clone.rs - source
Description: Source of the Rust file `library/core/src/clone.rs`.

#
core/
clone.rs

    1//! The `Clone` trait for types that cannot be 'implicitly copied'.
    2//!
    3//! In Rust, some simple types are "implicitly copyable" and when you
    4//! assign them or pass them as arguments, the receiver will get a copy,
    5//! leaving the original value in place. These types do not require
    6//! allocation to copy and do not have finalizers (i.e., they do not
    7//! contain owned boxes or implement [`Drop`]), so the compiler considers
    8//! them cheap and safe to copy. For other types copies must be made
    9//! explicitly, by convention implementing the [`Clone`] trait and calling
    10//! the [`clone`] method.
    11//!
    12//! [`clone`]: Clone::clone
    13//!
    14//! Basic usage example:
    15//!
    16//! ```
    17//! let s = String::new(); // String type implements Clone
    18//! let copy = s.clone(); // so we can clone it
    19//! ```
    20//!
    21//! To easily implement the Clone trait, you can also use
    22//! `#[derive(Clone)]`. Example:
    23//!
    24//! ```
    25//! #[derive(Clone)] // we add the Clone trait to Morpheus struct
    26//! struct Morpheus {
    27//!    blue_pill: f32,
    28//!    red_pill: i64,
    29//! }
    30//!
    31//! fn main() {
    32//!    let f = Morpheus { blue_pill: 0.0, red_pill: 0 };
    33//!    let copy = f.clone(); // and now we can clone it!
    34//! }
    35//! ```
    36
    37#![stable(feature = "rust1", since = "1.0.0")]
    38
    39use crate::marker::{Destruct, PointeeSized};
    40
    41mod uninit;
    42
    43/// A common trait that allows explicit creation of a duplicate value.
    44///
    45/// Calling [`clone`] always produces a new value.
    46/// However, for types that are references to other data (such as smart pointers or references),
    47/// the new value may still point to the same underlying data, rather than duplicating it.
    48/// See [`Clone::clone`] for more details.
    49///
    50/// This distinction is especially important when using `#[derive(Clone)]` on structs containing
    51/// smart pointers like `Arc<Mutex<T>>` - the cloned struct will share mutable state with the
    52/// original.
    53///
    54/// Differs from [`Copy`] in that [`Copy`] is implicit and an inexpensive bit-wise copy, while
    55/// `Clone` is always explicit and may or may not be expensive. [`Copy`] has no methods, so you
    56/// cannot change its behavior, but when implementing `Clone`, the `clone` method you provide
    57/// may run arbitrary code.
    58///
    59/// Since `Clone` is a supertrait of [`Copy`], any type that implements `Copy` must also implement
    60/// `Clone`.
    61///
    62/// ## Derivable
    63///
    64/// This trait can be used with `#[derive]` if all fields are `Clone`. The `derive`d
    65/// implementation of [`Clone`] calls [`clone`] on each field.
    66///
    67/// [`clone`]: Clone::clone
    68///
    69/// For a generic struct, `#[derive]` implements `Clone` conditionally by adding bound `Clone` on
    70/// generic parameters.
    71///
    72/// ```
    73/// // `derive` implements Clone for Reading<T> when T is Clone.
    74/// #[derive(Clone)]
    75/// struct Reading<T> {
    76///     frequency: T,
    77/// }
    78/// ```
    79///
    80/// ## How can I implement `Clone`?
    81///
    82/// Types that are [`Copy`] should have a trivial implementation of `Clone`. More formally:
    83/// if `T: Copy`, `x: T`, and `y: &T`, then `let x = y.clone();` is equivalent to `let x = *y;`.
    84/// Manual implementations should be careful to uphold this invariant; however, unsafe code
    85/// must not rely on it to ensure memory safety.
    86///
    87/// An example is a generic struct holding a function pointer. In this case, the
    88/// implementation of `Clone` cannot be `derive`d, but can be implemented as:
    89///
    90/// ```
    91/// struct Generate<T>(fn() -> T);
    92///
    93/// impl<T> Copy for Generate<T> {}
    94///
    95/// impl<T> Clone for Generate<T> {
    96///     fn clone(&self) -> Self {
    97///         *self
    98///     }
    99/// }
    100/// ```
    101///
    102/// If we `derive`:
    103///
    104/// ```
    105/// #[derive(Copy, Clone)]
    106/// struct Generate<T>(fn() -> T);
    107/// ```
    108///
    109/// the auto-derived implementations will have unnecessary `T: Copy` and `T: Clone` bounds:
    110///
    111/// ```
    112/// # struct Generate<T>(fn() -> T);
    113///
    114/// // Automatically derived
    115/// impl<T: Copy> Copy for Generate<T> { }
    116///
    117/// // Automatically derived
    118/// impl<T: Clone> Clone for Generate<T> {
    119///     fn clone(&self) -> Generate<T> {
    120///         Generate(Clone::clone(&self.0))
    121///     }
    122/// }
    123/// ```
    124///
    125/// The bounds are unnecessary because clearly the function itself should be
    126/// copy- and cloneable even if its return type is not:
    127///
    128/// ```compile_fail,E0599
    129/// #[derive(Copy, Clone)]
    130/// struct Generate<T>(fn() -> T);
    131///
    132/// struct NotCloneable;
    133///
    134/// fn generate_not_cloneable() -> NotCloneable {
    135///     NotCloneable
    136/// }
    137///
    138/// Generate(generate_not_cloneable).clone(); // error: trait bounds were not satisfied
    139/// // Note: With the manual implementations the above line will compile.
    140/// ```
    141///
    142/// ## `Clone` and `PartialEq`/`Eq`
    143/// `Clone` is intended for the duplication of objects. Consequently, when implementing
    144/// both `Clone` and [`PartialEq`], the following property is expected to hold:
    145/// ```text
    146/// x == x -> x.clone() == x
    147/// ```
    148/// In other words, if an object compares equal to itself,
    149/// its clone must also compare equal to the original.
    150///
    151/// For types that also implement [`Eq`] – for which `x == x` always holds –
    152/// this implies that `x.clone() == x` must always be true.
    153/// Standard library collections such as
    154/// [`HashMap`], [`HashSet`], [`BTreeMap`], [`BTreeSet`] and [`BinaryHeap`]
    155/// rely on their keys respecting this property for correct behavior.
    156/// Furthermore, these collections require that cloning a key preserves the outcome of the
    157/// [`Hash`] and [`Ord`] methods. Thankfully, this follows automatically from `x.clone() == x`
    158/// if `Hash` and `Ord` are correctly implemented according to their own requirements.
    159///
    160/// When deriving both `Clone` and [`PartialEq`] using `#[derive(Clone, PartialEq)]`
    161/// or when additionally deriving [`Eq`] using `#[derive(Clone, PartialEq, Eq)]`,
    162/// then this property is automatically upheld – provided that it is satisfied by
    163/// the underlying types.
    164///
    165/// Violating this property is a logic error. The behavior resulting from a logic error is not
    166/// specified, but users of the trait must ensure that such logic errors do *not* result in
    167/// undefined behavior. This means that `unsafe` code **must not** rely on this property
    168/// being satisfied.
    169///
    170/// ## Additional implementors
    171///
    172/// In addition to the [implementors listed below][impls],
    173/// the following types also implement `Clone`:
    174///
    175/// * Function item types (i.e., the distinct types defined for each function)
    176/// * Function pointer types (e.g., `fn() -> i32`)
    177/// * Closure types, if they capture no value from the environment
    178///   or if all such captured values implement `Clone` themselves.
    179///   Note that variables captured by shared reference always implement `Clone`
    180///   (even if the referent doesn't),
    181///   while variables captured by mutable reference never implement `Clone`.
    182///
    183/// [`HashMap`]: ../../std/collections/struct.HashMap.html
    184/// [`HashSet`]: ../../std/collections/struct.HashSet.html
    185/// [`BTreeMap`]: ../../std/collections/struct.BTreeMap.html
    186/// [`BTreeSet`]: ../../std/collections/struct.BTreeSet.html
    187/// [`BinaryHeap`]: ../../std/collections/struct.BinaryHeap.html
    188/// [impls]: #implementors
    189#[stable(feature = "rust1", since = "1.0.0")]
    190#[lang = "clone"]
    191#[rustc_diagnostic_item = "Clone"]
    192#[rustc_trivial_field_reads]
    193#[rustc_const_unstable(feature = "const_clone", issue = "142757")]
    194pub const trait Clone: Sized {
    195    /// Returns a duplicate of the value.
    196    ///
    197    /// Note that what "duplicate" means varies by type:
    198    /// - For most types, this creates a deep, independent copy
    199    /// - For reference types like `&T`, this creates another reference to the same value
    200    /// - For smart pointers like [`Arc`] or [`Rc`], this increments the reference count
    201    ///   but still points to the same underlying data
    202    ///
    203    /// [`Arc`]: ../../std/sync/struct.Arc.html
    204    /// [`Rc`]: ../../std/rc/struct.Rc.html
    205    ///
    206    /// # Examples
    207    ///
    208    /// ```
    209    /// # #![allow(noop_method_call)]
    210    /// let hello = "Hello"; // &str implements Clone
    211    ///
    212    /// assert_eq!("Hello", hello.clone());
    213    /// ```
    214    ///
    215    /// Example with a reference-counted type:
    216    ///
    217    /// ```
    218    /// use std::sync::{Arc, Mutex};
    219    ///
    220    /// let data = Arc::new(Mutex::new(vec![1, 2, 3]));
    221    /// let data_clone = data.clone(); // Creates another Arc pointing to the same Mutex
    222    ///
    223    /// {
    224    ///     let mut lock = data.lock().unwrap();
    225    ///     lock.push(4);
    226    /// }
    227    ///
    228    /// // Changes are visible through the clone because they share the same underlying data
    229    /// assert_eq!(*data_clone.lock().unwrap(), vec![1, 2, 3, 4]);
    230    /// ```
    231    #[stable(feature = "rust1", since = "1.0.0")]
    232    #[must_use = "cloning is often expensive and is not expected to have side effects"]
    233    // Clone::clone is special because the compiler generates MIR to implement it for some types.
    234    // See InstanceKind::CloneShim.
    235    #[lang = "clone_fn"]
    236    fn clone(&self) -> Self;
    237
    238    /// Performs copy-assignment from `source`.
    239    ///
    240    /// `a.clone_from(&b)` is equivalent to `a = b.clone()` in functionality,
    241    /// but can be overridden to reuse the resources of `a` to avoid unnecessary
    242    /// allocations.
    243    #[inline]
    244    #[stable(feature = "rust1", since = "1.0.0")]
    245    fn clone_from(&mut self, source: &Self)
    246    where
    247        Self: [const] Destruct,
    248    {
    249        *self = source.clone()
    250    }
    251}
    252
    253/// Indicates that the `Clone` implementation is identical to copying the value.
    254///
    255/// This is used for some optimizations in the standard library, which specializes
    256/// on this trait to select faster implementations of functions such as
    257/// [`clone_from_slice`](slice::clone_from_slice). It is automatically implemented
    258/// when using `#[derive(Clone, Copy)]`.
    259///
    260/// Note that this trait does not imply that the type is `Copy`, because e.g.
    261/// `core::ops::Range<i32>` could soundly implement this trait.
    262///
    263/// # Safety
    264/// `Clone::clone` must be equivalent to copying the value, otherwise calling functions
    265/// such as `slice::clone_from_slice` can have undefined behaviour.
    266#[unstable(
    267    feature = "trivial_clone",
    268    reason = "this isn't part of any API guarantee",
    269    issue = "none"
    270)]
    271#[rustc_const_unstable(feature = "const_clone", issue = "142757")]
    272#[lang = "trivial_clone"]
    273// SAFETY:
    274// It is sound to specialize on this because the `clone` implementation cannot be
    275// lifetime-dependent. Therefore, if `TrivialClone` is implemented for any lifetime,
    276// its invariant holds whenever `Clone` is implemented, even if the actual
    277// `TrivialClone` bound would not be satisfied because of lifetime bounds.
    278#[rustc_unsafe_specialization_marker]
    279// If `#[derive(Clone, Clone, Copy)]` is written, there will be multiple
    280// implementations of `TrivialClone`. To keep it from appearing in error
    281// messages, make it a `#[marker]` trait.
    282#[marker]
    283pub const unsafe trait TrivialClone: [const] Clone {}
    284
    285/// Derive macro generating an impl of the trait `Clone`.
    286#[rustc_builtin_macro]
    287#[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
    288#[allow_internal_unstable(core_intrinsics, derive_clone_copy_internals, trivial_clone)]
    289pub macro Clone($item:item) {
    290    /* compiler built-in */
    291}
    292
    293/// Trait for objects whose [`Clone`] impl is lightweight (e.g. reference-counted)
    294///
    295/// Cloning an object implementing this trait should in general:
    296/// - be O(1) (constant) time regardless of the amount of data managed by the object,
    297/// - not require a memory allocation,
    298/// - not require copying more than roughly 64 bytes (a typical cache line size),
    299/// - not block the current thread,
    300/// - not have any semantic side effects (e.g. allocating a file descriptor), and
    301/// - not have overhead larger than a couple of atomic operations.
    302///
    303/// The `UseCloned` trait does not provide a method; instead, it indicates that
    304/// `Clone::clone` is lightweight, and allows the use of the `.use` syntax.
    305///
    306/// ## .use postfix syntax
    307///
    308/// Values can be `.use`d by adding `.use` postfix to the value you want to use.
    309///
    310/// ```ignore (this won't work until we land use)
    311/// fn foo(f: Foo) {
    312///     // if `Foo` implements `Copy` f would be copied into x.
    313///     // if `Foo` implements `UseCloned` f would be cloned into x.
    314///     // otherwise f would be moved into x.
    315///     let x = f.use;
    316///     // ...
    317/// }
    318/// ```
    319///
    320/// ## use closures
    321///
    322/// Use closures allow captured values to be automatically used.
    323/// This is similar to have a closure that you would call `.use` over each captured value.
    324#[unstable(feature = "ergonomic_clones", issue = "132290")]
    325#[lang = "use_cloned"]
    326pub trait UseCloned: Clone {
    327    // Empty.
    328}
    329
    330macro_rules! impl_use_cloned {
    331    ($($t:ty)*) => {
    332        $(
    333            #[unstable(feature = "ergonomic_clones", issue = "132290")]
    334            impl UseCloned for $t {}
    335        )*
    336    }
    337}
    338
    339impl_use_cloned! {
    340    usize u8 u16 u32 u64 u128
    341    isize i8 i16 i32 i64 i128
    342             f16 f32 f64 f128
    343    bool char
    344}
    345
    346// FIXME(aburka): these structs are used solely by #[derive] to
    347// assert that every component of a type implements Clone or Copy.
    348//
    349// These structs should never appear in user code.
    350#[doc(hidden)]
    351#[allow(missing_debug_implementations)]
    352#[unstable(
    353    feature = "derive_clone_copy_internals",
    354    reason = "deriving hack, should not be public",
    355    issue = "none"
    356)]
    357pub struct AssertParamIsClone<T: Clone + PointeeSized> {
    358    _field: crate::marker::PhantomData<T>,
    359}
    360#[doc(hidden)]
    361#[allow(missing_debug_implementations)]
    362#[unstable(
    363    feature = "derive_clone_copy_internals",
    364    reason = "deriving hack, should not be public",
    365    issue = "none"
    366)]
    367pub struct AssertParamIsCopy<T: Copy + PointeeSized> {
    368    _field: crate::marker::PhantomData<T>,
    369}
    370
    371/// A generalization of [`Clone`] to [dynamically-sized types][DST] stored in arbitrary containers.
    372///
    373/// This trait is implemented for all types implementing [`Clone`], [slices](slice) of all
    374/// such types, and other dynamically-sized types in the standard library.
    375/// You may also implement this trait to enable cloning custom DSTs
    376/// (structures containing dynamically-sized fields), or use it as a supertrait to enable
    377/// cloning a [trait object].
    378///
    379/// This trait is normally used via operations on container types which support DSTs,
    380/// so you should not typically need to call `.clone_to_uninit()` explicitly except when
    381/// implementing such a container or otherwise performing explicit management of an allocation,
    382/// or when implementing `CloneToUninit` itself.
    383///
    384/// # Safety
    385///
    386/// Implementations must ensure that when `.clone_to_uninit(dest)` returns normally rather than
    387/// panicking, it always leaves `*dest` initialized as a valid value of type `Self`.
    388///
    389/// # Examples
    390///
    391// FIXME(#126799): when `Box::clone` allows use of `CloneToUninit`, rewrite these examples with it
    392// since `Rc` is a distraction.
    393///
    394/// If you are defining a trait, you can add `CloneToUninit` as a supertrait to enable cloning of
    395/// `dyn` values of your trait:
    396///
    397/// ```
    398/// #![feature(clone_to_uninit)]
    399/// use std::rc::Rc;
    400///
    401/// trait Foo: std::fmt::Debug + std::clone::CloneToUninit {
    402///     fn modify(&mut self);
    403///     fn value(&self) -> i32;
    404/// }
    405///
    406/// impl Foo for i32 {
    407///     fn modify(&mut self) {
    408///         *self *= 10;
    409///     }
    410///     fn value(&self) -> i32 {
    411///         *self
    412///     }
    413/// }
    414///
    415/// let first: Rc<dyn Foo> = Rc::new(1234);
    416///
    417/// let mut second = first.clone();
    418/// Rc::make_mut(&mut second).modify(); // make_mut() will call clone_to_uninit()
    419///
    420/// assert_eq!(first.value(), 1234);
    421/// assert_eq!(second.value(), 12340);
    422/// ```
    423///
    424/// The following is an example of implementing `CloneToUninit` for a custom DST.
    425/// (It is essentially a limited form of what `derive(CloneToUninit)` would do,
    426/// if such a derive macro existed.)
    427///
    428/// ```
    429/// #![feature(clone_to_uninit)]
    430/// use std::clone::CloneToUninit;
    431/// use std::mem::offset_of;
    432/// use std::rc::Rc;
    433///
    434/// #[derive(PartialEq)]
    435/// struct MyDst<T: ?Sized> {
    436///     label: String,
    437///     contents: T,
    438/// }
    439///
    440/// unsafe impl<T: ?Sized + CloneToUninit> CloneToUninit for MyDst<T> {
    441///     unsafe fn clone_to_uninit(&self, dest: *mut u8) {
    442///         // The offset of `self.contents` is dynamic because it depends on the alignment of T
    443///         // which can be dynamic (if `T = dyn SomeTrait`). Therefore, we have to obtain it
    444///         // dynamically by examining `self`, rather than using `offset_of!`.
    445///         //
    446///         // SAFETY: `self` by definition points somewhere before `&self.contents` in the same
    447///         // allocation.
    448///         let offset_of_contents = unsafe {
    449///             (&raw const self.contents).byte_offset_from_unsigned(self)
    450///         };
    451///
    452///         // Clone the *sized* fields of `self` (just one, in this example).
    453///         // (By cloning this first and storing it temporarily in a local variable, we avoid
    454///         // leaking it in case of any panic, using the ordinary automatic cleanup of local
    455///         // variables. Such a leak would be sound, but undesirable.)
    456///         let label = self.label.clone();
    457///
    458///         // SAFETY: The caller must provide a `dest` such that these field offsets are valid
    459///         // to write to.
    460///         unsafe {
    461///             // Clone the unsized field directly from `self` to `dest`.
    462///             self.contents.clone_to_uninit(dest.add(offset_of_contents));
    463///
    464///             // Now write all the sized fields.
    465///             //
    466///             // Note that we only do this once all of the clone() and clone_to_uninit() calls
    467///             // have completed, and therefore we know that there are no more possible panics;
    468///             // this ensures no memory leaks in case of panic.
    469///             dest.add(offset_of!(Self, label)).cast::<String>().write(label);
    470///         }
    471///         // All fields of the struct have been initialized; therefore, the struct is initialized,
    472///         // and we have satisfied our `unsafe impl CloneToUninit` obligations.
    473///     }
    474/// }
    475///
    476/// fn main() {
    477///     // Construct MyDst<[u8; 4]>, then coerce to MyDst<[u8]>.
    478///     let first: Rc<MyDst<[u8]>> = Rc::new(MyDst {
    479///         label: String::from("hello"),
    480///         contents: [1, 2, 3, 4],
    481///     });
    482///
    483///     let mut second = first.clone();
    484///     // make_mut() will call clone_to_uninit().
    485///     for elem in Rc::make_mut(&mut second).contents.iter_mut() {
    486///         *elem *= 10;
    487///     }
    488///
    489///     assert_eq!(first.contents, [1, 2, 3, 4]);
    490///     assert_eq!(second.contents, [10, 20, 30, 40]);
    491///     assert_eq!(second.label, "hello");
    492/// }
    493/// ```
    494///
    495/// # See Also
    496///
    497/// * [`Clone::clone_from`] is a safe function which may be used instead when [`Self: Sized`](Sized)
    498///   and the destination is already initialized; it may be able to reuse allocations owned by
    499///   the destination, whereas `clone_to_uninit` cannot, since its destination is assumed to be
    500///   uninitialized.
    501/// * [`ToOwned`], which allocates a new destination container.
    502///
    503/// [`ToOwned`]: ../../std/borrow/trait.ToOwned.html
    504/// [DST]: https://doc.rust-lang.org/reference/dynamically-sized-types.html
    505/// [trait object]: https://doc.rust-lang.org/reference/types/trait-object.html
    506#[unstable(feature = "clone_to_uninit", issue = "126799")]
    507pub unsafe trait CloneToUninit {
    508    /// Performs copy-assignment from `self` to `dest`.
    509    ///
    510    /// This is analogous to `std::ptr::write(dest.cast(), self.clone())`,
    511    /// except that `Self` may be a dynamically-sized type ([`!Sized`](Sized)).
    512    ///
    513    /// Before this function is called, `dest` may point to uninitialized memory.
    514    /// After this function is called, `dest` will point to initialized memory; it will be
    515    /// sound to create a `&Self` reference from the pointer with the [pointer metadata]
    516    /// from `self`.
    517    ///
    518    /// # Safety
    519    ///
    520    /// Behavior is undefined if any of the following conditions are violated:
    521    ///
    522    /// * `dest` must be [valid] for writes for `size_of_val(self)` bytes.
    523    /// * `dest` must be properly aligned to `align_of_val(self)`.
    524    ///
    525    /// [valid]: crate::ptr#safety
    526    /// [pointer metadata]: crate::ptr::metadata()
    527    ///
    528    /// # Panics
    529    ///
    530    /// This function may panic. (For example, it might panic if memory allocation for a clone
    531    /// of a value owned by `self` fails.)
    532    /// If the call panics, then `*dest` should be treated as uninitialized memory; it must not be
    533    /// read or dropped, because even if it was previously valid, it may have been partially
    534    /// overwritten.
    535    ///
    536    /// The caller may wish to take care to deallocate the allocation pointed to by `dest`,
    537    /// if applicable, to avoid a memory leak (but this is not a requirement).
    538    ///
    539    /// Implementors should avoid leaking values by, upon unwinding, dropping all component values
    540    /// that might have already been created. (For example, if a `[Foo]` of length 3 is being
    541    /// cloned, and the second of the three calls to `Foo::clone()` unwinds, then the first `Foo`
    542    /// cloned should be dropped.)
    543    unsafe fn clone_to_uninit(&self, dest: *mut u8);
    544}
    545
    546#[unstable(feature = "clone_to_uninit", issue = "126799")]
    547unsafe impl<T: Clone> CloneToUninit for T {
    548    #[inline]
    549    unsafe fn clone_to_uninit(&self, dest: *mut u8) {
    550        // SAFETY: we're calling a specialization with the same contract
    551        unsafe { <T as self::uninit::CopySpec>::clone_one(self, dest.cast::<T>()) }
    552    }
    553}
    554
    555#[unstable(feature = "clone_to_uninit", issue = "126799")]
    556unsafe impl<T: Clone> CloneToUninit for [T] {
    557    #[inline]
    558    #[cfg_attr(debug_assertions, track_caller)]
    559    unsafe fn clone_to_uninit(&self, dest: *mut u8) {
    560        let dest: *mut [T] = dest.with_metadata_of(self);
    561        // SAFETY: we're calling a specialization with the same contract
    562        unsafe { <T as self::uninit::CopySpec>::clone_slice(self, dest) }
    563    }
    564}
    565
    566#[unstable(feature = "clone_to_uninit", issue = "126799")]
    567unsafe impl CloneToUninit for str {
    568    #[inline]
    569    #[cfg_attr(debug_assertions, track_caller)]
    570    unsafe fn clone_to_uninit(&self, dest: *mut u8) {
    571        // SAFETY: str is just a [u8] with UTF-8 invariant
    572        unsafe { self.as_bytes().clone_to_uninit(dest) }
    573    }
    574}
    575
    576#[unstable(feature = "clone_to_uninit", issue = "126799")]
    577unsafe impl CloneToUninit for crate::ffi::CStr {
    578    #[cfg_attr(debug_assertions, track_caller)]
    579    unsafe fn clone_to_uninit(&self, dest: *mut u8) {
    580        // SAFETY: For now, CStr is just a #[repr(trasnsparent)] [c_char] with some invariants.
    581        // And we can cast [c_char] to [u8] on all supported platforms (see: to_bytes_with_nul).
    582        // The pointer metadata properly preserves the length (so NUL is also copied).
    583        // See: `cstr_metadata_is_length_with_nul` in tests.
    584        unsafe { self.to_bytes_with_nul().clone_to_uninit(dest) }
    585    }
    586}
    587
    588#[unstable(feature = "bstr", issue = "134915")]
    589unsafe impl CloneToUninit for crate::bstr::ByteStr {
    590    #[inline]
    591    #[cfg_attr(debug_assertions, track_caller)]
    592    unsafe fn clone_to_uninit(&self, dst: *mut u8) {
    593        // SAFETY: ByteStr is a `#[repr(transparent)]` wrapper around `[u8]`
    594        unsafe { self.as_bytes().clone_to_uninit(dst) }
    595    }
    596}
    597
    598/// Implementations of `Clone` for primitive types.
    599///
    600/// Implementations that cannot be described in Rust
    601/// are implemented in `traits::SelectionContext::copy_clone_conditions()`
    602/// in `rustc_trait_selection`.
    603mod impls {
    604    use super::TrivialClone;
    605    use crate::marker::PointeeSized;
    606
    607    macro_rules! impl_clone {
    608        ($($t:ty)*) => {
    609            $(
    610                #[stable(feature = "rust1", since = "1.0.0")]
    611                #[rustc_const_unstable(feature = "const_clone", issue = "142757")]
    612                impl const Clone for $t {
    613                    #[inline(always)]
    614                    fn clone(&self) -> Self {
    615                        *self
    616                    }
    617                }
    618
    619                #[doc(hidden)]
    620                #[unstable(feature = "trivial_clone", issue = "none")]
    621                #[rustc_const_unstable(feature = "const_clone", issue = "142757")]
    622                unsafe impl const TrivialClone for $t {}
    623            )*
    624        }
    625    }
    626
    627    impl_clone! {
    628        usize u8 u16 u32 u64 u128
    629        isize i8 i16 i32 i64 i128
    630        f16 f32 f64 f128
    631        bool char
    632    }
    633
    634    #[unstable(feature = "never_type", issue = "35121")]
    635    #[rustc_const_unstable(feature = "const_clone", issue = "142757")]
    636    impl const Clone for ! {
    637        #[inline]
    638        fn clone(&self) -> Self {
    639            *self
    640        }
    641    }
    642
    643    #[doc(hidden)]
    644    #[unstable(feature = "trivial_clone", issue = "none")]
    645    #[rustc_const_unstable(feature = "const_clone", issue = "142757")]
    646    unsafe impl const TrivialClone for ! {}
    647
    648    #[stable(feature = "rust1", since = "1.0.0")]
    649    #[rustc_const_unstable(feature = "const_clone", issue = "142757")]
    650    impl<T: PointeeSized> const Clone for *const T {
    651        #[inline(always)]
    652        fn clone(&self) -> Self {
    653            *self
    654        }
    655    }
    656
    657    #[doc(hidden)]
    658    #[unstable(feature = "trivial_clone", issue = "none")]
    659    #[rustc_const_unstable(feature = "const_clone", issue = "142757")]
    660    unsafe impl<T: PointeeSized> const TrivialClone for *const T {}
    661
    662    #[stable(feature = "rust1", since = "1.0.0")]
    663    #[rustc_const_unstable(feature = "const_clone", issue = "142757")]
    664    impl<T: PointeeSized> const Clone for *mut T {
    665        #[inline(always)]
    666        fn clone(&self) -> Self {
    667            *self
    668        }
    669    }
    670
    671    #[doc(hidden)]
    672    #[unstable(feature = "trivial_clone", issue = "none")]
    673    #[rustc_const_unstable(feature = "const_clone", issue = "142757")]
    674    unsafe impl<T: PointeeSized> const TrivialClone for *mut T {}
    675
    676    /// Shared references can be cloned, but mutable references *cannot*!
    677    #[stable(feature = "rust1", since = "1.0.0")]
    678    #[rustc_const_unstable(feature = "const_clone", issue = "142757")]
    679    impl<T: PointeeSized> const Clone for &T {
    680        #[inline(always)]
    681        #[rustc_diagnostic_item = "noop_method_clone"]
    682        fn clone(&self) -> Self {
    683            self
    684        }
    685    }
    686
    687    #[doc(hidden)]
    688    #[unstable(feature = "trivial_clone", issue = "none")]
    689    #[rustc_const_unstable(feature = "const_clone", issue = "142757")]
    690    unsafe impl<T: PointeeSized> const TrivialClone for &T {}
    691
    692    /// Shared references can be cloned, but mutable references *cannot*!
    693    #[stable(feature = "rust1", since = "1.0.0")]
    694    impl<T: PointeeSized> !Clone for &mut T {}
    695}