Make CloneToUninit dyn-compatible

Author: zachs18

alloc/src/boxed.rs

@@ -1735,7 +1735,7 @@ 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).clone_to_uninit(boxed.as_mut_ptr());
+            (**self).clone_to_uninit(boxed.as_mut_ptr().cast());
             boxed.assume_init()
         }
     }

alloc/src/rc.rs

@@ -1876,7 +1876,7 @@ impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Rc<T, A> {
             // 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());
+                this_data_ref.clone_to_uninit(in_progress.data_ptr().cast());
                 // Cast type of pointer, now that it is initialized.
                 in_progress.into_rc()
             };

alloc/src/sync.rs

@@ -2272,7 +2272,7 @@ impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Arc<T, A> {
 
             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());
+                this_data_ref.clone_to_uninit(in_progress.data_ptr().cast());
                 // Cast type of pointer, now that it is initialized.
                 in_progress.into_arc()
             };

core/src/clone.rs

@@ -232,20 +232,20 @@ pub struct AssertParamIsCopy<T: Copy + ?Sized> {
 pub unsafe trait CloneToUninit {
     /// Performs copy-assignment from `self` to `dst`.
     ///
-    /// This is analogous to `std::ptr::write(dst, self.clone())`,
+    /// This is analogous to `std::ptr::write(dst.cast(), 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.
+    /// sound to create a `&Self` reference from the pointer with the [pointer metadata]
+    /// from `self`.
     ///
     /// # 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`.
+    /// * `dst` must be [valid] for writes for `std::mem::size_of_val(self)` bytes.
+    /// * `dst` must be properly aligned to `std::mem::align_of_val(self)`.
     ///
     /// [valid]: crate::ptr#safety
     /// [pointer metadata]: crate::ptr::metadata()
@@ -266,23 +266,24 @@ pub unsafe trait CloneToUninit {
     /// 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);
+    unsafe fn clone_to_uninit(&self, dst: *mut u8);
 }
 
 #[unstable(feature = "clone_to_uninit", issue = "126799")]
 unsafe impl<T: Clone> CloneToUninit for T {
     #[inline]
-    unsafe fn clone_to_uninit(&self, dst: *mut Self) {
+    unsafe fn clone_to_uninit(&self, dst: *mut u8) {
         // SAFETY: we're calling a specialization with the same contract
-        unsafe { <T as self::uninit::CopySpec>::clone_one(self, dst) }
+        unsafe { <T as self::uninit::CopySpec>::clone_one(self, dst.cast::<T>()) }
     }
 }
 
 #[unstable(feature = "clone_to_uninit", issue = "126799")]
 unsafe impl<T: Clone> CloneToUninit for [T] {
     #[inline]
     #[cfg_attr(debug_assertions, track_caller)]
-    unsafe fn clone_to_uninit(&self, dst: *mut Self) {
+    unsafe fn clone_to_uninit(&self, dst: *mut u8) {
+        let dst: *mut [T] = dst.with_metadata_of(self);
         // SAFETY: we're calling a specialization with the same contract
         unsafe { <T as self::uninit::CopySpec>::clone_slice(self, dst) }
     }
@@ -292,21 +293,21 @@ unsafe impl<T: Clone> CloneToUninit for [T] {
 unsafe impl CloneToUninit for str {
     #[inline]
     #[cfg_attr(debug_assertions, track_caller)]
-    unsafe fn clone_to_uninit(&self, dst: *mut Self) {
+    unsafe fn clone_to_uninit(&self, dst: *mut u8) {
         // SAFETY: str is just a [u8] with UTF-8 invariant
-        unsafe { self.as_bytes().clone_to_uninit(dst as *mut [u8]) }
+        unsafe { self.as_bytes().clone_to_uninit(dst) }
     }
 }
 
 #[unstable(feature = "clone_to_uninit", issue = "126799")]
 unsafe impl CloneToUninit for crate::ffi::CStr {
     #[cfg_attr(debug_assertions, track_caller)]
-    unsafe fn clone_to_uninit(&self, dst: *mut Self) {
+    unsafe fn clone_to_uninit(&self, dst: *mut u8) {
         // SAFETY: For now, CStr is just a #[repr(trasnsparent)] [c_char] with some invariants.
         // And we can cast [c_char] to [u8] on all supported platforms (see: to_bytes_with_nul).
-        // The pointer metadata properly preserves the length (NUL included).
+        // The pointer metadata properly preserves the length (so NUL is also copied).
         // See: `cstr_metadata_is_length_with_nul` in tests.
-        unsafe { self.to_bytes_with_nul().clone_to_uninit(dst as *mut [u8]) }
+        unsafe { self.to_bytes_with_nul().clone_to_uninit(dst) }
     }
 }
 

std/src/ffi/os_str.rs

@@ -112,7 +112,7 @@ impl crate::sealed::Sealed for OsString {}
 /// [conversions]: super#conversions
 #[cfg_attr(not(test), rustc_diagnostic_item = "OsStr")]
 #[stable(feature = "rust1", since = "1.0.0")]
-// `OsStr::from_inner` current implementation relies
+// `OsStr::from_inner` and `impl CloneToUninit for OsStr` current implementation relies
 // on `OsStr` being layout-compatible with `Slice`.
 // However, `OsStr` layout is considered an implementation detail and must not be relied upon.
 #[repr(transparent)]
@@ -1278,9 +1278,9 @@ impl Clone for Box<OsStr> {
 unsafe impl CloneToUninit for OsStr {
     #[inline]
     #[cfg_attr(debug_assertions, track_caller)]
-    unsafe fn clone_to_uninit(&self, dst: *mut Self) {
-        // SAFETY: we're just a wrapper around a platform-specific Slice
-        unsafe { self.inner.clone_to_uninit(&raw mut (*dst).inner) }
+    unsafe fn clone_to_uninit(&self, dst: *mut u8) {
+        // SAFETY: we're just a transparent wrapper around a platform-specific Slice
+        unsafe { self.inner.clone_to_uninit(dst) }
     }
 }
 

std/src/ffi/os_str/tests.rs

@@ -294,12 +294,12 @@ fn clone_to_uninit() {
     let a = OsStr::new("hello.txt");
 
     let mut storage = vec![MaybeUninit::<u8>::uninit(); size_of_val::<OsStr>(a)];
-    unsafe { a.clone_to_uninit(ptr::from_mut::<[_]>(storage.as_mut_slice()) as *mut OsStr) };
+    unsafe { a.clone_to_uninit(ptr::from_mut::<[_]>(storage.as_mut_slice()).cast()) };
     assert_eq!(a.as_encoded_bytes(), unsafe { MaybeUninit::slice_assume_init_ref(&storage) });
 
     let mut b: Box<OsStr> = OsStr::new("world.exe").into();
     assert_eq!(size_of_val::<OsStr>(a), size_of_val::<OsStr>(&b));
     assert_ne!(a, &*b);
-    unsafe { a.clone_to_uninit(ptr::from_mut::<OsStr>(&mut b)) };
+    unsafe { a.clone_to_uninit(ptr::from_mut::<OsStr>(&mut b).cast()) };
     assert_eq!(a, &*b);
 }

std/src/path.rs

@@ -2128,7 +2128,7 @@ impl AsRef<OsStr> for PathBuf {
 /// ```
 #[cfg_attr(not(test), rustc_diagnostic_item = "Path")]
 #[stable(feature = "rust1", since = "1.0.0")]
-// `Path::new` current implementation relies
+// `Path::new` and `impl CloneToUninit for Path` current implementation relies
 // on `Path` being layout-compatible with `OsStr`.
 // However, `Path` layout is considered an implementation detail and must not be relied upon.
 #[repr(transparent)]
@@ -3170,9 +3170,9 @@ impl Path {
 unsafe impl CloneToUninit for Path {
     #[inline]
     #[cfg_attr(debug_assertions, track_caller)]
-    unsafe fn clone_to_uninit(&self, dst: *mut Self) {
-        // SAFETY: Path is just a wrapper around OsStr
-        unsafe { self.inner.clone_to_uninit(&raw mut (*dst).inner) }
+    unsafe fn clone_to_uninit(&self, dst: *mut u8) {
+        // SAFETY: Path is just a transparent wrapper around OsStr
+        unsafe { self.inner.clone_to_uninit(dst) }
     }
 }
 

std/src/path/tests.rs

@@ -2068,14 +2068,14 @@ fn clone_to_uninit() {
     let a = Path::new("hello.txt");
 
     let mut storage = vec![MaybeUninit::<u8>::uninit(); size_of_val::<Path>(a)];
-    unsafe { a.clone_to_uninit(ptr::from_mut::<[_]>(storage.as_mut_slice()) as *mut Path) };
+    unsafe { a.clone_to_uninit(ptr::from_mut::<[_]>(storage.as_mut_slice()).cast()) };
     assert_eq!(a.as_os_str().as_encoded_bytes(), unsafe {
         MaybeUninit::slice_assume_init_ref(&storage)
     });
 
     let mut b: Box<Path> = Path::new("world.exe").into();
     assert_eq!(size_of_val::<Path>(a), size_of_val::<Path>(&b));
     assert_ne!(a, &*b);
-    unsafe { a.clone_to_uninit(ptr::from_mut::<Path>(&mut b)) };
+    unsafe { a.clone_to_uninit(ptr::from_mut::<Path>(&mut b).cast()) };
     assert_eq!(a, &*b);
 }

std/src/sys/os_str/bytes.rs

@@ -352,8 +352,8 @@ impl Slice {
 unsafe impl CloneToUninit for Slice {
     #[inline]
     #[cfg_attr(debug_assertions, track_caller)]
-    unsafe fn clone_to_uninit(&self, dst: *mut Self) {
-        // SAFETY: we're just a wrapper around [u8]
-        unsafe { self.inner.clone_to_uninit(&raw mut (*dst).inner) }
+    unsafe fn clone_to_uninit(&self, dst: *mut u8) {
+        // SAFETY: we're just a transparent wrapper around [u8]
+        unsafe { self.inner.clone_to_uninit(dst) }
     }
 }

std/src/sys/os_str/wtf8.rs

@@ -275,8 +275,8 @@ impl Slice {
 unsafe impl CloneToUninit for Slice {
     #[inline]
     #[cfg_attr(debug_assertions, track_caller)]
-    unsafe fn clone_to_uninit(&self, dst: *mut Self) {
-        // SAFETY: we're just a wrapper around Wtf8
-        unsafe { self.inner.clone_to_uninit(&raw mut (*dst).inner) }
+    unsafe fn clone_to_uninit(&self, dst: *mut u8) {
+        // SAFETY: we're just a transparent wrapper around Wtf8
+        unsafe { self.inner.clone_to_uninit(dst) }
     }
 }