Expand CloneToUninit documentation.

* Clarify relationship to `dyn` after #133003.
* Add an example of using it with `dyn` as #133003 enabled.
* Add an example of implementing it.
* Add links to Rust Reference for the mentioned concepts.
* Mention that its method should rarely be called.
* Replace parameter name `dst` with `dest` to avoids confusion between
  “DeSTination” and “Dynamically-Sized Type”.
* Various small corrections.

Author: kpreid

library/core/src/clone.rs

@@ -209,43 +209,146 @@ 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()` 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> {
+///     flag: bool,
+///     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!`.
+///         let offset_of_contents =
+///             (&raw const self.contents).byte_offset_from_unsigned(&raw const *self);
+///
+///         // Since `flag` implements `Copy`, we can just copy it.
+///         // We use `pointer::write()` instead of assignment because the destination must be
+///         // assumed to be uninitialized, whereas an assignment assumes it is initialized.
+///         dest.add(offset_of!(Self, flag)).cast::<bool>().write(self.flag);
+///
+///         // Note: if `flag` owned any resources (i.e. had a `Drop` implementation), then we
+///         // must prepare to drop it in case `self.contents.clone_to_uninit()` panics.
+///         // In this simple case, where we have exactly one field for which `mem::needs_drop()`
+///         // might be true (`contents`), we don’t need to care about cleanup or ordering.
+///         self.contents.clone_to_uninit(dest.add(offset_of_contents));
+///
+///         // 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 {
+///         flag: true,
+///         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]);
+/// }
+/// ```
+///
+/// # 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`.
     ///
     /// # Safety
     ///
     /// Behavior is undefined if any of the following conditions are violated:
     ///
-    /// * `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)`.
+    /// * `dest` must be [valid] for writes for `std::mem::size_of_val(self)` bytes.
+    /// * `dest` must be properly aligned to `std::mem::align_of_val(self)`.
     ///
     /// [valid]: crate::ptr#safety
     /// [pointer metadata]: crate::ptr::metadata()
@@ -254,60 +357,60 @@ 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`,
+    /// The caller may also need to take care to deallocate the allocation pointed to by `dest`,
     /// if applicable, to avoid a memory leak, and may need to take other precautions to ensure
     /// soundness in the presence of unwinding.
     ///
     /// 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>()) }
     }
 }
 
 #[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 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) }
     }
 }
 
 #[unstable(feature = "clone_to_uninit", issue = "126799")]
 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) }
     }
 }