src/liballoc/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.
   6 //!
   7 //! # Examples
   8 //!
   9 //! Move a value from the stack to the heap by creating a [`Box`]:
  10 //!
  11 //! ```
  12 //! let val: u8 = 5;
  13 //! let boxed: Box<u8> = Box::new(val);
  14 //! ```
  15 //!
  16 //! Move a value from a [`Box`] back to the stack by [dereferencing]:
  17 //!
  18 //! ```
  19 //! let boxed: Box<u8> = Box::new(5);
  20 //! let val: u8 = *boxed;
  21 //! ```
  22 //!
  23 //! Creating a recursive data structure:
  24 //!
  25 //! ```
  26 //! #[derive(Debug)]
  27 //! enum List<T> {
  28 //!     Cons(T, Box<List<T>>),
  29 //!     Nil,
  30 //! }
  31 //!
  32 //! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
  33 //! println!("{:?}", list);
  34 //! ```
  35 //!
  36 //! This will print `Cons(1, Cons(2, Nil))`.
  37 //!
  38 //! Recursive structures must be boxed, because if the definition of `Cons`
  39 //! looked like this:
  40 //!
  41 //! ```compile_fail,E0072
  42 //! # enum List<T> {
  43 //! Cons(T, List<T>),
  44 //! # }
  45 //! ```
  46 //!
  47 //! It wouldn't work. This is because the size of a `List` depends on how many
  48 //! elements are in the list, and so we don't know how much memory to allocate
  49 //! for a `Cons`. By introducing a [`Box<T>`], which has a defined size, we know how
  50 //! big `Cons` needs to be.
  51 //!
  52 //! # Memory layout
  53 //!
  54 //! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
  55 //! its allocation. It is valid to convert both ways between a [`Box`] and a
  56 //! raw pointer allocated with the [`Global`] allocator, given that the
  57 //! [`Layout`] used with the allocator is correct for the type. More precisely,
  58 //! a `value: *mut T` that has been allocated with the [`Global`] allocator
  59 //! with `Layout::for_value(&*value)` may be converted into a box using
  60 //! [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut
  61 //! T` obtained from [`Box::<T>::into_raw`] may be deallocated using the
  62 //! [`Global`] allocator with [`Layout::for_value(&*value)`].
  63 //!
  64 //! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented
  65 //! as a single pointer and is also ABI-compatible with C pointers
  66 //! (i.e. the C type `T*`). This means that if you have extern "C"
  67 //! Rust functions that will be called from C, you can define those
  68 //! Rust functions using `Box<T>` types, and use `T*` as corresponding
  69 //! type on the C side. As an example, consider this C header which
  70 //! declares functions that create and destroy some kind of `Foo`
  71 //! value:
  72 //!
  73 //! ```c
  74 //! /* C header */
  75 //!
  76 //! /* Returns ownership to the caller */
  77 //! struct Foo* foo_new(void);
  78 //!
  79 //! /* Takes ownership from the caller; no-op when invoked with NULL */
  80 //! void foo_delete(struct Foo*);
  81 //! ```
  82 //!
  83 //! These two functions might be implemented in Rust as follows. Here, the
  84 //! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
  85 //! the ownership constraints. Note also that the nullable argument to
  86 //! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
  87 //! cannot be null.
  88 //!
  89 //! ```
  90 //! #[repr(C)]
  91 //! pub struct Foo;
  92 //!
  93 //! #[no_mangle]
  94 //! pub extern "C" fn foo_new() -> Box<Foo> {
  95 //!     Box::new(Foo)
  96 //! }
  97 //!
  98 //! #[no_mangle]
  99 //! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
 100 //! ```
 101 //!
 102 //! Even though `Box<T>` has the same representation and C ABI as a C pointer,
 103 //! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
 104 //! and expect things to work. `Box<T>` values will always be fully aligned,
 105 //! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
 106 //! free the value with the global allocator. In general, the best practice
 107 //! is to only use `Box<T>` for pointers that originated from the global
 108 //! allocator.
 109 //!
 110 //! **Important.** At least at present, you should avoid using
 111 //! `Box<T>` types for functions that are defined in C but invoked
 112 //! from Rust. In those cases, you should directly mirror the C types
 113 //! as closely as possible. Using types like `Box<T>` where the C
 114 //! definition is just using `T*` can lead to undefined behavior, as
 115 //! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
 116 //!
 117 //! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
 118 //! [dereferencing]: ../../std/ops/trait.Deref.html
 119 //! [`Box`]: struct.Box.html
 120 //! [`Box<T>`]: struct.Box.html
 121 //! [`Box::<T>::from_raw(value)`]: struct.Box.html#method.from_raw
 122 //! [`Box::<T>::into_raw`]: struct.Box.html#method.into_raw
 123 //! [`Global`]: ../alloc/struct.Global.html
 124 //! [`Layout`]: ../alloc/struct.Layout.html
 125 //! [`Layout::for_value(&*value)`]: ../alloc/struct.Layout.html#method.for_value
 126
 127 #![stable(feature = "rust1", since = "1.0.0")]
 128
 129 use core::any::Any;
 130 use core::array::LengthAtMost32;
 131 use core::borrow;
 132 use core::cmp::Ordering;
 133 use core::convert::{From, TryFrom};
 134 use core::fmt;
 135 use core::future::Future;
 136 use core::hash::{Hash, Hasher};
 137 use core::iter::{Iterator, FromIterator, FusedIterator};
 138 use core::marker::{Unpin, Unsize};
 139 use core::mem;
 140 use core::pin::Pin;
 141 use core::ops::{
 142     CoerceUnsized, DispatchFromDyn, Deref, DerefMut, Receiver, Generator, GeneratorState
 143 };
 144 use core::ptr::{self, NonNull, Unique};
 145 use core::slice;
 146 use core::task::{Context, Poll};
 147
 148 use crate::alloc::{self, Global, Alloc};
 149 use crate::vec::Vec;
 150 use crate::raw_vec::RawVec;
 151 use crate::str::from_boxed_utf8_unchecked;
 152
 153 /// A pointer type for heap allocation.
 154 ///
 155 /// See the [module-level documentation](../../std/boxed/index.html) for more.
 156 #[lang = "owned_box"]
 157 #[fundamental]
 158 #[stable(feature = "rust1", since = "1.0.0")]
 159 pub struct Box<T: ?Sized>(Unique<T>);
 160
 161 impl<T> Box<T> {
 162     /// Allocates memory on the heap and then places `x` into it.
 163     ///
 164     /// This doesn't actually allocate if `T` is zero-sized.
 165     ///
 166     /// # Examples
 167     ///
 168     /// ```
 169     /// let five = Box::new(5);
 170     /// ```
 171     #[stable(feature = "rust1", since = "1.0.0")]
 172     #[inline(always)]
 173     pub fn new(x: T) -> Box<T> {
 174         box x
 175     }
 176
 177     /// Constructs a new box with uninitialized contents.
 178     ///
 179     /// # Examples
 180     ///
 181     /// ```
 182     /// #![feature(new_uninit)]
 183     ///
 184     /// let mut five = Box::<u32>::new_uninit();
 185     ///
 186     /// let five = unsafe {
 187     ///     // Deferred initialization:
 188     ///     five.as_mut_ptr().write(5);
 189     ///
 190     ///     five.assume_init()
 191     /// };
 192     ///
 193     /// assert_eq!(*five, 5)
 194     /// ```
 195     #[unstable(feature = "new_uninit", issue = "63291")]
 196     pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
 197         let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
 198         if layout.size() == 0 {
 199             return Box(NonNull::dangling().into())
 200         }
 201         let ptr = unsafe {
 202             Global.alloc(layout)
 203                 .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
 204         };
 205         Box(ptr.cast().into())
 206     }
 207
 208     /// Constructs a new `Box` with uninitialized contents, with the memory
 209     /// being filled with `0` bytes.
 210     ///
 211     /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
 212     /// of this method.
 213     ///
 214     /// # Examples
 215     ///
 216     /// ```
 217     /// #![feature(new_uninit)]
 218     ///
 219     /// let zero = Box::<u32>::new_zeroed();
 220     /// let zero = unsafe { zero.assume_init() };
 221     ///
 222     /// assert_eq!(*zero, 0)
 223     /// ```
 224     ///
 225     /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
 226     #[unstable(feature = "new_uninit", issue = "63291")]
 227     pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
 228         unsafe {
 229             let mut uninit = Self::new_uninit();
 230             ptr::write_bytes::<T>(uninit.as_mut_ptr(), 0, 1);
 231             uninit
 232         }
 233     }
 234
 235     /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
 236     /// `x` will be pinned in memory and unable to be moved.
 237     #[stable(feature = "pin", since = "1.33.0")]
 238     #[inline(always)]
 239     pub fn pin(x: T) -> Pin<Box<T>> {
 240         (box x).into()
 241     }
 242 }
 243
 244 impl<T> Box<[T]> {
 245     /// Constructs a new boxed slice with uninitialized contents.
 246     ///
 247     /// # Examples
 248     ///
 249     /// ```
 250     /// #![feature(new_uninit)]
 251     ///
 252     /// let mut values = Box::<[u32]>::new_uninit_slice(3);
 253     ///
 254     /// let values = unsafe {
 255     ///     // Deferred initialization:
 256     ///     values[0].as_mut_ptr().write(1);
 257     ///     values[1].as_mut_ptr().write(2);
 258     ///     values[2].as_mut_ptr().write(3);
 259     ///
 260     ///     values.assume_init()
 261     /// };
 262     ///
 263     /// assert_eq!(*values, [1, 2, 3])
 264     /// ```
 265     #[unstable(feature = "new_uninit", issue = "63291")]
 266     pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
 267         let layout = alloc::Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
 268         let ptr = if layout.size() == 0 {
 269             NonNull::dangling()
 270         } else {
 271             unsafe {
 272                 Global.alloc(layout)
 273                     .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
 274                     .cast()
 275             }
 276         };
 277         let slice = unsafe { slice::from_raw_parts_mut(ptr.as_ptr(), len) };
 278         Box(Unique::from(slice))
 279     }
 280 }
 281
 282 impl<T> Box<mem::MaybeUninit<T>> {
 283     /// Converts to `Box<T>`.
 284     ///
 285     /// # Safety
 286     ///
 287     /// As with [`MaybeUninit::assume_init`],
 288     /// it is up to the caller to guarantee that the value
 289     /// really is in an initialized state.
 290     /// Calling this when the content is not yet fully initialized
 291     /// causes immediate undefined behavior.
 292     ///
 293     /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
 294     ///
 295     /// # Examples
 296     ///
 297     /// ```
 298     /// #![feature(new_uninit)]
 299     ///
 300     /// let mut five = Box::<u32>::new_uninit();
 301     ///
 302     /// let five: Box<u32> = unsafe {
 303     ///     // Deferred initialization:
 304     ///     five.as_mut_ptr().write(5);
 305     ///
 306     ///     five.assume_init()
 307     /// };
 308     ///
 309     /// assert_eq!(*five, 5)
 310     /// ```
 311     #[unstable(feature = "new_uninit", issue = "63291")]
 312     #[inline]
 313     pub unsafe fn assume_init(self) -> Box<T> {
 314         Box(Box::into_unique(self).cast())
 315     }
 316 }
 317
 318 impl<T> Box<[mem::MaybeUninit<T>]> {
 319     /// Converts to `Box<[T]>`.
 320     ///
 321     /// # Safety
 322     ///
 323     /// As with [`MaybeUninit::assume_init`],
 324     /// it is up to the caller to guarantee that the values
 325     /// really are in an initialized state.
 326     /// Calling this when the content is not yet fully initialized
 327     /// causes immediate undefined behavior.
 328     ///
 329     /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
 330     ///
 331     /// # Examples
 332     ///
 333     /// ```
 334     /// #![feature(new_uninit)]
 335     ///
 336     /// let mut values = Box::<[u32]>::new_uninit_slice(3);
 337     ///
 338     /// let values = unsafe {
 339     ///     // Deferred initialization:
 340     ///     values[0].as_mut_ptr().write(1);
 341     ///     values[1].as_mut_ptr().write(2);
 342     ///     values[2].as_mut_ptr().write(3);
 343     ///
 344     ///     values.assume_init()
 345     /// };
 346     ///
 347     /// assert_eq!(*values, [1, 2, 3])
 348     /// ```
 349     #[unstable(feature = "new_uninit", issue = "63291")]
 350     #[inline]
 351     pub unsafe fn assume_init(self) -> Box<[T]> {
 352         Box(Unique::new_unchecked(Box::into_raw(self) as _))
 353     }
 354 }
 355
 356 impl<T: ?Sized> Box<T> {
 357     /// Constructs a box from a raw pointer.
 358     ///
 359     /// After calling this function, the raw pointer is owned by the
 360     /// resulting `Box`. Specifically, the `Box` destructor will call
 361     /// the destructor of `T` and free the allocated memory. For this
 362     /// to be safe, the memory must have been allocated in accordance
 363     /// with the [memory layout] used by `Box` .
 364     ///
 365     /// # Safety
 366     ///
 367     /// This function is unsafe because improper use may lead to
 368     /// memory problems. For example, a double-free may occur if the
 369     /// function is called twice on the same raw pointer.
 370     ///
 371     /// # Examples
 372     /// Recreate a `Box` which was previously converted to a raw pointer
 373     /// using [`Box::into_raw`]:
 374     /// ```
 375     /// let x = Box::new(5);
 376     /// let ptr = Box::into_raw(x);
 377     /// let x = unsafe { Box::from_raw(ptr) };
 378     /// ```
 379     /// Manually create a `Box` from scratch by using the global allocator:
 380     /// ```
 381     /// use std::alloc::{alloc, Layout};
 382     ///
 383     /// unsafe {
 384     ///     let ptr = alloc(Layout::new::<i32>()) as *mut i32;
 385     ///     *ptr = 5;
 386     ///     let x = Box::from_raw(ptr);
 387     /// }
 388     /// ```
 389     ///
 390     /// [memory layout]: index.html#memory-layout
 391     /// [`Layout`]: ../alloc/struct.Layout.html
 392     /// [`Box::into_raw`]: struct.Box.html#method.into_raw
 393     #[stable(feature = "box_raw", since = "1.4.0")]
 394     #[inline]
 395     pub unsafe fn from_raw(raw: *mut T) -> Self {
 396         Box(Unique::new_unchecked(raw))
 397     }
 398
 399     /// Consumes the `Box`, returning a wrapped raw pointer.
 400     ///
 401     /// The pointer will be properly aligned and non-null.
 402     ///
 403     /// After calling this function, the caller is responsible for the
 404     /// memory previously managed by the `Box`. In particular, the
 405     /// caller should properly destroy `T` and release the memory, taking
 406     /// into account the [memory layout] used by `Box`. The easiest way to
 407     /// do this is to convert the raw pointer back into a `Box` with the
 408     /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
 409     /// the cleanup.
 410     ///
 411     /// Note: this is an associated function, which means that you have
 412     /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
 413     /// is so that there is no conflict with a method on the inner type.
 414     ///
 415     /// # Examples
 416     /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
 417     /// for automatic cleanup:
 418     /// ```
 419     /// let x = Box::new(String::from("Hello"));
 420     /// let ptr = Box::into_raw(x);
 421     /// let x = unsafe { Box::from_raw(ptr) };
 422     /// ```
 423     /// Manual cleanup by explicitly running the destructor and deallocating
 424     /// the memory:
 425     /// ```
 426     /// use std::alloc::{dealloc, Layout};
 427     /// use std::ptr;
 428     ///
 429     /// let x = Box::new(String::from("Hello"));
 430     /// let p = Box::into_raw(x);
 431     /// unsafe {
 432     ///     ptr::drop_in_place(p);
 433     ///     dealloc(p as *mut u8, Layout::new::<String>());
 434     /// }
 435     /// ```
 436     ///
 437     /// [memory layout]: index.html#memory-layout
 438     /// [`Box::from_raw`]: struct.Box.html#method.from_raw
 439     #[stable(feature = "box_raw", since = "1.4.0")]
 440     #[inline]
 441     pub fn into_raw(b: Box<T>) -> *mut T {
 442         Box::into_raw_non_null(b).as_ptr()
 443     }
 444
 445     /// Consumes the `Box`, returning the wrapped pointer as `NonNull<T>`.
 446     ///
 447     /// After calling this function, the caller is responsible for the
 448     /// memory previously managed by the `Box`. In particular, the
 449     /// caller should properly destroy `T` and release the memory. The
 450     /// easiest way to do so is to convert the `NonNull<T>` pointer
 451     /// into a raw pointer and back into a `Box` with the [`Box::from_raw`]
 452     /// function.
 453     ///
 454     /// Note: this is an associated function, which means that you have
 455     /// to call it as `Box::into_raw_non_null(b)`
 456     /// instead of `b.into_raw_non_null()`. This
 457     /// is so that there is no conflict with a method on the inner type.
 458     ///
 459     /// [`Box::from_raw`]: struct.Box.html#method.from_raw
 460     ///
 461     /// # Examples
 462     ///
 463     /// ```
 464     /// #![feature(box_into_raw_non_null)]
 465     ///
 466     /// let x = Box::new(5);
 467     /// let ptr = Box::into_raw_non_null(x);
 468     ///
 469     /// // Clean up the memory by converting the NonNull pointer back
 470     /// // into a Box and letting the Box be dropped.
 471     /// let x = unsafe { Box::from_raw(ptr.as_ptr()) };
 472     /// ```
 473     #[unstable(feature = "box_into_raw_non_null", issue = "47336")]
 474     #[inline]
 475     pub fn into_raw_non_null(b: Box<T>) -> NonNull<T> {
 476         Box::into_unique(b).into()
 477     }
 478
 479     #[unstable(feature = "ptr_internals", issue = "0", reason = "use into_raw_non_null instead")]
 480     #[inline]
 481     #[doc(hidden)]
 482     pub fn into_unique(b: Box<T>) -> Unique<T> {
 483         let mut unique = b.0;
 484         mem::forget(b);
 485         // Box is kind-of a library type, but recognized as a "unique pointer" by
 486         // Stacked Borrows.  This function here corresponds to "reborrowing to
 487         // a raw pointer", but there is no actual reborrow here -- so
 488         // without some care, the pointer we are returning here still carries
 489         // the tag of `b`, with `Unique` permission.
 490         // We round-trip through a mutable reference to avoid that.
 491         unsafe { Unique::new_unchecked(unique.as_mut() as *mut T) }
 492     }
 493
 494     /// Consumes and leaks the `Box`, returning a mutable reference,
 495     /// `&'a mut T`. Note that the type `T` must outlive the chosen lifetime
 496     /// `'a`. If the type has only static references, or none at all, then this
 497     /// may be chosen to be `'static`.
 498     ///
 499     /// This function is mainly useful for data that lives for the remainder of
 500     /// the program's life. Dropping the returned reference will cause a memory
 501     /// leak. If this is not acceptable, the reference should first be wrapped
 502     /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
 503     /// then be dropped which will properly destroy `T` and release the
 504     /// allocated memory.
 505     ///
 506     /// Note: this is an associated function, which means that you have
 507     /// to call it as `Box::leak(b)` instead of `b.leak()`. This
 508     /// is so that there is no conflict with a method on the inner type.
 509     ///
 510     /// [`Box::from_raw`]: struct.Box.html#method.from_raw
 511     ///
 512     /// # Examples
 513     ///
 514     /// Simple usage:
 515     ///
 516     /// ```
 517     /// let x = Box::new(41);
 518     /// let static_ref: &'static mut usize = Box::leak(x);
 519     /// *static_ref += 1;
 520     /// assert_eq!(*static_ref, 42);
 521     /// ```
 522     ///
 523     /// Unsized data:
 524     ///
 525     /// ```
 526     /// let x = vec![1, 2, 3].into_boxed_slice();
 527     /// let static_ref = Box::leak(x);
 528     /// static_ref[0] = 4;
 529     /// assert_eq!(*static_ref, [4, 2, 3]);
 530     /// ```
 531     #[stable(feature = "box_leak", since = "1.26.0")]
 532     #[inline]
 533     pub fn leak<'a>(b: Box<T>) -> &'a mut T
 534     where
 535         T: 'a // Technically not needed, but kept to be explicit.
 536     {
 537         unsafe { &mut *Box::into_raw(b) }
 538     }
 539
 540     /// Converts a `Box<T>` into a `Pin<Box<T>>`
 541     ///
 542     /// This conversion does not allocate on the heap and happens in place.
 543     ///
 544     /// This is also available via [`From`].
 545     #[unstable(feature = "box_into_pin", issue = "62370")]
 546     pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>> {
 547         // It's not possible to move or replace the insides of a `Pin<Box<T>>`
 548         // when `T: !Unpin`,  so it's safe to pin it directly without any
 549         // additional requirements.
 550         unsafe { Pin::new_unchecked(boxed) }
 551     }
 552 }
 553
 554 #[stable(feature = "rust1", since = "1.0.0")]
 555 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
 556     fn drop(&mut self) {
 557         // FIXME: Do nothing, drop is currently performed by compiler.
 558     }
 559 }
 560
 561 #[stable(feature = "rust1", since = "1.0.0")]
 562 impl<T: Default> Default for Box<T> {
 563     /// Creates a `Box<T>`, with the `Default` value for T.
 564     fn default() -> Box<T> {
 565         box Default::default()
 566     }
 567 }
 568
 569 #[stable(feature = "rust1", since = "1.0.0")]
 570 impl<T> Default for Box<[T]> {
 571     fn default() -> Box<[T]> {
 572         Box::<[T; 0]>::new([])
 573     }
 574 }
 575
 576 #[stable(feature = "default_box_extra", since = "1.17.0")]
 577 impl Default for Box<str> {
 578     fn default() -> Box<str> {
 579         unsafe { from_boxed_utf8_unchecked(Default::default()) }
 580     }
 581 }
 582
 583 #[stable(feature = "rust1", since = "1.0.0")]
 584 impl<T: Clone> Clone for Box<T> {
 585     /// Returns a new box with a `clone()` of this box's contents.
 586     ///
 587     /// # Examples
 588     ///
 589     /// ```
 590     /// let x = Box::new(5);
 591     /// let y = x.clone();
 592     ///
 593     /// // The value is the same
 594     /// assert_eq!(x, y);
 595     ///
 596     /// // But they are unique objects
 597     /// assert_ne!(&*x as *const i32, &*y as *const i32);
 598     /// ```
 599     #[rustfmt::skip]
 600     #[inline]
 601     fn clone(&self) -> Box<T> {
 602         box { (**self).clone() }
 603     }
 604
 605     /// Copies `source`'s contents into `self` without creating a new allocation.
 606     ///
 607     /// # Examples
 608     ///
 609     /// ```
 610     /// let x = Box::new(5);
 611     /// let mut y = Box::new(10);
 612     /// let yp: *const i32 = &*y;
 613     ///
 614     /// y.clone_from(&x);
 615     ///
 616     /// // The value is the same
 617     /// assert_eq!(x, y);
 618     ///
 619     /// // And no allocation occurred
 620     /// assert_eq!(yp, &*y);
 621     /// ```
 622     #[inline]
 623     fn clone_from(&mut self, source: &Box<T>) {
 624         (**self).clone_from(&(**source));
 625     }
 626 }
 627
 628
 629 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 630 impl Clone for Box<str> {
 631     fn clone(&self) -> Self {
 632         // this makes a copy of the data
 633         let buf: Box<[u8]> = self.as_bytes().into();
 634         unsafe {
 635             from_boxed_utf8_unchecked(buf)
 636         }
 637     }
 638 }
 639
 640 #[stable(feature = "rust1", since = "1.0.0")]
 641 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
 642     #[inline]
 643     fn eq(&self, other: &Box<T>) -> bool {
 644         PartialEq::eq(&**self, &**other)
 645     }
 646     #[inline]
 647     fn ne(&self, other: &Box<T>) -> bool {
 648         PartialEq::ne(&**self, &**other)
 649     }
 650 }
 651 #[stable(feature = "rust1", since = "1.0.0")]
 652 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
 653     #[inline]
 654     fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
 655         PartialOrd::partial_cmp(&**self, &**other)
 656     }
 657     #[inline]
 658     fn lt(&self, other: &Box<T>) -> bool {
 659         PartialOrd::lt(&**self, &**other)
 660     }
 661     #[inline]
 662     fn le(&self, other: &Box<T>) -> bool {
 663         PartialOrd::le(&**self, &**other)
 664     }
 665     #[inline]
 666     fn ge(&self, other: &Box<T>) -> bool {
 667         PartialOrd::ge(&**self, &**other)
 668     }
 669     #[inline]
 670     fn gt(&self, other: &Box<T>) -> bool {
 671         PartialOrd::gt(&**self, &**other)
 672     }
 673 }
 674 #[stable(feature = "rust1", since = "1.0.0")]
 675 impl<T: ?Sized + Ord> Ord for Box<T> {
 676     #[inline]
 677     fn cmp(&self, other: &Box<T>) -> Ordering {
 678         Ord::cmp(&**self, &**other)
 679     }
 680 }
 681 #[stable(feature = "rust1", since = "1.0.0")]
 682 impl<T: ?Sized + Eq> Eq for Box<T> {}
 683
 684 #[stable(feature = "rust1", since = "1.0.0")]
 685 impl<T: ?Sized + Hash> Hash for Box<T> {
 686     fn hash<H: Hasher>(&self, state: &mut H) {
 687         (**self).hash(state);
 688     }
 689 }
 690
 691 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
 692 impl<T: ?Sized + Hasher> Hasher for Box<T> {
 693     fn finish(&self) -> u64 {
 694         (**self).finish()
 695     }
 696     fn write(&mut self, bytes: &[u8]) {
 697         (**self).write(bytes)
 698     }
 699     fn write_u8(&mut self, i: u8) {
 700         (**self).write_u8(i)
 701     }
 702     fn write_u16(&mut self, i: u16) {
 703         (**self).write_u16(i)
 704     }
 705     fn write_u32(&mut self, i: u32) {
 706         (**self).write_u32(i)
 707     }
 708     fn write_u64(&mut self, i: u64) {
 709         (**self).write_u64(i)
 710     }
 711     fn write_u128(&mut self, i: u128) {
 712         (**self).write_u128(i)
 713     }
 714     fn write_usize(&mut self, i: usize) {
 715         (**self).write_usize(i)
 716     }
 717     fn write_i8(&mut self, i: i8) {
 718         (**self).write_i8(i)
 719     }
 720     fn write_i16(&mut self, i: i16) {
 721         (**self).write_i16(i)
 722     }
 723     fn write_i32(&mut self, i: i32) {
 724         (**self).write_i32(i)
 725     }
 726     fn write_i64(&mut self, i: i64) {
 727         (**self).write_i64(i)
 728     }
 729     fn write_i128(&mut self, i: i128) {
 730         (**self).write_i128(i)
 731     }
 732     fn write_isize(&mut self, i: isize) {
 733         (**self).write_isize(i)
 734     }
 735 }
 736
 737 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
 738 impl<T> From<T> for Box<T> {
 739     /// Converts a generic type `T` into a `Box<T>`
 740     ///
 741     /// The conversion allocates on the heap and moves `t`
 742     /// from the stack into it.
 743     ///
 744     /// # Examples
 745     /// ```rust
 746     /// let x = 5;
 747     /// let boxed = Box::new(5);
 748     ///
 749     /// assert_eq!(Box::from(x), boxed);
 750     /// ```
 751     fn from(t: T) -> Self {
 752         Box::new(t)
 753     }
 754 }
 755
 756 #[stable(feature = "pin", since = "1.33.0")]
 757 impl<T: ?Sized> From<Box<T>> for Pin<Box<T>> {
 758     /// Converts a `Box<T>` into a `Pin<Box<T>>`
 759     ///
 760     /// This conversion does not allocate on the heap and happens in place.
 761     fn from(boxed: Box<T>) -> Self {
 762         Box::into_pin(boxed)
 763     }
 764 }
 765
 766 #[stable(feature = "box_from_slice", since = "1.17.0")]
 767 impl<T: Copy> From<&[T]> for Box<[T]> {
 768     /// Converts a `&[T]` into a `Box<[T]>`
 769     ///
 770     /// This conversion allocates on the heap
 771     /// and performs a copy of `slice`.
 772     ///
 773     /// # Examples
 774     /// ```rust
 775     /// // create a &[u8] which will be used to create a Box<[u8]>
 776     /// let slice: &[u8] = &[104, 101, 108, 108, 111];
 777     /// let boxed_slice: Box<[u8]> = Box::from(slice);
 778     ///
 779     /// println!("{:?}", boxed_slice);
 780     /// ```
 781     fn from(slice: &[T]) -> Box<[T]> {
 782         let len = slice.len();
 783         let buf = RawVec::with_capacity(len);
 784         unsafe {
 785             ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
 786             buf.into_box()
 787         }
 788     }
 789 }
 790
 791 #[stable(feature = "box_from_slice", since = "1.17.0")]
 792 impl From<&str> for Box<str> {
 793     /// Converts a `&str` into a `Box<str>`
 794     ///
 795     /// This conversion allocates on the heap
 796     /// and performs a copy of `s`.
 797     ///
 798     /// # Examples
 799     /// ```rust
 800     /// let boxed: Box<str> = Box::from("hello");
 801     /// println!("{}", boxed);
 802     /// ```
 803     #[inline]
 804     fn from(s: &str) -> Box<str> {
 805         unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
 806     }
 807 }
 808
 809 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
 810 impl From<Box<str>> for Box<[u8]> {
 811     /// Converts a `Box<str>>` into a `Box<[u8]>`
 812     ///
 813     /// This conversion does not allocate on the heap and happens in place.
 814     ///
 815     /// # Examples
 816     /// ```rust
 817     /// // create a Box<str> which will be used to create a Box<[u8]>
 818     /// let boxed: Box<str> = Box::from("hello");
 819     /// let boxed_str: Box<[u8]> = Box::from(boxed);
 820     ///
 821     /// // create a &[u8] which will be used to create a Box<[u8]>
 822     /// let slice: &[u8] = &[104, 101, 108, 108, 111];
 823     /// let boxed_slice = Box::from(slice);
 824     ///
 825     /// assert_eq!(boxed_slice, boxed_str);
 826     /// ```
 827     #[inline]
 828     fn from(s: Box<str>) -> Self {
 829         unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
 830     }
 831 }
 832
 833 #[unstable(feature = "boxed_slice_try_from", issue = "0")]
 834 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]>
 835 where
 836     [T; N]: LengthAtMost32,
 837 {
 838     type Error = Box<[T]>;
 839
 840     fn try_from(boxed_slice: Box<[T]>) -> Result<Self, Self::Error> {
 841         if boxed_slice.len() == N {
 842             Ok(unsafe { Box::from_raw(Box::into_raw(boxed_slice) as *mut [T; N]) })
 843         } else {
 844             Err(boxed_slice)
 845         }
 846     }
 847 }
 848
 849 impl Box<dyn Any> {
 850     #[inline]
 851     #[stable(feature = "rust1", since = "1.0.0")]
 852     /// Attempt to downcast the box to a concrete type.
 853     ///
 854     /// # Examples
 855     ///
 856     /// ```
 857     /// use std::any::Any;
 858     ///
 859     /// fn print_if_string(value: Box<dyn Any>) {
 860     ///     if let Ok(string) = value.downcast::<String>() {
 861     ///         println!("String ({}): {}", string.len(), string);
 862     ///     }
 863     /// }
 864     ///
 865     /// let my_string = "Hello World".to_string();
 866     /// print_if_string(Box::new(my_string));
 867     /// print_if_string(Box::new(0i8));
 868     /// ```
 869     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any>> {
 870         if self.is::<T>() {
 871             unsafe {
 872                 let raw: *mut dyn Any = Box::into_raw(self);
 873                 Ok(Box::from_raw(raw as *mut T))
 874             }
 875         } else {
 876             Err(self)
 877         }
 878     }
 879 }
 880
 881 impl Box<dyn Any + Send> {
 882     #[inline]
 883     #[stable(feature = "rust1", since = "1.0.0")]
 884     /// Attempt to downcast the box to a concrete type.
 885     ///
 886     /// # Examples
 887     ///
 888     /// ```
 889     /// use std::any::Any;
 890     ///
 891     /// fn print_if_string(value: Box<dyn Any + Send>) {
 892     ///     if let Ok(string) = value.downcast::<String>() {
 893     ///         println!("String ({}): {}", string.len(), string);
 894     ///     }
 895     /// }
 896     ///
 897     /// let my_string = "Hello World".to_string();
 898     /// print_if_string(Box::new(my_string));
 899     /// print_if_string(Box::new(0i8));
 900     /// ```
 901     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<dyn Any + Send>> {
 902         <Box<dyn Any>>::downcast(self).map_err(|s| unsafe {
 903             // reapply the Send marker
 904             Box::from_raw(Box::into_raw(s) as *mut (dyn Any + Send))
 905         })
 906     }
 907 }
 908
 909 #[stable(feature = "rust1", since = "1.0.0")]
 910 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
 911     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 912         fmt::Display::fmt(&**self, f)
 913     }
 914 }
 915
 916 #[stable(feature = "rust1", since = "1.0.0")]
 917 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
 918     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 919         fmt::Debug::fmt(&**self, f)
 920     }
 921 }
 922
 923 #[stable(feature = "rust1", since = "1.0.0")]
 924 impl<T: ?Sized> fmt::Pointer for Box<T> {
 925     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 926         // It's not possible to extract the inner Uniq directly from the Box,
 927         // instead we cast it to a *const which aliases the Unique
 928         let ptr: *const T = &**self;
 929         fmt::Pointer::fmt(&ptr, f)
 930     }
 931 }
 932
 933 #[stable(feature = "rust1", since = "1.0.0")]
 934 impl<T: ?Sized> Deref for Box<T> {
 935     type Target = T;
 936
 937     fn deref(&self) -> &T {
 938         &**self
 939     }
 940 }
 941
 942 #[stable(feature = "rust1", since = "1.0.0")]
 943 impl<T: ?Sized> DerefMut for Box<T> {
 944     fn deref_mut(&mut self) -> &mut T {
 945         &mut **self
 946     }
 947 }
 948
 949 #[unstable(feature = "receiver_trait", issue = "0")]
 950 impl<T: ?Sized> Receiver for Box<T> {}
 951
 952 #[stable(feature = "rust1", since = "1.0.0")]
 953 impl<I: Iterator + ?Sized> Iterator for Box<I> {
 954     type Item = I::Item;
 955     fn next(&mut self) -> Option<I::Item> {
 956         (**self).next()
 957     }
 958     fn size_hint(&self) -> (usize, Option<usize>) {
 959         (**self).size_hint()
 960     }
 961     fn nth(&mut self, n: usize) -> Option<I::Item> {
 962         (**self).nth(n)
 963     }
 964     fn last(self) -> Option<I::Item> {
 965         BoxIter::last(self)
 966     }
 967 }
 968
 969 trait BoxIter {
 970     type Item;
 971     fn last(self) -> Option<Self::Item>;
 972 }
 973
 974 impl<I: Iterator + ?Sized> BoxIter for Box<I> {
 975     type Item = I::Item;
 976     default fn last(self) -> Option<I::Item> {
 977         #[inline]
 978         fn some<T>(_: Option<T>, x: T) -> Option<T> {
 979             Some(x)
 980         }
 981
 982         self.fold(None, some)
 983     }
 984 }
 985
 986 /// Specialization for sized `I`s that uses `I`s implementation of `last()`
 987 /// instead of the default.
 988 #[stable(feature = "rust1", since = "1.0.0")]
 989 impl<I: Iterator> BoxIter for Box<I> {
 990     fn last(self) -> Option<I::Item> {
 991         (*self).last()
 992     }
 993 }
 994
 995 #[stable(feature = "rust1", since = "1.0.0")]
 996 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
 997     fn next_back(&mut self) -> Option<I::Item> {
 998         (**self).next_back()
 999     }
1000     fn nth_back(&mut self, n: usize) -> Option<I::Item> {
1001         (**self).nth_back(n)
1002     }
1003 }
1004 #[stable(feature = "rust1", since = "1.0.0")]
1005 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
1006     fn len(&self) -> usize {
1007         (**self).len()
1008     }
1009     fn is_empty(&self) -> bool {
1010         (**self).is_empty()
1011     }
1012 }
1013
1014 #[stable(feature = "fused", since = "1.26.0")]
1015 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
1016
1017 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1018 impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F> {
1019     type Output = <F as FnOnce<A>>::Output;
1020
1021     extern "rust-call" fn call_once(self, args: A) -> Self::Output {
1022         <F as FnOnce<A>>::call_once(*self, args)
1023     }
1024 }
1025
1026 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1027 impl<A, F: FnMut<A> + ?Sized> FnMut<A> for Box<F> {
1028     extern "rust-call" fn call_mut(&mut self, args: A) -> Self::Output {
1029         <F as FnMut<A>>::call_mut(self, args)
1030     }
1031 }
1032
1033 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1034 impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F> {
1035     extern "rust-call" fn call(&self, args: A) -> Self::Output {
1036         <F as Fn<A>>::call(self, args)
1037     }
1038 }
1039
1040 #[unstable(feature = "coerce_unsized", issue = "27732")]
1041 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
1042
1043 #[unstable(feature = "dispatch_from_dyn", issue = "0")]
1044 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
1045
1046 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
1047 impl<A> FromIterator<A> for Box<[A]> {
1048     fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
1049         iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
1050     }
1051 }
1052
1053 #[stable(feature = "box_slice_clone", since = "1.3.0")]
1054 impl<T: Clone> Clone for Box<[T]> {
1055     fn clone(&self) -> Self {
1056         let mut new = BoxBuilder {
1057             data: RawVec::with_capacity(self.len()),
1058             len: 0,
1059         };
1060
1061         let mut target = new.data.ptr();
1062
1063         for item in self.iter() {
1064             unsafe {
1065                 ptr::write(target, item.clone());
1066                 target = target.offset(1);
1067             };
1068
1069             new.len += 1;
1070         }
1071
1072         return unsafe { new.into_box() };
1073
1074         // Helper type for responding to panics correctly.
1075         struct BoxBuilder<T> {
1076             data: RawVec<T>,
1077             len: usize,
1078         }
1079
1080         impl<T> BoxBuilder<T> {
1081             unsafe fn into_box(self) -> Box<[T]> {
1082                 let raw = ptr::read(&self.data);
1083                 mem::forget(self);
1084                 raw.into_box()
1085             }
1086         }
1087
1088         impl<T> Drop for BoxBuilder<T> {
1089             fn drop(&mut self) {
1090                 let mut data = self.data.ptr();
1091                 let max = unsafe { data.add(self.len) };
1092
1093                 while data != max {
1094                     unsafe {
1095                         ptr::read(data);
1096                         data = data.offset(1);
1097                     }
1098                 }
1099             }
1100         }
1101     }
1102 }
1103
1104 #[stable(feature = "box_borrow", since = "1.1.0")]
1105 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
1106     fn borrow(&self) -> &T {
1107         &**self
1108     }
1109 }
1110
1111 #[stable(feature = "box_borrow", since = "1.1.0")]
1112 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
1113     fn borrow_mut(&mut self) -> &mut T {
1114         &mut **self
1115     }
1116 }
1117
1118 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1119 impl<T: ?Sized> AsRef<T> for Box<T> {
1120     fn as_ref(&self) -> &T {
1121         &**self
1122     }
1123 }
1124
1125 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
1126 impl<T: ?Sized> AsMut<T> for Box<T> {
1127     fn as_mut(&mut self) -> &mut T {
1128         &mut **self
1129     }
1130 }
1131
1132 /* Nota bene
1133  *
1134  *  We could have chosen not to add this impl, and instead have written a
1135  *  function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
1136  *  because Box<T> implements Unpin even when T does not, as a result of
1137  *  this impl.
1138  *
1139  *  We chose this API instead of the alternative for a few reasons:
1140  *      - Logically, it is helpful to understand pinning in regard to the
1141  *        memory region being pointed to. For this reason none of the
1142  *        standard library pointer types support projecting through a pin
1143  *        (Box<T> is the only pointer type in std for which this would be
1144  *        safe.)
1145  *      - It is in practice very useful to have Box<T> be unconditionally
1146  *        Unpin because of trait objects, for which the structural auto
1147  *        trait functionality does not apply (e.g., Box<dyn Foo> would
1148  *        otherwise not be Unpin).
1149  *
1150  *  Another type with the same semantics as Box but only a conditional
1151  *  implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
1152  *  could have a method to project a Pin<T> from it.
1153  */
1154 #[stable(feature = "pin", since = "1.33.0")]
1155 impl<T: ?Sized> Unpin for Box<T> { }
1156
1157 #[unstable(feature = "generator_trait", issue = "43122")]
1158 impl<G: ?Sized + Generator + Unpin> Generator for Box<G> {
1159     type Yield = G::Yield;
1160     type Return = G::Return;
1161
1162     fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
1163         G::resume(Pin::new(&mut *self))
1164     }
1165 }
1166
1167 #[unstable(feature = "generator_trait", issue = "43122")]
1168 impl<G: ?Sized + Generator> Generator for Pin<Box<G>> {
1169     type Yield = G::Yield;
1170     type Return = G::Return;
1171
1172     fn resume(mut self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return> {
1173         G::resume((*self).as_mut())
1174     }
1175 }
1176
1177 #[stable(feature = "futures_api", since = "1.36.0")]
1178 impl<F: ?Sized + Future + Unpin> Future for Box<F> {
1179     type Output = F::Output;
1180
1181     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
1182         F::poll(Pin::new(&mut *self), cx)
1183     }
1184 }