library/alloc/src/boxed.rs

   1 //! A pointer type for heap allocation.
   2 //!
   3 //! [`Box<T>`], casually referred to as a 'box', provides the simplest form of
   4 //! heap allocation in Rust. Boxes provide ownership for this allocation, and
   5 //! drop their contents when they go out of scope. Boxes also ensure that they
   6 //! never allocate more than `isize::MAX` bytes.
   7 //!
   8 //! # Examples
   9 //!
  10 //! Move a value from the stack to the heap by creating a [`Box`]:
  11 //!
  12 //! ```
  13 //! let val: u8 = 5;
  14 //! let boxed: Box<u8> = Box::new(val);
  15 //! ```
  16 //!
  17 //! Move a value from a [`Box`] back to the stack by [dereferencing]:
  18 //!
  19 //! ```
  20 //! let boxed: Box<u8> = Box::new(5);
  21 //! let val: u8 = *boxed;
  22 //! ```
  23 //!
  24 //! Creating a recursive data structure:
  25 //!
  26 //! ```
  27 //! #[derive(Debug)]
  28 //! enum List<T> {
  29 //!     Cons(T, Box<List<T>>),
  30 //!     Nil,
  31 //! }
  32 //!
  33 //! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
  34 //! println!("{:?}", list);
  35 //! ```
  36 //!
  37 //! This will print `Cons(1, Cons(2, Nil))`.
  38 //!
  39 //! Recursive structures must be boxed, because if the definition of `Cons`
  40 //! looked like this:
  41 //!
  42 //! ```compile_fail,E0072
  43 //! # enum List<T> {
  44 //! Cons(T, List<T>),
  45 //! # }
  46 //! ```
  47 //!
  48 //! It wouldn't work. This is because the size of a `List` depends on how many
  49 //! elements are in the list, and so we don't know how much memory to allocate
  50 //! for a `Cons`. By introducing a [`Box<T>`], which has a defined size, we know how
  51 //! big `Cons` needs to be.
  52 //!
  53 //! # Memory layout
  54 //!
  55 //! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
  56 //! its allocation. It is valid to convert both ways between a [`Box`] and a
  57 //! raw pointer allocated with the [`Global`] allocator, given that the
  58 //! [`Layout`] used with the allocator is correct for the type. More precisely,
  59 //! a `value: *mut T` that has been allocated with the [`Global`] allocator
  60 //! with `Layout::for_value(&*value)` may be converted into a box using
  61 //! [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut
  62 //! T` obtained from [`Box::<T>::into_raw`] may be deallocated using the
  63 //! [`Global`] allocator with [`Layout::for_value(&*value)`].
  64 //!
  65 //! For zero-sized values, the `Box` pointer still has to be [valid] for reads
  66 //! and writes and sufficiently aligned. In particular, casting any aligned
  67 //! non-zero integer literal to a raw pointer produces a valid pointer, but a
  68 //! pointer pointing into previously allocated memory that since got freed is
  69 //! not valid. The recommended way to build a Box to a ZST if `Box::new` cannot
  70 //! be used is to use [`ptr::NonNull::dangling`].
  71 //!
  72 //! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented
  73 //! as a single pointer and is also ABI-compatible with C pointers
  74 //! (i.e. the C type `T*`). This means that if you have extern "C"
  75 //! Rust functions that will be called from C, you can define those
  76 //! Rust functions using `Box<T>` types, and use `T*` as corresponding
  77 //! type on the C side. As an example, consider this C header which
  78 //! declares functions that create and destroy some kind of `Foo`
  79 //! value:
  80 //!
  81 //! ```c
  82 //! /* C header */
  83 //!
  84 //! /* Returns ownership to the caller */
  85 //! struct Foo* foo_new(void);
  86 //!
  87 //! /* Takes ownership from the caller; no-op when invoked with NULL */
  88 //! void foo_delete(struct Foo*);
  89 //! ```
  90 //!
  91 //! These two functions might be implemented in Rust as follows. Here, the
  92 //! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
  93 //! the ownership constraints. Note also that the nullable argument to
  94 //! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
  95 //! cannot be null.
  96 //!
  97 //! ```
  98 //! #[repr(C)]
  99 //! pub struct Foo;
 100 //!
 101 //! #[no_mangle]
 102 //! pub extern "C" fn foo_new() -> Box<Foo> {
 103 //!     Box::new(Foo)
 104 //! }
 105 //!
 106 //! #[no_mangle]
 107 //! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
 108 //! ```
 109 //!
 110 //! Even though `Box<T>` has the same representation and C ABI as a C pointer,
 111 //! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
 112 //! and expect things to work. `Box<T>` values will always be fully aligned,
 113 //! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
 114 //! free the value with the global allocator. In general, the best practice
 115 //! is to only use `Box<T>` for pointers that originated from the global
 116 //! allocator.
 117 //!
 118 //! **Important.** At least at present, you should avoid using
 119 //! `Box<T>` types for functions that are defined in C but invoked
 120 //! from Rust. In those cases, you should directly mirror the C types
 121 //! as closely as possible. Using types like `Box<T>` where the C
 122 //! definition is just using `T*` can lead to undefined behavior, as
 123 //! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
 124 //!
 125 //! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
 126 //! [dereferencing]: core::ops::Deref
 127 //! [`Box::<T>::from_raw(value)`]: Box::from_raw
 128 //! [`Global`]: crate::alloc::Global
 129 //! [`Layout`]: crate::alloc::Layout
 130 //! [`Layout::for_value(&*value)`]: crate::alloc::Layout::for_value
 131 //! [valid]: ptr#safety
 132
 133 #![stable(feature = "rust1", since = "1.0.0")]
 134
 135 use core::any::Any;
 136 use core::borrow;
 137 use core::cmp::Ordering;
 138 use core::convert::{From, TryFrom};
 139 use core::fmt;
 140 use core::future::Future;
 141 use core::hash::{Hash, Hasher};
 142 use core::iter::{FromIterator, FusedIterator, Iterator};
 143 use core::marker::{Unpin, Unsize};
 144 use core::mem;
 145 use core::ops::{
 146     CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Generator, GeneratorState, Receiver,
 147 };
 148 use core::pin::Pin;
 149 use core::ptr::{self, Unique};
 150 use core::stream::Stream;
 151 use core::task::{Context, Poll};
 152
 153 use crate::alloc::{handle_alloc_error, AllocError, Allocator, Global, Layout, WriteCloneIntoRaw};
 154 use crate::borrow::Cow;
 155 use crate::raw_vec::RawVec;
 156 use crate::str::from_boxed_utf8_unchecked;
 157 use crate::vec::Vec;
 158
 159 /// A pointer type for heap allocation.
 160 ///
 161 /// See the [module-level documentation](../../std/boxed/index.html) for more.
 162 #[lang = "owned_box"]
 163 #[fundamental]
 164 #[stable(feature = "rust1", since = "1.0.0")]
 165 pub struct Box<
 166     T: ?Sized,
 167     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
 168 >(Unique<T>, A);
 169
 170 impl<T> Box<T> {
 171     /// Allocates memory on the heap and then places `x` into it.
 172     ///
 173     /// This doesn't actually allocate if `T` is zero-sized.
 174     ///
 175     /// # Examples
 176     ///
 177     /// ```
 178     /// let five = Box::new(5);
 179     /// ```
 180     #[inline(always)]
 181     #[doc(alias = "alloc")]
 182     #[doc(alias = "malloc")]
 183     #[stable(feature = "rust1", since = "1.0.0")]
 184     pub fn new(x: T) -> Self {
 185         box x
 186     }
 187
 188     /// Constructs a new box with uninitialized contents.
 189     ///
 190     /// # Examples
 191     ///
 192     /// ```
 193     /// #![feature(new_uninit)]
 194     ///
 195     /// let mut five = Box::<u32>::new_uninit();
 196     ///
 197     /// let five = unsafe {
 198     ///     // Deferred initialization:
 199     ///     five.as_mut_ptr().write(5);
 200     ///
 201     ///     five.assume_init()
 202     /// };
 203     ///
 204     /// assert_eq!(*five, 5)
 205     /// ```
 206     #[unstable(feature = "new_uninit", issue = "63291")]
 207     #[inline]
 208     pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
 209         Self::new_uninit_in(Global)
 210     }
 211
 212     /// Constructs a new `Box` with uninitialized contents, with the memory
 213     /// being filled with `0` bytes.
 214     ///
 215     /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
 216     /// of this method.
 217     ///
 218     /// # Examples
 219     ///
 220     /// ```
 221     /// #![feature(new_uninit)]
 222     ///
 223     /// let zero = Box::<u32>::new_zeroed();
 224     /// let zero = unsafe { zero.assume_init() };
 225     ///
 226     /// assert_eq!(*zero, 0)
 227     /// ```
 228     ///
 229     /// [zeroed]: mem::MaybeUninit::zeroed
 230     #[inline]
 231     #[doc(alias = "calloc")]
 232     #[unstable(feature = "new_uninit", issue = "63291")]
 233     pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
 234         Self::new_zeroed_in(Global)
 235     }
 236
 237     /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
 238     /// `x` will be pinned in memory and unable to be moved.
 239     #[stable(feature = "pin", since = "1.33.0")]
 240     #[inline(always)]
 241     pub fn pin(x: T) -> Pin<Box<T>> {
 242         (box x).into()
 243     }
 244
 245     /// Allocates memory on the heap then places `x` into it,
 246     /// returning an error if the allocation fails
 247     ///
 248     /// This doesn't actually allocate if `T` is zero-sized.
 249     ///
 250     /// # Examples
 251     ///
 252     /// ```
 253     /// #![feature(allocator_api)]
 254     ///
 255     /// let five = Box::try_new(5)?;
 256     /// # Ok::<(), std::alloc::AllocError>(())
 257     /// ```
 258     #[unstable(feature = "allocator_api", issue = "32838")]
 259     #[inline]
 260     pub fn try_new(x: T) -> Result<Self, AllocError> {
 261         Self::try_new_in(x, Global)
 262     }
 263
 264     /// Constructs a new box with uninitialized contents on the heap,
 265     /// returning an error if the allocation fails
 266     ///
 267     /// # Examples
 268     ///
 269     /// ```
 270     /// #![feature(allocator_api, new_uninit)]
 271     ///
 272     /// let mut five = Box::<u32>::try_new_uninit()?;
 273     ///
 274     /// let five = unsafe {
 275     ///     // Deferred initialization:
 276     ///     five.as_mut_ptr().write(5);
 277     ///
 278     ///     five.assume_init()
 279     /// };
 280     ///
 281     /// assert_eq!(*five, 5);
 282     /// # Ok::<(), std::alloc::AllocError>(())
 283     /// ```
 284     #[unstable(feature = "allocator_api", issue = "32838")]
 285     // #[unstable(feature = "new_uninit", issue = "63291")]
 286     #[inline]
 287     pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
 288         Box::try_new_uninit_in(Global)
 289     }
 290
 291     /// Constructs a new `Box` with uninitialized contents, with the memory
 292     /// being filled with `0` bytes on the heap
 293     ///
 294     /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
 295     /// of this method.
 296     ///
 297     /// # Examples
 298     ///
 299     /// ```
 300     /// #![feature(allocator_api, new_uninit)]
 301     ///
 302     /// let zero = Box::<u32>::try_new_zeroed()?;
 303     /// let zero = unsafe { zero.assume_init() };
 304     ///
 305     /// assert_eq!(*zero, 0);
 306     /// # Ok::<(), std::alloc::AllocError>(())
 307     /// ```
 308     ///
 309     /// [zeroed]: mem::MaybeUninit::zeroed
 310     #[unstable(feature = "allocator_api", issue = "32838")]
 311     // #[unstable(feature = "new_uninit", issue = "63291")]
 312     #[inline]
 313     pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
 314         Box::try_new_zeroed_in(Global)
 315     }
 316 }
 317
 318 impl<T, A: Allocator> Box<T, A> {
 319     /// Allocates memory in the given allocator then places `x` into it.
 320     ///
 321     /// This doesn't actually allocate if `T` is zero-sized.
 322     ///
 323     /// # Examples
 324     ///
 325     /// ```
 326     /// #![feature(allocator_api)]
 327     ///
 328     /// use std::alloc::System;
 329     ///
 330     /// let five = Box::new_in(5, System);
 331     /// ```
 332     #[unstable(feature = "allocator_api", issue = "32838")]
 333     #[inline]
 334     pub fn new_in(x: T, alloc: A) -> Self {
 335         let mut boxed = Self::new_uninit_in(alloc);
 336         unsafe {
 337             boxed.as_mut_ptr().write(x);
 338             boxed.assume_init()
 339         }
 340     }
 341
 342     /// Allocates memory in the given allocator then places `x` into it,
 343     /// returning an error if the allocation fails
 344     ///
 345     /// This doesn't actually allocate if `T` is zero-sized.
 346     ///
 347     /// # Examples
 348     ///
 349     /// ```
 350     /// #![feature(allocator_api)]
 351     ///
 352     /// use std::alloc::System;
 353     ///
 354     /// let five = Box::try_new_in(5, System)?;
 355     /// # Ok::<(), std::alloc::AllocError>(())
 356     /// ```
 357     #[unstable(feature = "allocator_api", issue = "32838")]
 358     #[inline]
 359     pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError> {
 360         let mut boxed = Self::try_new_uninit_in(alloc)?;
 361         unsafe {
 362             boxed.as_mut_ptr().write(x);
 363             Ok(boxed.assume_init())
 364         }
 365     }
 366
 367     /// Constructs a new box with uninitialized contents in the provided allocator.
 368     ///
 369     /// # Examples
 370     ///
 371     /// ```
 372     /// #![feature(allocator_api, new_uninit)]
 373     ///
 374     /// use std::alloc::System;
 375     ///
 376     /// let mut five = Box::<u32, _>::new_uninit_in(System);
 377     ///
 378     /// let five = unsafe {
 379     ///     // Deferred initialization:
 380     ///     five.as_mut_ptr().write(5);
 381     ///
 382     ///     five.assume_init()
 383     /// };
 384     ///
 385     /// assert_eq!(*five, 5)
 386     /// ```
 387     #[unstable(feature = "allocator_api", issue = "32838")]
 388     // #[unstable(feature = "new_uninit", issue = "63291")]
 389     pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
 390         let layout = Layout::new::<mem::MaybeUninit<T>>();
 391         // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
 392         // That would make code size bigger.
 393         match Box::try_new_uninit_in(alloc) {
 394             Ok(m) => m,
 395             Err(_) => handle_alloc_error(layout),
 396         }
 397     }
 398
 399     /// Constructs a new box with uninitialized contents in the provided allocator,
 400     /// returning an error if the allocation fails
 401     ///
 402     /// # Examples
 403     ///
 404     /// ```
 405     /// #![feature(allocator_api, new_uninit)]
 406     ///
 407     /// use std::alloc::System;
 408     ///
 409     /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
 410     ///
 411     /// let five = unsafe {
 412     ///     // Deferred initialization:
 413     ///     five.as_mut_ptr().write(5);
 414     ///
 415     ///     five.assume_init()
 416     /// };
 417     ///
 418     /// assert_eq!(*five, 5);
 419     /// # Ok::<(), std::alloc::AllocError>(())
 420     /// ```
 421     #[unstable(feature = "allocator_api", issue = "32838")]
 422     // #[unstable(feature = "new_uninit", issue = "63291")]
 423     pub fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError> {
 424         let layout = Layout::new::<mem::MaybeUninit<T>>();
 425         let ptr = alloc.allocate(layout)?.cast();
 426         unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
 427     }
 428
 429     /// Constructs a new `Box` with uninitialized contents, with the memory
 430     /// being filled with `0` bytes in the provided allocator.
 431     ///
 432     /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
 433     /// of this method.
 434     ///
 435     /// # Examples
 436     ///
 437     /// ```
 438     /// #![feature(allocator_api, new_uninit)]
 439     ///
 440     /// use std::alloc::System;
 441     ///
 442     /// let zero = Box::<u32, _>::new_zeroed_in(System);
 443     /// let zero = unsafe { zero.assume_init() };
 444     ///
 445     /// assert_eq!(*zero, 0)
 446     /// ```
 447     ///
 448     /// [zeroed]: mem::MaybeUninit::zeroed
 449     #[unstable(feature = "allocator_api", issue = "32838")]
 450     // #[unstable(feature = "new_uninit", issue = "63291")]
 451     pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
 452         let layout = Layout::new::<mem::MaybeUninit<T>>();
 453         // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
 454         // That would make code size bigger.
 455         match Box::try_new_zeroed_in(alloc) {
 456             Ok(m) => m,
 457             Err(_) => handle_alloc_error(layout),
 458         }
 459     }
 460
 461     /// Constructs a new `Box` with uninitialized contents, with the memory
 462     /// being filled with `0` bytes in the provided allocator,
 463     /// returning an error if the allocation fails,
 464     ///
 465     /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
 466     /// of this method.
 467     ///
 468     /// # Examples
 469     ///
 470     /// ```
 471     /// #![feature(allocator_api, new_uninit)]
 472     ///
 473     /// use std::alloc::System;
 474     ///
 475     /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
 476     /// let zero = unsafe { zero.assume_init() };
 477     ///
 478     /// assert_eq!(*zero, 0);
 479     /// # Ok::<(), std::alloc::AllocError>(())
 480     /// ```
 481     ///
 482     /// [zeroed]: mem::MaybeUninit::zeroed
 483     #[unstable(feature = "allocator_api", issue = "32838")]
 484     // #[unstable(feature = "new_uninit", issue = "63291")]
 485     pub fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError> {
 486         let layout = Layout::new::<mem::MaybeUninit<T>>();
 487         let ptr = alloc.allocate_zeroed(layout)?.cast();
 488         unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
 489     }
 490
 491     /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement `Unpin`, then
 492     /// `x` will be pinned in memory and unable to be moved.
 493     #[unstable(feature = "allocator_api", issue = "32838")]
 494     #[inline(always)]
 495     pub fn pin_in(x: T, alloc: A) -> Pin<Self>
 496     where
 497         A: 'static,
 498     {
 499         Self::new_in(x, alloc).into()
 500     }
 501
 502     /// Converts a `Box<T>` into a `Box<[T]>`
 503     ///
 504     /// This conversion does not allocate on the heap and happens in place.
 505     #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
 506     pub fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
 507         let (raw, alloc) = Box::into_raw_with_allocator(boxed);
 508         unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
 509     }
 510
 511     /// Consumes the `Box`, returning the wrapped value.
 512     ///
 513     /// # Examples
 514     ///
 515     /// ```
 516     /// #![feature(box_into_inner)]
 517     ///
 518     /// let c = Box::new(5);
 519     ///
 520     /// assert_eq!(Box::into_inner(c), 5);
 521     /// ```
 522     #[unstable(feature = "box_into_inner", issue = "80437")]
 523     #[inline]
 524     pub fn into_inner(boxed: Self) -> T {
 525         *boxed
 526     }
 527 }
 528
 529 impl<T> Box<[T]> {
 530     /// Constructs a new boxed slice with uninitialized contents.
 531     ///
 532     /// # Examples
 533     ///
 534     /// ```
 535     /// #![feature(new_uninit)]
 536     ///
 537     /// let mut values = Box::<[u32]>::new_uninit_slice(3);
 538     ///
 539     /// let values = unsafe {
 540     ///     // Deferred initialization:
 541     ///     values[0].as_mut_ptr().write(1);
 542     ///     values[1].as_mut_ptr().write(2);
 543     ///     values[2].as_mut_ptr().write(3);
 544     ///
 545     ///     values.assume_init()
 546     /// };
 547     ///
 548     /// assert_eq!(*values, [1, 2, 3])
 549     /// ```
 550     #[unstable(feature = "new_uninit", issue = "63291")]
 551     pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
 552         unsafe { RawVec::with_capacity(len).into_box(len) }
 553     }
 554
 555     /// Constructs a new boxed slice with uninitialized contents, with the memory
 556     /// being filled with `0` bytes.
 557     ///
 558     /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
 559     /// of this method.
 560     ///
 561     /// # Examples
 562     ///
 563     /// ```
 564     /// #![feature(new_uninit)]
 565     ///
 566     /// let values = Box::<[u32]>::new_zeroed_slice(3);
 567     /// let values = unsafe { values.assume_init() };
 568     ///
 569     /// assert_eq!(*values, [0, 0, 0])
 570     /// ```
 571     ///
 572     /// [zeroed]: mem::MaybeUninit::zeroed
 573     #[unstable(feature = "new_uninit", issue = "63291")]
 574     pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
 575         unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
 576     }
 577 }
 578
 579 impl<T, A: Allocator> Box<[T], A> {
 580     /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
 581     ///
 582     /// # Examples
 583     ///
 584     /// ```
 585     /// #![feature(allocator_api, new_uninit)]
 586     ///
 587     /// use std::alloc::System;
 588     ///
 589     /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
 590     ///
 591     /// let values = unsafe {
 592     ///     // Deferred initialization:
 593     ///     values[0].as_mut_ptr().write(1);
 594     ///     values[1].as_mut_ptr().write(2);
 595     ///     values[2].as_mut_ptr().write(3);
 596     ///
 597     ///     values.assume_init()
 598     /// };
 599     ///
 600     /// assert_eq!(*values, [1, 2, 3])
 601     /// ```
 602     #[unstable(feature = "allocator_api", issue = "32838")]
 603     // #[unstable(feature = "new_uninit", issue = "63291")]
 604     pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
 605         unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
 606     }
 607
 608     /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
 609     /// with the memory being filled with `0` bytes.
 610     ///
 611     /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
 612     /// of this method.
 613     ///
 614     /// # Examples
 615     ///
 616     /// ```
 617     /// #![feature(allocator_api, new_uninit)]
 618     ///
 619     /// use std::alloc::System;
 620     ///
 621     /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
 622     /// let values = unsafe { values.assume_init() };
 623     ///
 624     /// assert_eq!(*values, [0, 0, 0])
 625     /// ```
 626     ///
 627     /// [zeroed]: mem::MaybeUninit::zeroed
 628     #[unstable(feature = "allocator_api", issue = "32838")]
 629     // #[unstable(feature = "new_uninit", issue = "63291")]
 630     pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
 631         unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
 632     }
 633 }
 634
 635 impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
 636     /// Converts to `Box<T, A>`.
 637     ///
 638     /// # Safety
 639     ///
 640     /// As with [`MaybeUninit::assume_init`],
 641     /// it is up to the caller to guarantee that the value
 642     /// really is in an initialized state.
 643     /// Calling this when the content is not yet fully initialized
 644     /// causes immediate undefined behavior.
 645     ///
 646     /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
 647     ///
 648     /// # Examples
 649     ///
 650     /// ```
 651     /// #![feature(new_uninit)]
 652     ///
 653     /// let mut five = Box::<u32>::new_uninit();
 654     ///
 655     /// let five: Box<u32> = unsafe {
 656     ///     // Deferred initialization:
 657     ///     five.as_mut_ptr().write(5);
 658     ///
 659     ///     five.assume_init()
 660     /// };
 661     ///
 662     /// assert_eq!(*five, 5)
 663     /// ```
 664     #[unstable(feature = "new_uninit", issue = "63291")]
 665     #[inline]
 666     pub unsafe fn assume_init(self) -> Box<T, A> {
 667         let (raw, alloc) = Box::into_raw_with_allocator(self);
 668         unsafe { Box::from_raw_in(raw as *mut T, alloc) }
 669     }
 670 }
 671
 672 impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
 673     /// Converts to `Box<[T], A>`.
 674     ///
 675     /// # Safety
 676     ///
 677     /// As with [`MaybeUninit::assume_init`],
 678     /// it is up to the caller to guarantee that the values
 679     /// really are in an initialized state.
 680     /// Calling this when the content is not yet fully initialized
 681     /// causes immediate undefined behavior.
 682     ///
 683     /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
 684     ///
 685     /// # Examples
 686     ///
 687     /// ```
 688     /// #![feature(new_uninit)]
 689     ///
 690     /// let mut values = Box::<[u32]>::new_uninit_slice(3);
 691     ///
 692     /// let values = unsafe {
 693     ///     // Deferred initialization:
 694     ///     values[0].as_mut_ptr().write(1);
 695     ///     values[1].as_mut_ptr().write(2);
 696     ///     values[2].as_mut_ptr().write(3);
 697     ///
 698     ///     values.assume_init()
 699     /// };
 700     ///
 701     /// assert_eq!(*values, [1, 2, 3])
 702     /// ```
 703     #[unstable(feature = "new_uninit", issue = "63291")]
 704     #[inline]
 705     pub unsafe fn assume_init(self) -> Box<[T], A> {
 706         let (raw, alloc) = Box::into_raw_with_allocator(self);
 707         unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
 708     }
 709 }
 710
 711 impl<T: ?Sized> Box<T> {
 712     /// Constructs a box from a raw pointer.
 713     ///
 714     /// After calling this function, the raw pointer is owned by the
 715     /// resulting `Box`. Specifically, the `Box` destructor will call
 716     /// the destructor of `T` and free the allocated memory. For this
 717     /// to be safe, the memory must have been allocated in accordance
 718     /// with the [memory layout] used by `Box` .
 719     ///
 720     /// # Safety
 721     ///
 722     /// This function is unsafe because improper use may lead to
 723     /// memory problems. For example, a double-free may occur if the
 724     /// function is called twice on the same raw pointer.
 725     ///
 726     /// The safety conditions are described in the [memory layout] section.
 727     ///
 728     /// # Examples
 729     ///
 730     /// Recreate a `Box` which was previously converted to a raw pointer
 731     /// using [`Box::into_raw`]:
 732     /// ```
 733     /// let x = Box::new(5);
 734     /// let ptr = Box::into_raw(x);
 735     /// let x = unsafe { Box::from_raw(ptr) };
 736     /// ```
 737     /// Manually create a `Box` from scratch by using the global allocator:
 738     /// ```
 739     /// use std::alloc::{alloc, Layout};
 740     ///
 741     /// unsafe {
 742     ///     let ptr = alloc(Layout::new::<i32>()) as *mut i32;
 743     ///     // In general .write is required to avoid attempting to destruct
 744     ///     // the (uninitialized) previous contents of `ptr`, though for this
 745     ///     // simple example `*ptr = 5` would have worked as well.
 746     ///     ptr.write(5);
 747     ///     let x = Box::from_raw(ptr);
 748     /// }
 749     /// ```
 750     ///
 751     /// [memory layout]: self#memory-layout
 752     /// [`Layout`]: crate::Layout
 753     #[stable(feature = "box_raw", since = "1.4.0")]
 754     #[inline]
 755     pub unsafe fn from_raw(raw: *mut T) -> Self {
 756         unsafe { Self::from_raw_in(raw, Global) }
 757     }
 758 }
 759
 760 impl<T: ?Sized, A: Allocator> Box<T, A> {
 761     /// Constructs a box from a raw pointer in the given allocator.
 762     ///
 763     /// After calling this function, the raw pointer is owned by the
 764     /// resulting `Box`. Specifically, the `Box` destructor will call
 765     /// the destructor of `T` and free the allocated memory. For this
 766     /// to be safe, the memory must have been allocated in accordance
 767     /// with the [memory layout] used by `Box` .
 768     ///
 769     /// # Safety
 770     ///
 771     /// This function is unsafe because improper use may lead to
 772     /// memory problems. For example, a double-free may occur if the
 773     /// function is called twice on the same raw pointer.
 774     ///
 775     ///
 776     /// # Examples
 777     ///
 778     /// Recreate a `Box` which was previously converted to a raw pointer
 779     /// using [`Box::into_raw_with_allocator`]:
 780     /// ```
 781     /// #![feature(allocator_api)]
 782     ///
 783     /// use std::alloc::System;
 784     ///
 785     /// let x = Box::new_in(5, System);
 786     /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
 787     /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
 788     /// ```
 789     /// Manually create a `Box` from scratch by using the system allocator:
 790     /// ```
 791     /// #![feature(allocator_api, slice_ptr_get)]
 792     ///
 793     /// use std::alloc::{Allocator, Layout, System};
 794     ///
 795     /// unsafe {
 796     ///     let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
 797     ///     // In general .write is required to avoid attempting to destruct
 798     ///     // the (uninitialized) previous contents of `ptr`, though for this
 799     ///     // simple example `*ptr = 5` would have worked as well.
 800     ///     ptr.write(5);
 801     ///     let x = Box::from_raw_in(ptr, System);
 802     /// }
 803     /// # Ok::<(), std::alloc::AllocError>(())
 804     /// ```
 805     ///
 806     /// [memory layout]: self#memory-layout
 807     /// [`Layout`]: crate::Layout
 808     #[unstable(feature = "allocator_api", issue = "32838")]
 809     #[inline]
 810     pub unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
 811         Box(unsafe { Unique::new_unchecked(raw) }, alloc)
 812     }
 813
 814     /// Consumes the `Box`, returning a wrapped raw pointer.
 815     ///
 816     /// The pointer will be properly aligned and non-null.
 817     ///
 818     /// After calling this function, the caller is responsible for the
 819     /// memory previously managed by the `Box`. In particular, the
 820     /// caller should properly destroy `T` and release the memory, taking
 821     /// into account the [memory layout] used by `Box`. The easiest way to
 822     /// do this is to convert the raw pointer back into a `Box` with the
 823     /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
 824     /// the cleanup.
 825     ///
 826     /// Note: this is an associated function, which means that you have
 827     /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
 828     /// is so that there is no conflict with a method on the inner type.
 829     ///
 830     /// # Examples
 831     /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
 832     /// for automatic cleanup:
 833     /// ```
 834     /// let x = Box::new(String::from("Hello"));
 835     /// let ptr = Box::into_raw(x);
 836     /// let x = unsafe { Box::from_raw(ptr) };
 837     /// ```
 838     /// Manual cleanup by explicitly running the destructor and deallocating
 839     /// the memory:
 840     /// ```
 841     /// use std::alloc::{dealloc, Layout};
 842     /// use std::ptr;
 843     ///
 844     /// let x = Box::new(String::from("Hello"));
 845     /// let p = Box::into_raw(x);
 846     /// unsafe {
 847     ///     ptr::drop_in_place(p);
 848     ///     dealloc(p as *mut u8, Layout::new::<String>());
 849     /// }
 850     /// ```
 851     ///
 852     /// [memory layout]: self#memory-layout
 853     #[stable(feature = "box_raw", since = "1.4.0")]
 854     #[inline]
 855     pub fn into_raw(b: Self) -> *mut T {
 856         Self::into_raw_with_allocator(b).0
 857     }
 858
 859     /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
 860     ///
 861     /// The pointer will be properly aligned and non-null.
 862     ///
 863     /// After calling this function, the caller is responsible for the
 864     /// memory previously managed by the `Box`. In particular, the
 865     /// caller should properly destroy `T` and release the memory, taking
 866     /// into account the [memory layout] used by `Box`. The easiest way to
 867     /// do this is to convert the raw pointer back into a `Box` with the
 868     /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
 869     /// the cleanup.
 870     ///
 871     /// Note: this is an associated function, which means that you have
 872     /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
 873     /// is so that there is no conflict with a method on the inner type.
 874     ///
 875     /// # Examples
 876     /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
 877     /// for automatic cleanup:
 878     /// ```
 879     /// #![feature(allocator_api)]
 880     ///
 881     /// use std::alloc::System;
 882     ///
 883     /// let x = Box::new_in(String::from("Hello"), System);
 884     /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
 885     /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
 886     /// ```
 887     /// Manual cleanup by explicitly running the destructor and deallocating
 888     /// the memory:
 889     /// ```
 890     /// #![feature(allocator_api)]
 891     ///
 892     /// use std::alloc::{Allocator, Layout, System};
 893     /// use std::ptr::{self, NonNull};
 894     ///
 895     /// let x = Box::new_in(String::from("Hello"), System);
 896     /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
 897     /// unsafe {
 898     ///     ptr::drop_in_place(ptr);
 899     ///     let non_null = NonNull::new_unchecked(ptr);
 900     ///     alloc.deallocate(non_null.cast(), Layout::new::<String>());
 901     /// }
 902     /// ```
 903     ///
 904     /// [memory layout]: self#memory-layout
 905     #[unstable(feature = "allocator_api", issue = "32838")]
 906     #[inline]
 907     pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
 908         let (leaked, alloc) = Box::into_unique(b);
 909         (leaked.as_ptr(), alloc)
 910     }
 911
 912     #[unstable(
 913         feature = "ptr_internals",
 914         issue = "none",
 915         reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
 916     )]
 917     #[inline]
 918     #[doc(hidden)]
 919     pub fn into_unique(b: Self) -> (Unique<T>, A) {
 920         // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
 921         // raw pointer for the type system. Turning it directly into a raw pointer would not be
 922         // recognized as "releasing" the unique pointer to permit aliased raw accesses,
 923         // so all raw pointer methods have to go through `Box::leak`. Turning *that* to a raw pointer
 924         // behaves correctly.
 925         let alloc = unsafe { ptr::read(&b.1) };
 926         (Unique::from(Box::leak(b)), alloc)
 927     }
 928
 929     /// Returns a reference to the underlying allocator.
 930     ///
 931     /// Note: this is an associated function, which means that you have
 932     /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
 933     /// is so that there is no conflict with a method on the inner type.
 934     #[unstable(feature = "allocator_api", issue = "32838")]
 935     #[inline]
 936     pub fn allocator(b: &Self) -> &A {
 937         &b.1
 938     }
 939
 940     /// Consumes and leaks the `Box`, returning a mutable reference,
 941     /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
 942     /// `'a`. If the type has only static references, or none at all, then this
 943     /// may be chosen to be `'static`.
 944     ///
 945     /// This function is mainly useful for data that lives for the remainder of
 946     /// the program's life. Dropping the returned reference will cause a memory
 947     /// leak. If this is not acceptable, the reference should first be wrapped
 948     /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
 949     /// then be dropped which will properly destroy `T` and release the
 950     /// allocated memory.
 951     ///
 952     /// Note: this is an associated function, which means that you have
 953     /// to call it as `Box::leak(b)` instead of `b.leak()`. This
 954     /// is so that there is no conflict with a method on the inner type.
 955     ///
 956     /// # Examples
 957     ///
 958     /// Simple usage:
 959     ///
 960     /// ```
 961     /// let x = Box::new(41);
 962     /// let static_ref: &'static mut usize = Box::leak(x);
 963     /// *static_ref += 1;
 964     /// assert_eq!(*static_ref, 42);
 965     /// ```
 966     ///
 967     /// Unsized data:
 968     ///
 969     /// ```
 970     /// let x = vec![1, 2, 3].into_boxed_slice();
 971     /// let static_ref = Box::leak(x);
 972     /// static_ref[0] = 4;
 973     /// assert_eq!(*static_ref, [4, 2, 3]);
 974     /// ```
 975     #[stable(feature = "box_leak", since = "1.26.0")]
 976     #[inline]
 977     pub fn leak<'a>(b: Self) -> &'a mut T
 978     where
 979         A: 'a,
 980     {
 981         unsafe { &mut *mem::ManuallyDrop::new(b).0.as_ptr() }
 982     }
 983
 984     /// Converts a `Box<T>` into a `Pin<Box<T>>`
 985     ///
 986     /// This conversion does not allocate on the heap and happens in place.
 987     ///
 988     /// This is also available via [`From`].
 989     #[unstable(feature = "box_into_pin", issue = "62370")]
 990     pub fn into_pin(boxed: Self) -> Pin<Self>
 991     where
 992         A: 'static,
 993     {
 994         // It's not possible to move or replace the insides of a `Pin<Box<T>>`
 995         // when `T: !Unpin`,  so it's safe to pin it directly without any
 996         // additional requirements.
 997         unsafe { Pin::new_unchecked(boxed) }
 998     }
 999 }
1000
1001 #[stable(feature = "rust1", since = "1.0.0")]
1002 unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
1003     fn drop(&mut self) {
1004         // FIXME: Do nothing, drop is currently performed by compiler.
1005     }
1006 }
1007
1008 #[stable(feature = "rust1", since = "1.0.0")]
1009 impl<T: Default> Default for Box<T> {
1010     /// Creates a `Box<T>`, with the `Default` value for T.
1011     fn default() -> Self {
1012         box T::default()
1013     }
1014 }
1015
1016 #[stable(feature = "rust1", since = "1.0.0")]
1017 impl<T> Default for Box<[T]> {
1018     fn default() -> Self {
1019         Box::<[T; 0]>::new([])
1020     }
1021 }
1022
1023 #[stable(feature = "default_box_extra", since = "1.17.0")]
1024 impl Default for Box<str> {
1025     fn default() -> Self {
1026         unsafe { from_boxed_utf8_unchecked(Default::default()) }
1027     }
1028 }
1029
1030 #[stable(feature = "rust1", since = "1.0.0")]
1031 impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
1032     /// Returns a new box with a `clone()` of this box's contents.
1033     ///
1034     /// # Examples
1035     ///
1036     /// ```
1037     /// let x = Box::new(5);
1038     /// let y = x.clone();
1039     ///
1040     /// // The value is the same
1041     /// assert_eq!(x, y);
1042     ///
1043     /// // But they are unique objects
1044     /// assert_ne!(&*x as *const i32, &*y as *const i32);
1045     /// ```
1046     #[inline]
1047     fn clone(&self) -> Self {
1048         // Pre-allocate memory to allow writing the cloned value directly.
1049         let mut boxed = Self::new_uninit_in(self.1.clone());
1050         unsafe {
1051             (**self).write_clone_into_raw(boxed.as_mut_ptr());
1052             boxed.assume_init()
1053         }
1054     }
1055
1056     /// Copies `source`'s contents into `self` without creating a new allocation.
1057     ///
1058     /// # Examples
1059     ///
1060     /// ```
1061     /// let x = Box::new(5);
1062     /// let mut y = Box::new(10);
1063     /// let yp: *const i32 = &*y;
1064     ///
1065     /// y.clone_from(&x);
1066     ///
1067     /// // The value is the same
1068     /// assert_eq!(x, y);
1069     ///
1070     /// // And no allocation occurred
1071     /// assert_eq!(yp, &*y);
1072     /// ```
1073     #[inline]
1074     fn clone_from(&mut self, source: &Self) {
1075         (**self).clone_from(&(**source));
1076     }
1077 }
1078
1079 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1080 impl Clone for Box<str> {
1081     fn clone(&self) -> Self {
1082         // this makes a copy of the data
1083         let buf: Box<[u8]> = self.as_bytes().into();
1084         unsafe { from_boxed_utf8_unchecked(buf) }
1085     }
1086 }
1087
1088 #[stable(feature = "rust1", since = "1.0.0")]
1089 impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
1090     #[inline]
1091     fn eq(&self, other: &Self) -> bool {
1092         PartialEq::eq(&**self, &**other)
1093     }
1094     #[inline]
1095     fn ne(&self, other: &Self) -> bool {
1096         PartialEq::ne(&**self, &**other)
1097     }
1098 }
1099 #[stable(feature = "rust1", since = "1.0.0")]
1100 impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
1101     #[inline]
1102     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1103         PartialOrd::partial_cmp(&**self, &**other)
1104     }
1105     #[inline]
1106     fn lt(&self, other: &Self) -> bool {
1107         PartialOrd::lt(&**self, &**other)
1108     }
1109     #[inline]
1110     fn le(&self, other: &Self) -> bool {
1111         PartialOrd::le(&**self, &**other)
1112     }
1113     #[inline]
1114     fn ge(&self, other: &Self) -> bool {
1115         PartialOrd::ge(&**self, &**other)
1116     }
1117     #[inline]
1118     fn gt(&self, other: &Self) -> bool {
1119         PartialOrd::gt(&**self, &**other)
1120     }
1121 }
1122 #[stable(feature = "rust1", since = "1.0.0")]
1123 impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
1124     #[inline]
1125     fn cmp(&self, other: &Self) -> Ordering {
1126         Ord::cmp(&**self, &**other)
1127     }
1128 }
1129 #[stable(feature = "rust1", since = "1.0.0")]
1130 impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}
1131
1132 #[stable(feature = "rust1", since = "1.0.0")]
1133 impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
1134     fn hash<H: Hasher>(&self, state: &mut H) {
1135         (**self).hash(state);
1136     }
1137 }
1138
1139 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
1140 impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
1141     fn finish(&self) -> u64 {
1142         (**self).finish()
1143     }
1144     fn write(&mut self, bytes: &[u8]) {
1145         (**self).write(bytes)
1146     }
1147     fn write_u8(&mut self, i: u8) {
1148         (**self).write_u8(i)
1149     }
1150     fn write_u16(&mut self, i: u16) {
1151         (**self).write_u16(i)
1152     }
1153     fn write_u32(&mut self, i: u32) {
1154         (**self).write_u32(i)
1155     }
1156     fn write_u64(&mut self, i: u64) {
1157         (**self).write_u64(i)
1158     }
1159     fn write_u128(&mut self, i: u128) {
1160         (**self).write_u128(i)
1161     }
1162     fn write_usize(&mut self, i: usize) {
1163         (**self).write_usize(i)
1164     }
1165     fn write_i8(&mut self, i: i8) {
1166         (**self).write_i8(i)
1167     }
1168     fn write_i16(&mut self, i: i16) {
1169         (**self).write_i16(i)
1170     }
1171     fn write_i32(&mut self, i: i32) {
1172         (**self).write_i32(i)
1173     }
1174     fn write_i64(&mut self, i: i64) {
1175         (**self).write_i64(i)
1176     }
1177     fn write_i128(&mut self, i: i128) {
1178         (**self).write_i128(i)
1179     }
1180     fn write_isize(&mut self, i: isize) {
1181         (**self).write_isize(i)
1182     }
1183 }
1184
1185 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1186 impl<T> From<T> for Box<T> {
1187     /// Converts a generic type `T` into a `Box<T>`
1188     ///
1189     /// The conversion allocates on the heap and moves `t`
1190     /// from the stack into it.
1191     ///
1192     /// # Examples
1193     /// ```rust
1194     /// let x = 5;
1195     /// let boxed = Box::new(5);
1196     ///
1197     /// assert_eq!(Box::from(x), boxed);
1198     /// ```
1199     fn from(t: T) -> Self {
1200         Box::new(t)
1201     }
1202 }
1203
1204 #[stable(feature = "pin", since = "1.33.0")]
1205 impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Pin<Box<T, A>>
1206 where
1207     A: 'static,
1208 {
1209     /// Converts a `Box<T>` into a `Pin<Box<T>>`
1210     ///
1211     /// This conversion does not allocate on the heap and happens in place.
1212     fn from(boxed: Box<T, A>) -> Self {
1213         Box::into_pin(boxed)
1214     }
1215 }
1216
1217 #[stable(feature = "box_from_slice", since = "1.17.0")]
1218 impl<T: Copy> From<&[T]> for Box<[T]> {
1219     /// Converts a `&[T]` into a `Box<[T]>`
1220     ///
1221     /// This conversion allocates on the heap
1222     /// and performs a copy of `slice`.
1223     ///
1224     /// # Examples
1225     /// ```rust
1226     /// // create a &[u8] which will be used to create a Box<[u8]>
1227     /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1228     /// let boxed_slice: Box<[u8]> = Box::from(slice);
1229     ///
1230     /// println!("{:?}", boxed_slice);
1231     /// ```
1232     fn from(slice: &[T]) -> Box<[T]> {
1233         let len = slice.len();
1234         let buf = RawVec::with_capacity(len);
1235         unsafe {
1236             ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
1237             buf.into_box(slice.len()).assume_init()
1238         }
1239     }
1240 }
1241
1242 #[stable(feature = "box_from_cow", since = "1.45.0")]
1243 impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
1244     #[inline]
1245     fn from(cow: Cow<'_, [T]>) -> Box<[T]> {
1246         match cow {
1247             Cow::Borrowed(slice) => Box::from(slice),
1248             Cow::Owned(slice) => Box::from(slice),
1249         }
1250     }
1251 }
1252
1253 #[stable(feature = "box_from_slice", since = "1.17.0")]
1254 impl From<&str> for Box<str> {
1255     /// Converts a `&str` into a `Box<str>`
1256     ///
1257     /// This conversion allocates on the heap
1258     /// and performs a copy of `s`.
1259     ///
1260     /// # Examples
1261     /// ```rust
1262     /// let boxed: Box<str> = Box::from("hello");
1263     /// println!("{}", boxed);
1264     /// ```
1265     #[inline]
1266     fn from(s: &str) -> Box<str> {
1267         unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
1268     }
1269 }
1270
1271 #[stable(feature = "box_from_cow", since = "1.45.0")]
1272 impl From<Cow<'_, str>> for Box<str> {
1273     #[inline]
1274     fn from(cow: Cow<'_, str>) -> Box<str> {
1275         match cow {
1276             Cow::Borrowed(s) => Box::from(s),
1277             Cow::Owned(s) => Box::from(s),
1278         }
1279     }
1280 }
1281
1282 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
1283 impl<A: Allocator> From<Box<str, A>> for Box<[u8], A> {
1284     /// Converts a `Box<str>` into a `Box<[u8]>`
1285     ///
1286     /// This conversion does not allocate on the heap and happens in place.
1287     ///
1288     /// # Examples
1289     /// ```rust
1290     /// // create a Box<str> which will be used to create a Box<[u8]>
1291     /// let boxed: Box<str> = Box::from("hello");
1292     /// let boxed_str: Box<[u8]> = Box::from(boxed);
1293     ///
1294     /// // create a &[u8] which will be used to create a Box<[u8]>
1295     /// let slice: &[u8] = &[104, 101, 108, 108, 111];
1296     /// let boxed_slice = Box::from(slice);
1297     ///
1298     /// assert_eq!(boxed_slice, boxed_str);
1299     /// ```
1300     #[inline]
1301     fn from(s: Box<str, A>) -> Self {
1302         let (raw, alloc) = Box::into_raw_with_allocator(s);
1303         unsafe { Box::from_raw_in(raw as *mut [u8], alloc) }
1304     }
1305 }
1306
1307 #[stable(feature = "box_from_array", since = "1.45.0")]
1308 impl<T, const N: usize> From<[T; N]> for Box<[T]> {
1309     /// Converts a `[T; N]` into a `Box<[T]>`
1310     ///
1311     /// This conversion moves the array to newly heap-allocated memory.
1312     ///
1313     /// # Examples
1314     /// ```rust
1315     /// let boxed: Box<[u8]> = Box::from([4, 2]);
1316     /// println!("{:?}", boxed);
1317     /// ```
1318     fn from(array: [T; N]) -> Box<[T]> {
1319         box array
1320     }
1321 }
1322
1323 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
1324 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]> {
1325     type Error = Box<[T]>;
1326
1327     fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
1328         if boxed_slice.len() == N {
1329             Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
1330         } else {
1331             Err(boxed_slice)
1332         }
1333     }
1334 }
1335
1336 impl<A: Allocator> Box<dyn Any, A> {
1337     #[inline]
1338     #[stable(feature = "rust1", since = "1.0.0")]
1339     /// Attempt to downcast the box to a concrete type.
1340     ///
1341     /// # Examples
1342     ///
1343     /// ```
1344     /// use std::any::Any;
1345     ///
1346     /// fn print_if_string(value: Box<dyn Any>) {
1347     ///     if let Ok(string) = value.downcast::<String>() {
1348     ///         println!("String ({}): {}", string.len(), string);
1349     ///     }
1350     /// }
1351     ///
1352     /// let my_string = "Hello World".to_string();
1353     /// print_if_string(Box::new(my_string));
1354     /// print_if_string(Box::new(0i8));
1355     /// ```
1356     pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1357         if self.is::<T>() {
1358             unsafe {
1359                 let (raw, alloc): (*mut dyn Any, _) = Box::into_raw_with_allocator(self);
1360                 Ok(Box::from_raw_in(raw as *mut T, alloc))
1361             }
1362         } else {
1363             Err(self)
1364         }
1365     }
1366 }
1367
1368 impl<A: Allocator> Box<dyn Any + Send, A> {
1369     #[inline]
1370     #[stable(feature = "rust1", since = "1.0.0")]
1371     /// Attempt to downcast the box to a concrete type.
1372     ///
1373     /// # Examples
1374     ///
1375     /// ```
1376     /// use std::any::Any;
1377     ///
1378     /// fn print_if_string(value: Box<dyn Any + Send>) {
1379     ///     if let Ok(string) = value.downcast::<String>() {
1380     ///         println!("String ({}): {}", string.len(), string);
1381     ///     }
1382     /// }
1383     ///
1384     /// let my_string = "Hello World".to_string();
1385     /// print_if_string(Box::new(my_string));
1386     /// print_if_string(Box::new(0i8));
1387     /// ```
1388     pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1389         if self.is::<T>() {
1390             unsafe {
1391                 let (raw, alloc): (*mut (dyn Any + Send), _) = Box::into_raw_with_allocator(self);
1392                 Ok(Box::from_raw_in(raw as *mut T, alloc))
1393             }
1394         } else {
1395             Err(self)
1396         }
1397     }
1398 }
1399
1400 impl<A: Allocator> Box<dyn Any + Send + Sync, A> {
1401     #[inline]
1402     #[stable(feature = "box_send_sync_any_downcast", since = "1.51.0")]
1403     /// Attempt to downcast the box to a concrete type.
1404     ///
1405     /// # Examples
1406     ///
1407     /// ```
1408     /// use std::any::Any;
1409     ///
1410     /// fn print_if_string(value: Box<dyn Any + Send + Sync>) {
1411     ///     if let Ok(string) = value.downcast::<String>() {
1412     ///         println!("String ({}): {}", string.len(), string);
1413     ///     }
1414     /// }
1415     ///
1416     /// let my_string = "Hello World".to_string();
1417     /// print_if_string(Box::new(my_string));
1418     /// print_if_string(Box::new(0i8));
1419     /// ```
1420     pub fn downcast<T: Any>(self) -> Result<Box<T, A>, Self> {
1421         if self.is::<T>() {
1422             unsafe {
1423                 let (raw, alloc): (*mut (dyn Any + Send + Sync), _) =
1424                     Box::into_raw_with_allocator(self);
1425                 Ok(Box::from_raw_in(raw as *mut T, alloc))
1426             }
1427         } else {
1428             Err(self)
1429         }
1430     }
1431 }
1432
1433 #[stable(feature = "rust1", since = "1.0.0")]
1434 impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
1435     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1436         fmt::Display::fmt(&**self, f)
1437     }
1438 }
1439
1440 #[stable(feature = "rust1", since = "1.0.0")]
1441 impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
1442     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1443         fmt::Debug::fmt(&**self, f)
1444     }
1445 }
1446
1447 #[stable(feature = "rust1", since = "1.0.0")]
1448 impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
1449     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1450         // It's not possible to extract the inner Uniq directly from the Box,
1451         // instead we cast it to a *const which aliases the Unique
1452         let ptr: *const T = &**self;
1453         fmt::Pointer::fmt(&ptr, f)
1454     }
1455 }
1456
1457 #[stable(feature = "rust1", since = "1.0.0")]
1458 impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
1459     type Target = T;
1460
1461     fn deref(&self) -> &T {
1462         &**self
1463     }
1464 }
1465
1466 #[stable(feature = "rust1", since = "1.0.0")]
1467 impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
1468     fn deref_mut(&mut self) -> &mut T {
1469         &mut **self
1470     }
1471 }
1472
1473 #[unstable(feature = "receiver_trait", issue = "none")]
1474 impl<T: ?Sized, A: Allocator> Receiver for Box<T, A> {}
1475
1476 #[stable(feature = "rust1", since = "1.0.0")]
1477 impl<I: Iterator + ?Sized, A: Allocator> Iterator for Box<I, A> {
1478     type Item = I::Item;
1479     fn next(&mut self) -> Option<I::Item> {
1480         (**self).next()
1481     }
1482     fn size_hint(&self) -> (usize, Option<usize>) {
1483         (**self).size_hint()
1484     }
1485     fn nth(&mut self, n: usize) -> Option<I::Item> {
1486         (**self).nth(n)
1487     }
1488     fn last(self) -> Option<I::Item> {
1489         BoxIter::last(self)
1490     }
1491 }
1492
1493 trait BoxIter {
1494     type Item;
1495     fn last(self) -> Option<Self::Item>;
1496 }
1497
1498 impl<I: Iterator + ?Sized, A: Allocator> BoxIter for Box<I, A> {
1499     type Item = I::Item;
1500     default fn last(self) -> Option<I::Item> {
1501         #[inline]
1502         fn some<T>(_: Option<T>, x: T) -> Option<T> {
1503             Some(x)
1504         }
1505
1506         self.fold(None, some)
1507     }
1508 }
1509
1510 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
1511 /// instead of the default.
1512 #[stable(feature = "rust1", since = "1.0.0")]
1513 impl<I: Iterator, A: Allocator> BoxIter for Box<I, A> {
1514     fn last(self) -> Option<I::Item> {
1515         (*self).last()
1516     }
1517 }
1518
1519 #[stable(feature = "rust1", since = "1.0.0")]
1520 impl<I: DoubleEndedIterator + ?Sized, A: Allocator> DoubleEndedIterator for Box<I, A> {
1521     fn next_back(&mut self) -> Option<I::Item> {
1522         (**self).next_back()
1523     }
1524     fn nth_back(&mut self, n: usize) -> Option<I::Item> {
1525         (**self).nth_back(n)
1526     }
1527 }
1528 #[stable(feature = "rust1", since = "1.0.0")]
1529 impl<I: ExactSizeIterator + ?Sized, A: Allocator> ExactSizeIterator for Box<I, A> {
1530     fn len(&self) -> usize {
1531         (**self).len()
1532     }
1533     fn is_empty(&self) -> bool {
1534         (**self).is_empty()
1535     }
1536 }
1537
1538 #[stable(feature = "fused", since = "1.26.0")]
1539 impl<I: FusedIterator + ?Sized, A: Allocator> FusedIterator for Box<I, A> {}
1540
1541 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1542 impl<Args, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
1543     type Output = <F as FnOnce<Args>>::Output;
1544
1545     extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
1546         <F as FnOnce<Args>>::call_once(*self, args)
1547     }
1548 }
1549
1550 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1551 impl<Args, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
1552     extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
1553         <F as FnMut<Args>>::call_mut(self, args)
1554     }
1555 }
1556
1557 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1558 impl<Args, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
1559     extern "rust-call" fn call(&self, args: Args) -> Self::Output {
1560         <F as Fn<Args>>::call(self, args)
1561     }
1562 }
1563
1564 #[unstable(feature = "coerce_unsized", issue = "27732")]
1565 impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
1566
1567 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
1568 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
1569
1570 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
1571 impl<I> FromIterator<I> for Box<[I]> {
1572     fn from_iter<T: IntoIterator<Item = I>>(iter: T) -> Self {
1573         iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
1574     }
1575 }
1576
1577 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1578 impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
1579     fn clone(&self) -> Self {
1580         let alloc = Box::allocator(self).clone();
1581         self.to_vec_in(alloc).into_boxed_slice()
1582     }
1583
1584     fn clone_from(&mut self, other: &Self) {
1585         if self.len() == other.len() {
1586             self.clone_from_slice(&other);
1587         } else {
1588             *self = other.clone();
1589         }
1590     }
1591 }
1592
1593 #[stable(feature = "box_borrow", since = "1.1.0")]
1594 impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Box<T, A> {
1595     fn borrow(&self) -> &T {
1596         &**self
1597     }
1598 }
1599
1600 #[stable(feature = "box_borrow", since = "1.1.0")]
1601 impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for Box<T, A> {
1602     fn borrow_mut(&mut self) -> &mut T {
1603         &mut **self
1604     }
1605 }
1606
1607 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1608 impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
1609     fn as_ref(&self) -> &T {
1610         &**self
1611     }
1612 }
1613
1614 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1615 impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
1616     fn as_mut(&mut self) -> &mut T {
1617         &mut **self
1618     }
1619 }
1620
1621 /* Nota bene
1622  *
1623  *  We could have chosen not to add this impl, and instead have written a
1624  *  function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
1625  *  because Box<T> implements Unpin even when T does not, as a result of
1626  *  this impl.
1627  *
1628  *  We chose this API instead of the alternative for a few reasons:
1629  *      - Logically, it is helpful to understand pinning in regard to the
1630  *        memory region being pointed to. For this reason none of the
1631  *        standard library pointer types support projecting through a pin
1632  *        (Box<T> is the only pointer type in std for which this would be
1633  *        safe.)
1634  *      - It is in practice very useful to have Box<T> be unconditionally
1635  *        Unpin because of trait objects, for which the structural auto
1636  *        trait functionality does not apply (e.g., Box<dyn Foo> would
1637  *        otherwise not be Unpin).
1638  *
1639  *  Another type with the same semantics as Box but only a conditional
1640  *  implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
1641  *  could have a method to project a Pin<T> from it.
1642  */
1643 #[stable(feature = "pin", since = "1.33.0")]
1644 impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> where A: 'static {}
1645
1646 #[unstable(feature = "generator_trait", issue = "43122")]
1647 impl<G: ?Sized + Generator<R> + Unpin, R, A: Allocator> Generator<R> for Box<G, A>
1648 where
1649     A: 'static,
1650 {
1651     type Yield = G::Yield;
1652     type Return = G::Return;
1653
1654     fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1655         G::resume(Pin::new(&mut *self), arg)
1656     }
1657 }
1658
1659 #[unstable(feature = "generator_trait", issue = "43122")]
1660 impl<G: ?Sized + Generator<R>, R, A: Allocator> Generator<R> for Pin<Box<G, A>>
1661 where
1662     A: 'static,
1663 {
1664     type Yield = G::Yield;
1665     type Return = G::Return;
1666
1667     fn resume(mut self: Pin<&mut Self>, arg: R) -> GeneratorState<Self::Yield, Self::Return> {
1668         G::resume((*self).as_mut(), arg)
1669     }
1670 }
1671
1672 #[stable(feature = "futures_api", since = "1.36.0")]
1673 impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A>
1674 where
1675     A: 'static,
1676 {
1677     type Output = F::Output;
1678
1679     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
1680         F::poll(Pin::new(&mut *self), cx)
1681     }
1682 }
1683
1684 #[unstable(feature = "async_stream", issue = "79024")]
1685 impl<S: ?Sized + Stream + Unpin> Stream for Box<S> {
1686     type Item = S::Item;
1687
1688     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
1689         Pin::new(&mut **self).poll_next(cx)
1690     }
1691
1692     fn size_hint(&self) -> (usize, Option<usize>) {
1693         (**self).size_hint()
1694     }
1695 }