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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494
//! A module for working with borrowed data.
#![stable(feature = "rust1", since = "1.0.0")]
use core::cmp::Ordering;
use core::hash::{Hash, Hasher};
use core::ops::Deref;
#[cfg(not(no_global_oom_handling))]
use core::ops::{Add, AddAssign};
#[stable(feature = "rust1", since = "1.0.0")]
pub use core::borrow::{Borrow, BorrowMut};
use crate::fmt;
#[cfg(not(no_global_oom_handling))]
use crate::string::String;
use Cow::*;
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, B: ?Sized> Borrow<B> for Cow<'a, B>
where
B: ToOwned,
{
fn borrow(&self) -> &B {
&**self
}
}
/// A generalization of `Clone` to borrowed data.
///
/// Some types make it possible to go from borrowed to owned, usually by
/// implementing the `Clone` trait. But `Clone` works only for going from `&T`
/// to `T`. The `ToOwned` trait generalizes `Clone` to construct owned data
/// from any borrow of a given type.
#[cfg_attr(not(test), rustc_diagnostic_item = "ToOwned")]
#[stable(feature = "rust1", since = "1.0.0")]
pub trait ToOwned {
/// The resulting type after obtaining ownership.
#[stable(feature = "rust1", since = "1.0.0")]
type Owned: Borrow<Self>;
/// Creates owned data from borrowed data, usually by cloning.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let s: &str = "a";
/// let ss: String = s.to_owned();
///
/// let v: &[i32] = &[1, 2];
/// let vv: Vec<i32> = v.to_owned();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[must_use = "cloning is often expensive and is not expected to have side effects"]
#[cfg_attr(not(test), rustc_diagnostic_item = "to_owned_method")]
fn to_owned(&self) -> Self::Owned;
/// Uses borrowed data to replace owned data, usually by cloning.
///
/// This is borrow-generalized version of [`Clone::clone_from`].
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let mut s: String = String::new();
/// "hello".clone_into(&mut s);
///
/// let mut v: Vec<i32> = Vec::new();
/// [1, 2][..].clone_into(&mut v);
/// ```
#[stable(feature = "toowned_clone_into", since = "1.63.0")]
fn clone_into(&self, target: &mut Self::Owned) {
*target = self.to_owned();
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> ToOwned for T
where
T: Clone,
{
type Owned = T;
fn to_owned(&self) -> T {
self.clone()
}
fn clone_into(&self, target: &mut T) {
target.clone_from(self);
}
}
/// A clone-on-write smart pointer.
///
/// The type `Cow` is a smart pointer providing clone-on-write functionality: it
/// can enclose and provide immutable access to borrowed data, and clone the
/// data lazily when mutation or ownership is required. The type is designed to
/// work with general borrowed data via the `Borrow` trait.
///
/// `Cow` implements `Deref`, which means that you can call
/// non-mutating methods directly on the data it encloses. If mutation
/// is desired, `to_mut` will obtain a mutable reference to an owned
/// value, cloning if necessary.
///
/// If you need reference-counting pointers, note that
/// [`Rc::make_mut`][crate::rc::Rc::make_mut] and
/// [`Arc::make_mut`][crate::sync::Arc::make_mut] can provide clone-on-write
/// functionality as well.
///
/// # Examples
///
/// ```
/// use std::borrow::Cow;
///
/// fn abs_all(input: &mut Cow<'_, [i32]>) {
/// for i in 0..input.len() {
/// let v = input[i];
/// if v < 0 {
/// // Clones into a vector if not already owned.
/// input.to_mut()[i] = -v;
/// }
/// }
/// }
///
/// // No clone occurs because `input` doesn't need to be mutated.
/// let slice = [0, 1, 2];
/// let mut input = Cow::from(&slice[..]);
/// abs_all(&mut input);
///
/// // Clone occurs because `input` needs to be mutated.
/// let slice = [-1, 0, 1];
/// let mut input = Cow::from(&slice[..]);
/// abs_all(&mut input);
///
/// // No clone occurs because `input` is already owned.
/// let mut input = Cow::from(vec![-1, 0, 1]);
/// abs_all(&mut input);
/// ```
///
/// Another example showing how to keep `Cow` in a struct:
///
/// ```
/// use std::borrow::Cow;
///
/// struct Items<'a, X> where [X]: ToOwned<Owned = Vec<X>> {
/// values: Cow<'a, [X]>,
/// }
///
/// impl<'a, X: Clone + 'a> Items<'a, X> where [X]: ToOwned<Owned = Vec<X>> {
/// fn new(v: Cow<'a, [X]>) -> Self {
/// Items { values: v }
/// }
/// }
///
/// // Creates a container from borrowed values of a slice
/// let readonly = [1, 2];
/// let borrowed = Items::new((&readonly[..]).into());
/// match borrowed {
/// Items { values: Cow::Borrowed(b) } => println!("borrowed {b:?}"),
/// _ => panic!("expect borrowed value"),
/// }
///
/// let mut clone_on_write = borrowed;
/// // Mutates the data from slice into owned vec and pushes a new value on top
/// clone_on_write.values.to_mut().push(3);
/// println!("clone_on_write = {:?}", clone_on_write.values);
///
/// // The data was mutated. Let's check it out.
/// match clone_on_write {
/// Items { values: Cow::Owned(_) } => println!("clone_on_write contains owned data"),
/// _ => panic!("expect owned data"),
/// }
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "Cow")]
pub enum Cow<'a, B: ?Sized + 'a>
where
B: ToOwned,
{
/// Borrowed data.
#[stable(feature = "rust1", since = "1.0.0")]
Borrowed(#[stable(feature = "rust1", since = "1.0.0")] &'a B),
/// Owned data.
#[stable(feature = "rust1", since = "1.0.0")]
Owned(#[stable(feature = "rust1", since = "1.0.0")] <B as ToOwned>::Owned),
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<B: ?Sized + ToOwned> Clone for Cow<'_, B> {
fn clone(&self) -> Self {
match *self {
Borrowed(b) => Borrowed(b),
Owned(ref o) => {
let b: &B = o.borrow();
Owned(b.to_owned())
}
}
}
fn clone_from(&mut self, source: &Self) {
match (self, source) {
(&mut Owned(ref mut dest), &Owned(ref o)) => o.borrow().clone_into(dest),
(t, s) => *t = s.clone(),
}
}
}
impl<B: ?Sized + ToOwned> Cow<'_, B> {
/// Returns true if the data is borrowed, i.e. if `to_mut` would require additional work.
///
/// # Examples
///
/// ```
/// #![feature(cow_is_borrowed)]
/// use std::borrow::Cow;
///
/// let cow = Cow::Borrowed("moo");
/// assert!(cow.is_borrowed());
///
/// let bull: Cow<'_, str> = Cow::Owned("...moo?".to_string());
/// assert!(!bull.is_borrowed());
/// ```
#[unstable(feature = "cow_is_borrowed", issue = "65143")]
#[rustc_const_unstable(feature = "const_cow_is_borrowed", issue = "65143")]
pub const fn is_borrowed(&self) -> bool {
match *self {
Borrowed(_) => true,
Owned(_) => false,
}
}
/// Returns true if the data is owned, i.e. if `to_mut` would be a no-op.
///
/// # Examples
///
/// ```
/// #![feature(cow_is_borrowed)]
/// use std::borrow::Cow;
///
/// let cow: Cow<'_, str> = Cow::Owned("moo".to_string());
/// assert!(cow.is_owned());
///
/// let bull = Cow::Borrowed("...moo?");
/// assert!(!bull.is_owned());
/// ```
#[unstable(feature = "cow_is_borrowed", issue = "65143")]
#[rustc_const_unstable(feature = "const_cow_is_borrowed", issue = "65143")]
pub const fn is_owned(&self) -> bool {
!self.is_borrowed()
}
/// Acquires a mutable reference to the owned form of the data.
///
/// Clones the data if it is not already owned.
///
/// # Examples
///
/// ```
/// use std::borrow::Cow;
///
/// let mut cow = Cow::Borrowed("foo");
/// cow.to_mut().make_ascii_uppercase();
///
/// assert_eq!(
/// cow,
/// Cow::Owned(String::from("FOO")) as Cow<'_, str>
/// );
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn to_mut(&mut self) -> &mut <B as ToOwned>::Owned {
match *self {
Borrowed(borrowed) => {
*self = Owned(borrowed.to_owned());
match *self {
Borrowed(..) => unreachable!(),
Owned(ref mut owned) => owned,
}
}
Owned(ref mut owned) => owned,
}
}
/// Extracts the owned data.
///
/// Clones the data if it is not already owned.
///
/// # Examples
///
/// Calling `into_owned` on a `Cow::Borrowed` returns a clone of the borrowed data:
///
/// ```
/// use std::borrow::Cow;
///
/// let s = "Hello world!";
/// let cow = Cow::Borrowed(s);
///
/// assert_eq!(
/// cow.into_owned(),
/// String::from(s)
/// );
/// ```
///
/// Calling `into_owned` on a `Cow::Owned` returns the owned data. The data is moved out of the
/// `Cow` without being cloned.
///
/// ```
/// use std::borrow::Cow;
///
/// let s = "Hello world!";
/// let cow: Cow<'_, str> = Cow::Owned(String::from(s));
///
/// assert_eq!(
/// cow.into_owned(),
/// String::from(s)
/// );
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn into_owned(self) -> <B as ToOwned>::Owned {
match self {
Borrowed(borrowed) => borrowed.to_owned(),
Owned(owned) => owned,
}
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<B: ?Sized + ToOwned> Deref for Cow<'_, B>
where
B::Owned: Borrow<B>,
{
type Target = B;
fn deref(&self) -> &B {
match *self {
Borrowed(borrowed) => borrowed,
Owned(ref owned) => owned.borrow(),
}
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<B: ?Sized> Eq for Cow<'_, B> where B: Eq + ToOwned {}
#[stable(feature = "rust1", since = "1.0.0")]
impl<B: ?Sized> Ord for Cow<'_, B>
where
B: Ord + ToOwned,
{
#[inline]
fn cmp(&self, other: &Self) -> Ordering {
Ord::cmp(&**self, &**other)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, 'b, B: ?Sized, C: ?Sized> PartialEq<Cow<'b, C>> for Cow<'a, B>
where
B: PartialEq<C> + ToOwned,
C: ToOwned,
{
#[inline]
fn eq(&self, other: &Cow<'b, C>) -> bool {
PartialEq::eq(&**self, &**other)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, B: ?Sized> PartialOrd for Cow<'a, B>
where
B: PartialOrd + ToOwned,
{
#[inline]
fn partial_cmp(&self, other: &Cow<'a, B>) -> Option<Ordering> {
PartialOrd::partial_cmp(&**self, &**other)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<B: ?Sized> fmt::Debug for Cow<'_, B>
where
B: fmt::Debug + ToOwned<Owned: fmt::Debug>,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match *self {
Borrowed(ref b) => fmt::Debug::fmt(b, f),
Owned(ref o) => fmt::Debug::fmt(o, f),
}
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<B: ?Sized> fmt::Display for Cow<'_, B>
where
B: fmt::Display + ToOwned<Owned: fmt::Display>,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match *self {
Borrowed(ref b) => fmt::Display::fmt(b, f),
Owned(ref o) => fmt::Display::fmt(o, f),
}
}
}
#[stable(feature = "default", since = "1.11.0")]
impl<B: ?Sized> Default for Cow<'_, B>
where
B: ToOwned<Owned: Default>,
{
/// Creates an owned Cow<'a, B> with the default value for the contained owned value.
fn default() -> Self {
Owned(<B as ToOwned>::Owned::default())
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<B: ?Sized> Hash for Cow<'_, B>
where
B: Hash + ToOwned,
{
#[inline]
fn hash<H: Hasher>(&self, state: &mut H) {
Hash::hash(&**self, state)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + ToOwned> AsRef<T> for Cow<'_, T> {
fn as_ref(&self) -> &T {
self
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "cow_add", since = "1.14.0")]
impl<'a> Add<&'a str> for Cow<'a, str> {
type Output = Cow<'a, str>;
#[inline]
fn add(mut self, rhs: &'a str) -> Self::Output {
self += rhs;
self
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "cow_add", since = "1.14.0")]
impl<'a> Add<Cow<'a, str>> for Cow<'a, str> {
type Output = Cow<'a, str>;
#[inline]
fn add(mut self, rhs: Cow<'a, str>) -> Self::Output {
self += rhs;
self
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "cow_add", since = "1.14.0")]
impl<'a> AddAssign<&'a str> for Cow<'a, str> {
fn add_assign(&mut self, rhs: &'a str) {
if self.is_empty() {
*self = Cow::Borrowed(rhs)
} else if !rhs.is_empty() {
if let Cow::Borrowed(lhs) = *self {
let mut s = String::with_capacity(lhs.len() + rhs.len());
s.push_str(lhs);
*self = Cow::Owned(s);
}
self.to_mut().push_str(rhs);
}
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "cow_add", since = "1.14.0")]
impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str> {
fn add_assign(&mut self, rhs: Cow<'a, str>) {
if self.is_empty() {
*self = rhs
} else if !rhs.is_empty() {
if let Cow::Borrowed(lhs) = *self {
let mut s = String::with_capacity(lhs.len() + rhs.len());
s.push_str(lhs);
*self = Cow::Owned(s);
}
self.to_mut().push_str(&rhs);
}
}
}