sync.rs

@@ -22,7 +22,9 @@ use core::slice::from_raw_parts_mut;
 use core::sync::atomic;
 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
 
-use crate::alloc::{box_free, handle_alloc_error, AllocError, Allocator, Global, Layout};
+use crate::alloc::{
+    box_free, handle_alloc_error, AllocError, Allocator, Global, Layout, WriteCloneIntoRaw,
+};
 use crate::borrow::{Cow, ToOwned};
 use crate::boxed::Box;
 use crate::rc::is_dangling;
@@ -1369,8 +1371,14 @@ impl<T: Clone> Arc<T> {
         // weak count, there's no chance the ArcInner itself could be
         // deallocated.
         if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
-            // Another strong pointer exists; clone
-            *this = Arc::new((**this).clone());
+            // Another strong pointer exists, so we must clone.
+            // Pre-allocate memory to allow writing the cloned value directly.
+            let mut arc = Self::new_uninit();
+            unsafe {
+                let data = Arc::get_mut_unchecked(&mut arc);
+                (**this).write_clone_into_raw(data.as_mut_ptr());
+                *this = arc.assume_init();
+            }
         } 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
@@ -1386,17 +1394,14 @@ impl<T: Clone> Arc<T> {
 
             // Materialize our own implicit weak pointer, so that it can clean
             // up the ArcInner as needed.
-            let weak = Weak { ptr: this.ptr };
+            let _weak = Weak { ptr: this.ptr };
 
-            // mark the data itself as already deallocated
+            // Can just steal the data, all that's left is Weaks
+            let mut arc = Self::new_uninit();
             unsafe {
-                // there is no data race in the implicit write caused by `read`
-                // here (due to zeroing) because data is no longer accessed by
-                // other threads (due to there being no more strong refs at this
-                // point).
-                let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
-                mem::swap(this, &mut swap);
-                mem::forget(swap);
+                let data = Arc::get_mut_unchecked(&mut arc);
+                data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
+                ptr::write(this, arc.assume_init());
             }
         } else {
             // We were the sole reference of either kind; bump back up the