diff --git a/library/core/src/clone.rs b/library/core/src/clone.rs
index c777dd995a656..e0ac0bfc5289f 100644
--- a/library/core/src/clone.rs
+++ b/library/core/src/clone.rs
@@ -262,34 +262,150 @@ pub struct AssertParamIsCopy<T: Copy + ?Sized> {
_field: crate::marker::PhantomData<T>,
}
-/// A generalization of [`Clone`] to dynamically-sized types stored in arbitrary containers.
+/// A generalization of [`Clone`] to [dynamically-sized types][DST] 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).
+/// This trait is implemented for all types implementing [`Clone`], [slices](slice) of all
+/// such types, and other dynamically-sized types in the standard library.
+/// You may also implement this trait to enable cloning custom DSTs
+/// (structures containing dynamically-sized fields), or use it as a supertrait to enable
+/// cloning a [trait object].
+///
+/// This trait is normally used via operations on container types which support DSTs,
+/// so you should not typically need to call `.clone_to_uninit()` explicitly except when
+/// implementing such a container or otherwise performing explicit management of an allocation,
+/// or when implementing `CloneToUninit` itself.
///
/// # 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`.
+/// Implementations must ensure that when `.clone_to_uninit(dest)` returns normally rather than
+/// panicking, it always leaves `*dest` initialized as a valid value of type `Self`.
+///
+/// # Examples
+///
+// FIXME(#126799): when `Box::clone` allows use of `CloneToUninit`, rewrite these examples with it
+// since `Rc` is a distraction.
+///
+/// If you are defining a trait, you can add `CloneToUninit` as a supertrait to enable cloning of
+/// `dyn` values of your trait:
+///
+/// ```
+/// #![feature(clone_to_uninit)]
+/// use std::rc::Rc;
+///
+/// trait Foo: std::fmt::Debug + std::clone::CloneToUninit {
+/// fn modify(&mut self);
+/// fn value(&self) -> i32;
+/// }
+///
+/// impl Foo for i32 {
+/// fn modify(&mut self) {
+/// *self *= 10;
+/// }
+/// fn value(&self) -> i32 {
+/// *self
+/// }
+/// }
+///
+/// let first: Rc<dyn Foo> = Rc::new(1234);
+///
+/// let mut second = first.clone();
+/// Rc::make_mut(&mut second).modify(); // make_mut() will call clone_to_uninit()
+///
+/// assert_eq!(first.value(), 1234);
+/// assert_eq!(second.value(), 12340);
+/// ```
+///
+/// The following is an example of implementing `CloneToUninit` for a custom DST.
+/// (It is essentially a limited form of what `derive(CloneToUninit)` would do,
+/// if such a derive macro existed.)
///
-/// # See also
+/// ```
+/// #![feature(clone_to_uninit)]
+/// use std::clone::CloneToUninit;
+/// use std::mem::offset_of;
+/// use std::rc::Rc;
+///
+/// #[derive(PartialEq)]
+/// struct MyDst<T: ?Sized> {
+/// label: String,
+/// contents: T,
+/// }
///
-/// * [`Clone::clone_from`] is a safe function which may be used instead when `Self` is a [`Sized`]
+/// unsafe impl<T: ?Sized + CloneToUninit> CloneToUninit for MyDst<T> {
+/// unsafe fn clone_to_uninit(&self, dest: *mut u8) {
+/// // The offset of `self.contents` is dynamic because it depends on the alignment of T
+/// // which can be dynamic (if `T = dyn SomeTrait`). Therefore, we have to obtain it
+/// // dynamically by examining `self`, rather than using `offset_of!`.
+/// //
+/// // SAFETY: `self` by definition points somewhere before `&self.contents` in the same
+/// // allocation.
+/// let offset_of_contents = unsafe {
+/// (&raw const self.contents).byte_offset_from_unsigned(self)
+/// };
+///
+/// // Clone the *sized* fields of `self` (just one, in this example).
+/// // (By cloning this first and storing it temporarily in a local variable, we avoid
+/// // leaking it in case of any panic, using the ordinary automatic cleanup of local
+/// // variables. Such a leak would be sound, but undesirable.)
+/// let label = self.label.clone();
+///
+/// // SAFETY: The caller must provide a `dest` such that these field offsets are valid
+/// // to write to.
+/// unsafe {
+/// // Clone the unsized field directly from `self` to `dest`.
+/// self.contents.clone_to_uninit(dest.add(offset_of_contents));
+///
+/// // Now write all the sized fields.
+/// //
+/// // Note that we only do this once all of the clone() and clone_to_uninit() calls
+/// // have completed, and therefore we know that there are no more possible panics;
+/// // this ensures no memory leaks in case of panic.
+/// dest.add(offset_of!(Self, label)).cast::<String>().write(label);
+/// }
+/// // All fields of the struct have been initialized; therefore, the struct is initialized,
+/// // and we have satisfied our `unsafe impl CloneToUninit` obligations.
+/// }
+/// }
+///
+/// fn main() {
+/// // Construct MyDst<[u8; 4]>, then coerce to MyDst<[u8]>.
+/// let first: Rc<MyDst<[u8]>> = Rc::new(MyDst {
+/// label: String::from("hello"),
+/// contents: [1, 2, 3, 4],
+/// });
+///
+/// let mut second = first.clone();
+/// // make_mut() will call clone_to_uninit().
+/// for elem in Rc::make_mut(&mut second).contents.iter_mut() {
+/// *elem *= 10;
+/// }
+///
+/// assert_eq!(first.contents, [1, 2, 3, 4]);
+/// assert_eq!(second.contents, [10, 20, 30, 40]);
+/// assert_eq!(second.label, "hello");
+/// }
+/// ```
+///
+/// # See Also
+///
+/// * [`Clone::clone_from`] is a safe function which may be used instead when [`Self: Sized`](Sized)
/// and the destination is already initialized; it may be able to reuse allocations owned by
-/// the destination.
+/// the destination, whereas `clone_to_uninit` cannot, since its destination is assumed to be
+/// uninitialized.
/// * [`ToOwned`], which allocates a new destination container.
///
/// [`ToOwned`]: ../../std/borrow/trait.ToOwned.html
+/// [DST]: https://doc.rust-lang.org/reference/dynamically-sized-types.html
+/// [trait object]: https://doc.rust-lang.org/reference/types/trait-object.html
#[unstable(feature = "clone_to_uninit", issue = "126799")]
pub unsafe trait CloneToUninit {
- /// Performs copy-assignment from `self` to `dst`.
+ /// Performs copy-assignment from `self` to `dest`.
///
- /// This is analogous to `std::ptr::write(dst.cast(), self.clone())`,
- /// except that `self` may be a dynamically-sized type ([`!Sized`](Sized)).
+ /// This is analogous to `std::ptr::write(dest.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
+ /// Before this function is called, `dest` may point to uninitialized memory.
+ /// After this function is called, `dest` will point to initialized memory; it will be
/// sound to create a `&Self` reference from the pointer with the [pointer metadata]
/// from `self`.
///
@@ -297,8 +413,8 @@ pub unsafe trait CloneToUninit {
///
/// Behavior is undefined if any of the following conditions are violated:
///
- /// * `dst` must be [valid] for writes for `size_of_val(self)` bytes.
- /// * `dst` must be properly aligned to `align_of_val(self)`.
+ /// * `dest` must be [valid] for writes for `size_of_val(self)` bytes.
+ /// * `dest` must be properly aligned to `align_of_val(self)`.
///
/// [valid]: crate::ptr#safety
/// [pointer metadata]: crate::ptr::metadata()
@@ -307,27 +423,26 @@ pub unsafe trait CloneToUninit {
///
/// 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
+ /// If the call panics, then `*dest` 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.
+ /// The caller may wish to to take care to deallocate the allocation pointed to by `dest`,
+ /// if applicable, to avoid a memory leak (but this is not a requirement).
///
/// 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 u8);
+ unsafe fn clone_to_uninit(&self, dest: *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 u8) {
+ unsafe fn clone_to_uninit(&self, dest: *mut u8) {
// SAFETY: we're calling a specialization with the same contract
- unsafe { <T as self::uninit::CopySpec>::clone_one(self, dst.cast::<T>()) }
+ unsafe { <T as self::uninit::CopySpec>::clone_one(self, dest.cast::<T>()) }
}
}
@@ -335,10 +450,10 @@ unsafe impl<T: Clone> CloneToUninit for T {
unsafe impl<T: Clone> CloneToUninit for [T] {
#[inline]
#[cfg_attr(debug_assertions, track_caller)]
- unsafe fn clone_to_uninit(&self, dst: *mut u8) {
- let dst: *mut [T] = dst.with_metadata_of(self);
+ unsafe fn clone_to_uninit(&self, dest: *mut u8) {
+ let dest: *mut [T] = dest.with_metadata_of(self);
// SAFETY: we're calling a specialization with the same contract
- unsafe { <T as self::uninit::CopySpec>::clone_slice(self, dst) }
+ unsafe { <T as self::uninit::CopySpec>::clone_slice(self, dest) }
}
}
@@ -346,21 +461,21 @@ 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 u8) {
+ unsafe fn clone_to_uninit(&self, dest: *mut u8) {
// SAFETY: str is just a [u8] with UTF-8 invariant
- unsafe { self.as_bytes().clone_to_uninit(dst) }
+ unsafe { self.as_bytes().clone_to_uninit(dest) }
}
}
#[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 u8) {
+ unsafe fn clone_to_uninit(&self, dest: *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 (so NUL is also copied).
// See: `cstr_metadata_is_length_with_nul` in tests.
- unsafe { self.to_bytes_with_nul().clone_to_uninit(dst) }
+ unsafe { self.to_bytes_with_nul().clone_to_uninit(dest) }
}
}