//! The `Clone` trait for types that cannot be 'implicitly copied'.
//!
//! In Rust, some simple types are "implicitly copyable" and when you
//! assign them or pass them as arguments, the receiver will get a copy,
//! leaving the original value in place. These types do not require
//! allocation to copy and do not have finalizers (i.e., they do not
//! contain owned boxes or implement [`Drop`]), so the compiler considers
//! them cheap and safe to copy. For other types copies must be made
//! explicitly, by convention implementing the [`Clone`] trait and calling
//! the [`clone`] method.
//!
//! [`clone`]: Clone::clone
//!
//! Basic usage example:
//!
//! ```
//! let s = String::new(); // String type implements Clone
//! let copy = s.clone(); // so we can clone it
//! ```
//!
//! To easily implement the Clone trait, you can also use
//! `#[derive(Clone)]`. Example:
//!
//! ```
//! #[derive(Clone)] // we add the Clone trait to Morpheus struct
//! struct Morpheus {
//! blue_pill: f32,
//! red_pill: i64,
//! }
//!
//! fn main() {
//! let f = Morpheus { blue_pill: 0.0, red_pill: 0 };
//! let copy = f.clone(); // and now we can clone it!
//! }
//! ```
#![stable(feature = "rust1", since = "1.0.0")]
use crate::mem::{self, MaybeUninit};
use crate::ptr;
/// A common trait for the ability to explicitly duplicate an object.
///
/// Differs from [`Copy`] in that [`Copy`] is implicit and an inexpensive bit-wise copy, while
/// `Clone` is always explicit and may or may not be expensive. In order to enforce
/// these characteristics, Rust does not allow you to reimplement [`Copy`], but you
/// may reimplement `Clone` and run arbitrary code.
///
/// Since `Clone` is more general than [`Copy`], you can automatically make anything
/// [`Copy`] be `Clone` as well.
///
/// ## Derivable
///
/// This trait can be used with `#[derive]` if all fields are `Clone`. The `derive`d
/// implementation of [`Clone`] calls [`clone`] on each field.
///
/// [`clone`]: Clone::clone
///
/// For a generic struct, `#[derive]` implements `Clone` conditionally by adding bound `Clone` on
/// generic parameters.
///
/// ```
/// // `derive` implements Clone for Reading<T> when T is Clone.
/// #[derive(Clone)]
/// struct Reading<T> {
/// frequency: T,
/// }
/// ```
///
/// ## How can I implement `Clone`?
///
/// Types that are [`Copy`] should have a trivial implementation of `Clone`. More formally:
/// if `T: Copy`, `x: T`, and `y: &T`, then `let x = y.clone();` is equivalent to `let x = *y;`.
/// Manual implementations should be careful to uphold this invariant; however, unsafe code
/// must not rely on it to ensure memory safety.
///
/// An example is a generic struct holding a function pointer. In this case, the
/// implementation of `Clone` cannot be `derive`d, but can be implemented as:
///
/// ```
/// struct Generate<T>(fn() -> T);
///
/// impl<T> Copy for Generate<T> {}
///
/// impl<T> Clone for Generate<T> {
/// fn clone(&self) -> Self {
/// *self
/// }
/// }
/// ```
///
/// If we `derive`:
///
/// ```
/// #[derive(Copy, Clone)]
/// struct Generate<T>(fn() -> T);
/// ```
///
/// the auto-derived implementations will have unnecessary `T: Copy` and `T: Clone` bounds:
///
/// ```
/// # struct Generate<T>(fn() -> T);
///
/// // Automatically derived
/// impl<T: Copy> Copy for Generate<T> { }
///
/// // Automatically derived
/// impl<T: Clone> Clone for Generate<T> {
/// fn clone(&self) -> Generate<T> {
/// Generate(Clone::clone(&self.0))
/// }
/// }
/// ```
///
/// The bounds are unnecessary because clearly the function itself should be
/// copy- and cloneable even if its return type is not:
///
/// ```compile_fail,E0599
/// #[derive(Copy, Clone)]
/// struct Generate<T>(fn() -> T);
///
/// struct NotCloneable;
///
/// fn generate_not_cloneable() -> NotCloneable {
/// NotCloneable
/// }
///
/// Generate(generate_not_cloneable).clone(); // error: trait bounds were not satisfied
/// // Note: With the manual implementations the above line will compile.
/// ```
///
/// ## Additional implementors
///
/// In addition to the [implementors listed below][impls],
/// the following types also implement `Clone`:
///
/// * Function item types (i.e., the distinct types defined for each function)
/// * Function pointer types (e.g., `fn() -> i32`)
/// * Closure types, if they capture no value from the environment
/// or if all such captured values implement `Clone` themselves.
/// Note that variables captured by shared reference always implement `Clone`
/// (even if the referent doesn't),
/// while variables captured by mutable reference never implement `Clone`.
///
/// [impls]: #implementors
#[stable(feature = "rust1", since = "1.0.0")]
#[lang = "clone"]
#[rustc_diagnostic_item = "Clone"]
#[rustc_trivial_field_reads]
pub trait Clone: Sized {
/// Returns a copy of the value.
///
/// # Examples
///
/// ```
/// # #![allow(noop_method_call)]
/// let hello = "Hello"; // &str implements Clone
///
/// assert_eq!("Hello", hello.clone());
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[must_use = "cloning is often expensive and is not expected to have side effects"]
fn clone(&self) -> Self;
/// Performs copy-assignment from `source`.
///
/// `a.clone_from(&b)` is equivalent to `a = b.clone()` in functionality,
/// but can be overridden to reuse the resources of `a` to avoid unnecessary
/// allocations.
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn clone_from(&mut self, source: &Self) {
*self = source.clone()
}
}
/// Derive macro generating an impl of the trait `Clone`.
#[rustc_builtin_macro]
#[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
#[allow_internal_unstable(core_intrinsics, derive_clone_copy)]
pub macro Clone($item:item) {
/* compiler built-in */
}
// FIXME(aburka): these structs are used solely by #[derive] to
// assert that every component of a type implements Clone or Copy.
//
// These structs should never appear in user code.
#[doc(hidden)]
#[allow(missing_debug_implementations)]
#[unstable(
feature = "derive_clone_copy",
reason = "deriving hack, should not be public",
issue = "none"
)]
pub struct AssertParamIsClone<T: Clone + ?Sized> {
_field: crate:📑:PhantomData<T>,
}
#[doc(hidden)]
#[allow(missing_debug_implementations)]
#[unstable(
feature = "derive_clone_copy",
reason = "deriving hack, should not be public",
issue = "none"
)]
pub struct AssertParamIsCopy<T: Copy + ?Sized> {
_field: crate:📑:PhantomData<T>,
}
/// A generalization of [`Clone`] to dynamically-sized types 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).
///
/// # 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`.
///
/// # See also
///
/// * [`Clone::clone_from`] is a safe function which may be used instead when `Self` is a [`Sized`]
/// and the destination is already initialized; it may be able to reuse allocations owned by
/// the destination.
/// * [`ToOwned`], which allocates a new destination container.
///
/// [`ToOwned`]: ../../std/borrow/trait.ToOwned.html
#[unstable(feature = "clone_to_uninit", issue = "126799")]
pub unsafe trait CloneToUninit {
/// Performs copy-assignment from `self` to `dst`.
///
/// This is analogous to `std::ptr::write(dst, 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.
///
/// # 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`.
///
/// [valid]: ptr#safety
/// [pointer metadata]: crate::ptr::metadata()
///
/// # Panics
///
/// 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
/// 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.
///
/// 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 Self);
}
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl<T: Clone> CloneToUninit for T {
default unsafe fn clone_to_uninit(&self, dst: *mut Self) {
// SAFETY: The safety conditions of clone_to_uninit() are a superset of those of
// ptr::write().
unsafe {
// We hope the optimizer will figure out to create the cloned value in-place,
// skipping ever storing it on the stack and the copy to the destination.
ptr::write(dst, self.clone());
}
}
}
// Specialized implementation for types that are [`Copy`], not just [`Clone`],
// and can therefore be copied bitwise.
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl<T: Copy> CloneToUninit for T {
unsafe fn clone_to_uninit(&self, dst: *mut Self) {
// SAFETY: The safety conditions of clone_to_uninit() are a superset of those of
// ptr::copy_nonoverlapping().
unsafe {
ptr::copy_nonoverlapping(self, dst, 1);
}
}
}
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl<T: Clone> CloneToUninit for [T] {
#[cfg_attr(debug_assertions, track_caller)]
default unsafe fn clone_to_uninit(&self, dst: *mut Self) {
let len = self.len();
// This is the most likely mistake to make, so check it as a debug assertion.
debug_assert_eq!(
len,
dst.len(),
"clone_to_uninit() source and destination must have equal lengths",
);
// SAFETY: The produced `&mut` is valid because:
// * The caller is obligated to provide a pointer which is valid for writes.
// * All bytes pointed to are in MaybeUninit, so we don't care about the memory's
// initialization status.
let uninit_ref = unsafe { &mut *(dst as *mut [MaybeUninit<T>]) };
// Copy the elements
let mut initializing = InitializingSlice::from_fully_uninit(uninit_ref);
for element_ref in self.iter() {
// If the clone() panics, `initializing` will take care of the cleanup.
initializing.push(element_ref.clone());
}
// If we reach here, then the entire slice is initialized, and we've satisfied our
// responsibilities to the caller. Disarm the cleanup guard by forgetting it.
mem::forget(initializing);
}
}
#[unstable(feature = "clone_to_uninit", issue = "126799")]
unsafe impl<T: Copy> CloneToUninit for [T] {
#[cfg_attr(debug_assertions, track_caller)]
unsafe fn clone_to_uninit(&self, dst: *mut Self) {
let len = self.len();
// This is the most likely mistake to make, so check it as a debug assertion.
debug_assert_eq!(
len,
dst.len(),
"clone_to_uninit() source and destination must have equal lengths",
);
// SAFETY: The safety conditions of clone_to_uninit() are a superset of those of
// ptr::copy_nonoverlapping().
unsafe {
ptr::copy_nonoverlapping(self.as_ptr(), dst.as_mut_ptr(), len);
}
}
}
/// Ownership of a collection of values stored in a non-owned `[MaybeUninit<T>]`, some of which
/// are not yet initialized. This is sort of like a `Vec` that doesn't own its allocation.
/// Its responsibility is to provide cleanup on unwind by dropping the values that *are*
/// initialized, unless disarmed by forgetting.
///
/// This is a helper for `impl<T: Clone> CloneToUninit for [T]`.
struct InitializingSlice<'a, T> {
data: &'a mut [MaybeUninit<T>],
/// Number of elements of `*self.data` that are initialized.
initialized_len: usize,
}
impl<'a, T> InitializingSlice<'a, T> {
#[inline]
fn from_fully_uninit(data: &'a mut [MaybeUninit<T>]) -> Self {
Self { data, initialized_len: 0 }
}
/// Push a value onto the end of the initialized part of the slice.
///
/// # Panics
///
/// Panics if the slice is already fully initialized.
#[inline]
fn push(&mut self, value: T) {
MaybeUninit::write(&mut self.data[self.initialized_len], value);
self.initialized_len += 1;
}
}
impl<'a, T> Drop for InitializingSlice<'a, T> {
#[cold] // will only be invoked on unwind
fn drop(&mut self) {
let initialized_slice = ptr::slice_from_raw_parts_mut(
MaybeUninit::slice_as_mut_ptr(self.data),
self.initialized_len,
);
// SAFETY:
// * the pointer is valid because it was made from a mutable reference
// * `initialized_len` counts the initialized elements as an invariant of this type,
// so each of the pointed-to elements is initialized and may be dropped.
unsafe {
ptr::drop_in_place::<[T]>(initialized_slice);
}
}
}
/// Implementations of `Clone` for primitive types.
///
/// Implementations that cannot be described in Rust
/// are implemented in `traits::SelectionContext::copy_clone_conditions()`
/// in `rustc_trait_selection`.
mod impls {
macro_rules! impl_clone {
($($t:ty)*) => {
$(
#[stable(feature = "rust1", since = "1.0.0")]
impl Clone for $t {
#[inline(always)]
fn clone(&self) -> Self {
*self
}
}
)*
}
}
impl_clone! {
usize u8 u16 u32 u64 u128
isize i8 i16 i32 i64 i128
f16 f32 f64 f128
bool char
}
#[unstable(feature = "never_type", issue = "35121")]
impl Clone for ! {
#[inline]
fn clone(&self) -> Self {
*self
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Clone for *const T {
#[inline(always)]
fn clone(&self) -> Self {
*self
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Clone for *mut T {
#[inline(always)]
fn clone(&self) -> Self {
*self
}
}
/// Shared references can be cloned, but mutable references *cannot*!
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Clone for &T {
#[inline(always)]
#[rustc_diagnostic_item = "noop_method_clone"]
fn clone(&self) -> Self {
*self
}
}
/// Shared references can be cloned, but mutable references *cannot*!
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Clone for &mut T {}
}