1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271
//! 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")]
/// 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($item:item) {
/* compiler built-in */
}
// FIXME(aburka): these structs are used solely by #[derive] to
// assert that every component of a type implements Clone or Copy.
//
// These structs should never appear in user code.
#[doc(hidden)]
#[allow(missing_debug_implementations)]
#[unstable(
feature = "derive_clone_copy",
reason = "deriving hack, should not be public",
issue = "none"
)]
pub struct AssertParamIsClone<T: Clone + ?Sized> {
_field: crate::marker::PhantomData<T>,
}
#[doc(hidden)]
#[allow(missing_debug_implementations)]
#[unstable(
feature = "derive_clone_copy",
reason = "deriving hack, should not be public",
issue = "none"
)]
pub struct AssertParamIsCopy<T: Copy + ?Sized> {
_field: crate::marker::PhantomData<T>,
}
/// Implementations of `Clone` for primitive types.
///
/// Implementations that cannot be described in Rust
/// are implemented in `traits::SelectionContext::copy_clone_conditions()`
/// in `rustc_trait_selection`.
mod impls {
macro_rules! impl_clone {
($($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
f32 f64
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 {}
}