Generalize Box::clone

Author: a1phyr

library/alloc/src/boxed.rs

@@ -205,8 +205,6 @@ use core::task::{Context, Poll};
 use crate::alloc::handle_alloc_error;
 use crate::alloc::{AllocError, Allocator, Global, Layout};
 use crate::raw_vec::RawVec;
-#[cfg(not(no_global_oom_handling))]
-use crate::str::from_boxed_utf8_unchecked;
 
 /// Conversion related impls for `Box<_>` (`From`, `downcast`, etc)
 mod convert;
@@ -1733,7 +1731,7 @@ where
 
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
+impl<T: CloneToUninit + ?Sized, A: Allocator + Clone> Clone for Box<T, A> {
     /// Returns a new box with a `clone()` of this box's contents.
     ///
     /// # Examples
@@ -1751,14 +1749,21 @@ impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
     #[inline]
     fn clone(&self) -> Self {
         // Pre-allocate memory to allow writing the cloned value directly.
-        let mut boxed = Self::new_uninit_in(self.1.clone());
+        let mut boxed = unsafe {
+            uninit::UninitBox::<T, A>::new_for_metadata_in(
+                ptr::metadata(Self::as_ptr(self)),
+                self.1.clone(),
+            )
+        };
+
         unsafe {
             (**self).clone_to_uninit(boxed.as_mut_ptr().cast());
             boxed.assume_init()
         }
     }
 
-    /// Copies `source`'s contents into `self` without creating a new allocation.
+    /// Copies `source`'s contents into `self` without creating a new allocation,
+    /// so long as the two have the same metadata.
     ///
     /// # Examples
     ///
@@ -1777,55 +1782,16 @@ impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
     /// ```
     #[inline]
     fn clone_from(&mut self, source: &Self) {
-        (**self).clone_from(&(**source));
-    }
-}
-
-#[cfg(not(no_global_oom_handling))]
-#[stable(feature = "box_slice_clone", since = "1.3.0")]
-impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
-    fn clone(&self) -> Self {
-        let alloc = Box::allocator(self).clone();
-        self.to_vec_in(alloc).into_boxed_slice()
-    }
-
-    /// Copies `source`'s contents into `self` without creating a new allocation,
-    /// so long as the two are of the same length.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// let x = Box::new([5, 6, 7]);
-    /// let mut y = Box::new([8, 9, 10]);
-    /// let yp: *const [i32] = &*y;
-    ///
-    /// y.clone_from(&x);
-    ///
-    /// // The value is the same
-    /// assert_eq!(x, y);
-    ///
-    /// // And no allocation occurred
-    /// assert_eq!(yp, &*y);
-    /// ```
-    fn clone_from(&mut self, source: &Self) {
-        if self.len() == source.len() {
-            self.clone_from_slice(&source);
+        // The following comparison may fail for vtable pointers even if both
+        // types are the same, in which case we take the slow path.
+        if ptr::metadata(self) == ptr::metadata(source) {
+            unsafe { (**source).clone_to_init(Box::as_mut_ptr(self).cast()) };
         } else {
             *self = source.clone();
         }
     }
 }
 
-#[cfg(not(no_global_oom_handling))]
-#[stable(feature = "box_slice_clone", since = "1.3.0")]
-impl Clone for Box<str> {
-    fn clone(&self) -> Self {
-        // this makes a copy of the data
-        let buf: Box<[u8]> = self.as_bytes().into();
-        unsafe { from_boxed_utf8_unchecked(buf) }
-    }
-}
-
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
     #[inline]

library/alloc/src/bstr.rs

@@ -589,14 +589,6 @@ impl<'a> TryFrom<&'a ByteString> for &'a str {
 
 // Additional impls for `ByteStr` that require types from `alloc`:
 
-#[unstable(feature = "bstr", issue = "134915")]
-impl Clone for Box<ByteStr> {
-    #[inline]
-    fn clone(&self) -> Self {
-        Self::from(Box::<[u8]>::from(&self.0))
-    }
-}
-
 #[unstable(feature = "bstr", issue = "134915")]
 impl<'a> From<&'a ByteStr> for Cow<'a, ByteStr> {
     #[inline]

library/alloc/src/ffi/c_str.rs

@@ -851,14 +851,6 @@ impl TryFrom<CString> for String {
     }
 }
 
-#[stable(feature = "more_box_slice_clone", since = "1.29.0")]
-impl Clone for Box<CStr> {
-    #[inline]
-    fn clone(&self) -> Self {
-        (**self).into()
-    }
-}
-
 #[stable(feature = "box_from_c_string", since = "1.20.0")]
 impl From<CString> for Box<CStr> {
     /// Converts a [`CString`] into a <code>[Box]<[CStr]></code> without copying or allocating.

library/std/src/ffi/os_str.rs

@@ -1329,14 +1329,6 @@ impl From<OsString> for Box<OsStr> {
     }
 }
 
-#[stable(feature = "more_box_slice_clone", since = "1.29.0")]
-impl Clone for Box<OsStr> {
-    #[inline]
-    fn clone(&self) -> Self {
-        self.to_os_string().into_boxed_os_str()
-    }
-}
-
 #[unstable(feature = "clone_to_uninit", issue = "126799")]
 unsafe impl CloneToUninit for OsStr {
     #[inline]

library/std/src/path.rs

@@ -1825,14 +1825,6 @@ impl From<PathBuf> for Box<Path> {
     }
 }
 
-#[stable(feature = "more_box_slice_clone", since = "1.29.0")]
-impl Clone for Box<Path> {
-    #[inline]
-    fn clone(&self) -> Self {
-        self.to_path_buf().into_boxed_path()
-    }
-}
-
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + AsRef<OsStr>> From<&T> for PathBuf {
     /// Converts a borrowed [`OsStr`] to a [`PathBuf`].