src/liballoc/boxed.rs

   1 // Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! A pointer type for heap allocation.
  12 //!
  13 //! `Box<T>`, casually referred to as a 'box', provides the simplest form of
  14 //! heap allocation in Rust. Boxes provide ownership for this allocation, and
  15 //! drop their contents when they go out of scope.
  16 //!
  17 //! # Examples
  18 //!
  19 //! Creating a box:
  20 //!
  21 //! ```
  22 //! let x = Box::new(5);
  23 //! ```
  24 //!
  25 //! Creating a recursive data structure:
  26 //!
  27 //! ```
  28 //! #[derive(Debug)]
  29 //! enum List<T> {
  30 //!     Cons(T, Box<List<T>>),
  31 //!     Nil,
  32 //! }
  33 //!
  34 //! fn main() {
  35 //!     let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
  36 //!     println!("{:?}", list);
  37 //! }
  38 //! ```
  39 //!
  40 //! This will print `Cons(1, Cons(2, Nil))`.
  41 //!
  42 //! Recursive structures must be boxed, because if the definition of `Cons`
  43 //! looked like this:
  44 //!
  45 //! ```compile_fail,E0072
  46 //! # enum List<T> {
  47 //! Cons(T, List<T>),
  48 //! # }
  49 //! ```
  50 //!
  51 //! It wouldn't work. This is because the size of a `List` depends on how many
  52 //! elements are in the list, and so we don't know how much memory to allocate
  53 //! for a `Cons`. By introducing a `Box`, which has a defined size, we know how
  54 //! big `Cons` needs to be.
  55
  56 #![stable(feature = "rust1", since = "1.0.0")]
  57
  58 use heap::{Heap, Layout, Alloc};
  59 use raw_vec::RawVec;
  60
  61 use core::any::Any;
  62 use core::borrow;
  63 use core::cmp::Ordering;
  64 use core::fmt;
  65 use core::hash::{self, Hash};
  66 use core::iter::FusedIterator;
  67 use core::marker::{self, Unsize};
  68 use core::mem;
  69 use core::ops::{CoerceUnsized, Deref, DerefMut};
  70 use core::ops::{BoxPlace, Boxed, InPlace, Place, Placer};
  71 use core::ptr::{self, Unique};
  72 use core::convert::From;
  73 use str::from_boxed_utf8_unchecked;
  74
  75 /// A value that represents the heap. This is the default place that the `box`
  76 /// keyword allocates into when no place is supplied.
  77 ///
  78 /// The following two examples are equivalent:
  79 ///
  80 /// ```
  81 /// #![feature(box_heap)]
  82 ///
  83 /// #![feature(box_syntax, placement_in_syntax)]
  84 /// use std::boxed::HEAP;
  85 ///
  86 /// fn main() {
  87 ///     let foo: Box<i32> = in HEAP { 5 };
  88 ///     let foo = box 5;
  89 /// }
  90 /// ```
  91 #[unstable(feature = "box_heap",
  92            reason = "may be renamed; uncertain about custom allocator design",
  93            issue = "27779")]
  94 pub const HEAP: ExchangeHeapSingleton = ExchangeHeapSingleton { _force_singleton: () };
  95
  96 /// This the singleton type used solely for `boxed::HEAP`.
  97 #[unstable(feature = "box_heap",
  98            reason = "may be renamed; uncertain about custom allocator design",
  99            issue = "27779")]
 100 #[allow(missing_debug_implementations)]
 101 #[derive(Copy, Clone)]
 102 pub struct ExchangeHeapSingleton {
 103     _force_singleton: (),
 104 }
 105
 106 /// A pointer type for heap allocation.
 107 ///
 108 /// See the [module-level documentation](../../std/boxed/index.html) for more.
 109 #[lang = "owned_box"]
 110 #[fundamental]
 111 #[stable(feature = "rust1", since = "1.0.0")]
 112 pub struct Box<T: ?Sized>(Unique<T>);
 113
 114 /// `IntermediateBox` represents uninitialized backing storage for `Box`.
 115 ///
 116 /// FIXME (pnkfelix): Ideally we would just reuse `Box<T>` instead of
 117 /// introducing a separate `IntermediateBox<T>`; but then you hit
 118 /// issues when you e.g. attempt to destructure an instance of `Box`,
 119 /// since it is a lang item and so it gets special handling by the
 120 /// compiler.  Easier just to make this parallel type for now.
 121 ///
 122 /// FIXME (pnkfelix): Currently the `box` protocol only supports
 123 /// creating instances of sized types. This IntermediateBox is
 124 /// designed to be forward-compatible with a future protocol that
 125 /// supports creating instances of unsized types; that is why the type
 126 /// parameter has the `?Sized` generalization marker, and is also why
 127 /// this carries an explicit size. However, it probably does not need
 128 /// to carry the explicit alignment; that is just a work-around for
 129 /// the fact that the `align_of` intrinsic currently requires the
 130 /// input type to be Sized (which I do not think is strictly
 131 /// necessary).
 132 #[unstable(feature = "placement_in",
 133            reason = "placement box design is still being worked out.",
 134            issue = "27779")]
 135 #[allow(missing_debug_implementations)]
 136 pub struct IntermediateBox<T: ?Sized> {
 137     ptr: *mut u8,
 138     layout: Layout,
 139     marker: marker::PhantomData<*mut T>,
 140 }
 141
 142 #[unstable(feature = "placement_in",
 143            reason = "placement box design is still being worked out.",
 144            issue = "27779")]
 145 impl<T> Place<T> for IntermediateBox<T> {
 146     fn pointer(&mut self) -> *mut T {
 147         self.ptr as *mut T
 148     }
 149 }
 150
 151 unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
 152     let p = b.ptr as *mut T;
 153     mem::forget(b);
 154     mem::transmute(p)
 155 }
 156
 157 fn make_place<T>() -> IntermediateBox<T> {
 158     let layout = Layout::new::<T>();
 159
 160     let p = if layout.size() == 0 {
 161         mem::align_of::<T>() as *mut u8
 162     } else {
 163         unsafe {
 164             Heap.alloc(layout.clone()).unwrap_or_else(|err| {
 165                 Heap.oom(err)
 166             })
 167         }
 168     };
 169
 170     IntermediateBox {
 171         ptr: p,
 172         layout: layout,
 173         marker: marker::PhantomData,
 174     }
 175 }
 176
 177 #[unstable(feature = "placement_in",
 178            reason = "placement box design is still being worked out.",
 179            issue = "27779")]
 180 impl<T> BoxPlace<T> for IntermediateBox<T> {
 181     fn make_place() -> IntermediateBox<T> {
 182         make_place()
 183     }
 184 }
 185
 186 #[unstable(feature = "placement_in",
 187            reason = "placement box design is still being worked out.",
 188            issue = "27779")]
 189 impl<T> InPlace<T> for IntermediateBox<T> {
 190     type Owner = Box<T>;
 191     unsafe fn finalize(self) -> Box<T> {
 192         finalize(self)
 193     }
 194 }
 195
 196 #[unstable(feature = "placement_new_protocol", issue = "27779")]
 197 impl<T> Boxed for Box<T> {
 198     type Data = T;
 199     type Place = IntermediateBox<T>;
 200     unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> {
 201         finalize(b)
 202     }
 203 }
 204
 205 #[unstable(feature = "placement_in",
 206            reason = "placement box design is still being worked out.",
 207            issue = "27779")]
 208 impl<T> Placer<T> for ExchangeHeapSingleton {
 209     type Place = IntermediateBox<T>;
 210
 211     fn make_place(self) -> IntermediateBox<T> {
 212         make_place()
 213     }
 214 }
 215
 216 #[unstable(feature = "placement_in",
 217            reason = "placement box design is still being worked out.",
 218            issue = "27779")]
 219 impl<T: ?Sized> Drop for IntermediateBox<T> {
 220     fn drop(&mut self) {
 221         if self.layout.size() > 0 {
 222             unsafe {
 223                 Heap.dealloc(self.ptr, self.layout.clone())
 224             }
 225         }
 226     }
 227 }
 228
 229 impl<T> Box<T> {
 230     /// Allocates memory on the heap and then places `x` into it.
 231     ///
 232     /// This doesn't actually allocate if `T` is zero-sized.
 233     ///
 234     /// # Examples
 235     ///
 236     /// ```
 237     /// let five = Box::new(5);
 238     /// ```
 239     #[stable(feature = "rust1", since = "1.0.0")]
 240     #[inline(always)]
 241     pub fn new(x: T) -> Box<T> {
 242         box x
 243     }
 244 }
 245
 246 impl<T: ?Sized> Box<T> {
 247     /// Constructs a box from a raw pointer.
 248     ///
 249     /// After calling this function, the raw pointer is owned by the
 250     /// resulting `Box`. Specifically, the `Box` destructor will call
 251     /// the destructor of `T` and free the allocated memory. Since the
 252     /// way `Box` allocates and releases memory is unspecified, the
 253     /// only valid pointer to pass to this function is the one taken
 254     /// from another `Box` via the [`Box::into_raw`] function.
 255     ///
 256     /// This function is unsafe because improper use may lead to
 257     /// memory problems. For example, a double-free may occur if the
 258     /// function is called twice on the same raw pointer.
 259     ///
 260     /// [`Box::into_raw`]: struct.Box.html#method.into_raw
 261     ///
 262     /// # Examples
 263     ///
 264     /// ```
 265     /// let x = Box::new(5);
 266     /// let ptr = Box::into_raw(x);
 267     /// let x = unsafe { Box::from_raw(ptr) };
 268     /// ```
 269     #[stable(feature = "box_raw", since = "1.4.0")]
 270     #[inline]
 271     pub unsafe fn from_raw(raw: *mut T) -> Self {
 272         mem::transmute(raw)
 273     }
 274
 275     /// Consumes the `Box`, returning the wrapped raw pointer.
 276     ///
 277     /// After calling this function, the caller is responsible for the
 278     /// memory previously managed by the `Box`. In particular, the
 279     /// caller should properly destroy `T` and release the memory. The
 280     /// proper way to do so is to convert the raw pointer back into a
 281     /// `Box` with the [`Box::from_raw`] function.
 282     ///
 283     /// Note: this is an associated function, which means that you have
 284     /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
 285     /// is so that there is no conflict with a method on the inner type.
 286     ///
 287     /// [`Box::from_raw`]: struct.Box.html#method.from_raw
 288     ///
 289     /// # Examples
 290     ///
 291     /// ```
 292     /// let x = Box::new(5);
 293     /// let ptr = Box::into_raw(x);
 294     /// ```
 295     #[stable(feature = "box_raw", since = "1.4.0")]
 296     #[inline]
 297     pub fn into_raw(b: Box<T>) -> *mut T {
 298         unsafe { mem::transmute(b) }
 299     }
 300 }
 301
 302 #[stable(feature = "rust1", since = "1.0.0")]
 303 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
 304     fn drop(&mut self) {
 305         // FIXME: Do nothing, drop is currently performed by compiler.
 306     }
 307 }
 308
 309 #[stable(feature = "rust1", since = "1.0.0")]
 310 impl<T: Default> Default for Box<T> {
 311     /// Creates a `Box<T>`, with the `Default` value for T.
 312     fn default() -> Box<T> {
 313         box Default::default()
 314     }
 315 }
 316
 317 #[stable(feature = "rust1", since = "1.0.0")]
 318 impl<T> Default for Box<[T]> {
 319     fn default() -> Box<[T]> {
 320         Box::<[T; 0]>::new([])
 321     }
 322 }
 323
 324 #[stable(feature = "default_box_extra", since = "1.17.0")]
 325 impl Default for Box<str> {
 326     fn default() -> Box<str> {
 327         unsafe { from_boxed_utf8_unchecked(Default::default()) }
 328     }
 329 }
 330
 331 #[stable(feature = "rust1", since = "1.0.0")]
 332 impl<T: Clone> Clone for Box<T> {
 333     /// Returns a new box with a `clone()` of this box's contents.
 334     ///
 335     /// # Examples
 336     ///
 337     /// ```
 338     /// let x = Box::new(5);
 339     /// let y = x.clone();
 340     /// ```
 341     #[rustfmt_skip]
 342     #[inline]
 343     fn clone(&self) -> Box<T> {
 344         box { (**self).clone() }
 345     }
 346     /// Copies `source`'s contents into `self` without creating a new allocation.
 347     ///
 348     /// # Examples
 349     ///
 350     /// ```
 351     /// let x = Box::new(5);
 352     /// let mut y = Box::new(10);
 353     ///
 354     /// y.clone_from(&x);
 355     ///
 356     /// assert_eq!(*y, 5);
 357     /// ```
 358     #[inline]
 359     fn clone_from(&mut self, source: &Box<T>) {
 360         (**self).clone_from(&(**source));
 361     }
 362 }
 363
 364
 365 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 366 impl Clone for Box<str> {
 367     fn clone(&self) -> Self {
 368         let len = self.len();
 369         let buf = RawVec::with_capacity(len);
 370         unsafe {
 371             ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
 372             from_boxed_utf8_unchecked(buf.into_box())
 373         }
 374     }
 375 }
 376
 377 #[stable(feature = "rust1", since = "1.0.0")]
 378 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
 379     #[inline]
 380     fn eq(&self, other: &Box<T>) -> bool {
 381         PartialEq::eq(&**self, &**other)
 382     }
 383     #[inline]
 384     fn ne(&self, other: &Box<T>) -> bool {
 385         PartialEq::ne(&**self, &**other)
 386     }
 387 }
 388 #[stable(feature = "rust1", since = "1.0.0")]
 389 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
 390     #[inline]
 391     fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
 392         PartialOrd::partial_cmp(&**self, &**other)
 393     }
 394     #[inline]
 395     fn lt(&self, other: &Box<T>) -> bool {
 396         PartialOrd::lt(&**self, &**other)
 397     }
 398     #[inline]
 399     fn le(&self, other: &Box<T>) -> bool {
 400         PartialOrd::le(&**self, &**other)
 401     }
 402     #[inline]
 403     fn ge(&self, other: &Box<T>) -> bool {
 404         PartialOrd::ge(&**self, &**other)
 405     }
 406     #[inline]
 407     fn gt(&self, other: &Box<T>) -> bool {
 408         PartialOrd::gt(&**self, &**other)
 409     }
 410 }
 411 #[stable(feature = "rust1", since = "1.0.0")]
 412 impl<T: ?Sized + Ord> Ord for Box<T> {
 413     #[inline]
 414     fn cmp(&self, other: &Box<T>) -> Ordering {
 415         Ord::cmp(&**self, &**other)
 416     }
 417 }
 418 #[stable(feature = "rust1", since = "1.0.0")]
 419 impl<T: ?Sized + Eq> Eq for Box<T> {}
 420
 421 #[stable(feature = "rust1", since = "1.0.0")]
 422 impl<T: ?Sized + Hash> Hash for Box<T> {
 423     fn hash<H: hash::Hasher>(&self, state: &mut H) {
 424         (**self).hash(state);
 425     }
 426 }
 427
 428 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
 429 impl<T> From<T> for Box<T> {
 430     fn from(t: T) -> Self {
 431         Box::new(t)
 432     }
 433 }
 434
 435 #[stable(feature = "box_from_slice", since = "1.17.0")]
 436 impl<'a, T: Copy> From<&'a [T]> for Box<[T]> {
 437     fn from(slice: &'a [T]) -> Box<[T]> {
 438         let mut boxed = unsafe { RawVec::with_capacity(slice.len()).into_box() };
 439         boxed.copy_from_slice(slice);
 440         boxed
 441     }
 442 }
 443
 444 #[stable(feature = "box_from_slice", since = "1.17.0")]
 445 impl<'a> From<&'a str> for Box<str> {
 446     fn from(s: &'a str) -> Box<str> {
 447         unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
 448     }
 449 }
 450
 451 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
 452 impl From<Box<str>> for Box<[u8]> {
 453     fn from(s: Box<str>) -> Self {
 454         unsafe {
 455             mem::transmute(s)
 456         }
 457     }
 458 }
 459
 460 impl Box<Any> {
 461     #[inline]
 462     #[stable(feature = "rust1", since = "1.0.0")]
 463     /// Attempt to downcast the box to a concrete type.
 464     ///
 465     /// # Examples
 466     ///
 467     /// ```
 468     /// use std::any::Any;
 469     ///
 470     /// fn print_if_string(value: Box<Any>) {
 471     ///     if let Ok(string) = value.downcast::<String>() {
 472     ///         println!("String ({}): {}", string.len(), string);
 473     ///     }
 474     /// }
 475     ///
 476     /// fn main() {
 477     ///     let my_string = "Hello World".to_string();
 478     ///     print_if_string(Box::new(my_string));
 479     ///     print_if_string(Box::new(0i8));
 480     /// }
 481     /// ```
 482     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
 483         if self.is::<T>() {
 484             unsafe {
 485                 let raw: *mut Any = Box::into_raw(self);
 486                 Ok(Box::from_raw(raw as *mut T))
 487             }
 488         } else {
 489             Err(self)
 490         }
 491     }
 492 }
 493
 494 impl Box<Any + Send> {
 495     #[inline]
 496     #[stable(feature = "rust1", since = "1.0.0")]
 497     /// Attempt to downcast the box to a concrete type.
 498     ///
 499     /// # Examples
 500     ///
 501     /// ```
 502     /// use std::any::Any;
 503     ///
 504     /// fn print_if_string(value: Box<Any + Send>) {
 505     ///     if let Ok(string) = value.downcast::<String>() {
 506     ///         println!("String ({}): {}", string.len(), string);
 507     ///     }
 508     /// }
 509     ///
 510     /// fn main() {
 511     ///     let my_string = "Hello World".to_string();
 512     ///     print_if_string(Box::new(my_string));
 513     ///     print_if_string(Box::new(0i8));
 514     /// }
 515     /// ```
 516     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
 517         <Box<Any>>::downcast(self).map_err(|s| unsafe {
 518             // reapply the Send marker
 519             mem::transmute::<Box<Any>, Box<Any + Send>>(s)
 520         })
 521     }
 522 }
 523
 524 #[stable(feature = "rust1", since = "1.0.0")]
 525 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
 526     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 527         fmt::Display::fmt(&**self, f)
 528     }
 529 }
 530
 531 #[stable(feature = "rust1", since = "1.0.0")]
 532 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
 533     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 534         fmt::Debug::fmt(&**self, f)
 535     }
 536 }
 537
 538 #[stable(feature = "rust1", since = "1.0.0")]
 539 impl<T: ?Sized> fmt::Pointer for Box<T> {
 540     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 541         // It's not possible to extract the inner Uniq directly from the Box,
 542         // instead we cast it to a *const which aliases the Unique
 543         let ptr: *const T = &**self;
 544         fmt::Pointer::fmt(&ptr, f)
 545     }
 546 }
 547
 548 #[stable(feature = "rust1", since = "1.0.0")]
 549 impl<T: ?Sized> Deref for Box<T> {
 550     type Target = T;
 551
 552     fn deref(&self) -> &T {
 553         &**self
 554     }
 555 }
 556
 557 #[stable(feature = "rust1", since = "1.0.0")]
 558 impl<T: ?Sized> DerefMut for Box<T> {
 559     fn deref_mut(&mut self) -> &mut T {
 560         &mut **self
 561     }
 562 }
 563
 564 #[stable(feature = "rust1", since = "1.0.0")]
 565 impl<I: Iterator + ?Sized> Iterator for Box<I> {
 566     type Item = I::Item;
 567     fn next(&mut self) -> Option<I::Item> {
 568         (**self).next()
 569     }
 570     fn size_hint(&self) -> (usize, Option<usize>) {
 571         (**self).size_hint()
 572     }
 573     fn nth(&mut self, n: usize) -> Option<I::Item> {
 574         (**self).nth(n)
 575     }
 576 }
 577 #[stable(feature = "rust1", since = "1.0.0")]
 578 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
 579     fn next_back(&mut self) -> Option<I::Item> {
 580         (**self).next_back()
 581     }
 582 }
 583 #[stable(feature = "rust1", since = "1.0.0")]
 584 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
 585     fn len(&self) -> usize {
 586         (**self).len()
 587     }
 588     fn is_empty(&self) -> bool {
 589         (**self).is_empty()
 590     }
 591 }
 592
 593 #[unstable(feature = "fused", issue = "35602")]
 594 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
 595
 596
 597 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
 598 /// closure objects. The idea is that where one would normally store a
 599 /// `Box<FnOnce()>` in a data structure, you should use
 600 /// `Box<FnBox()>`. The two traits behave essentially the same, except
 601 /// that a `FnBox` closure can only be called if it is boxed. (Note
 602 /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
 603 /// closures become directly usable.)
 604 ///
 605 /// ### Example
 606 ///
 607 /// Here is a snippet of code which creates a hashmap full of boxed
 608 /// once closures and then removes them one by one, calling each
 609 /// closure as it is removed. Note that the type of the closures
 610 /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
 611 /// -> i32>`.
 612 ///
 613 /// ```
 614 /// #![feature(fnbox)]
 615 ///
 616 /// use std::boxed::FnBox;
 617 /// use std::collections::HashMap;
 618 ///
 619 /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
 620 ///     let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
 621 ///     map.insert(1, Box::new(|| 22));
 622 ///     map.insert(2, Box::new(|| 44));
 623 ///     map
 624 /// }
 625 ///
 626 /// fn main() {
 627 ///     let mut map = make_map();
 628 ///     for i in &[1, 2] {
 629 ///         let f = map.remove(&i).unwrap();
 630 ///         assert_eq!(f(), i * 22);
 631 ///     }
 632 /// }
 633 /// ```
 634 #[rustc_paren_sugar]
 635 #[unstable(feature = "fnbox",
 636            reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
 637 pub trait FnBox<A> {
 638     type Output;
 639
 640     fn call_box(self: Box<Self>, args: A) -> Self::Output;
 641 }
 642
 643 #[unstable(feature = "fnbox",
 644            reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
 645 impl<A, F> FnBox<A> for F
 646     where F: FnOnce<A>
 647 {
 648     type Output = F::Output;
 649
 650     fn call_box(self: Box<F>, args: A) -> F::Output {
 651         self.call_once(args)
 652     }
 653 }
 654
 655 #[unstable(feature = "fnbox",
 656            reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
 657 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a> {
 658     type Output = R;
 659
 660     extern "rust-call" fn call_once(self, args: A) -> R {
 661         self.call_box(args)
 662     }
 663 }
 664
 665 #[unstable(feature = "fnbox",
 666            reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
 667 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + Send + 'a> {
 668     type Output = R;
 669
 670     extern "rust-call" fn call_once(self, args: A) -> R {
 671         self.call_box(args)
 672     }
 673 }
 674
 675 #[unstable(feature = "coerce_unsized", issue = "27732")]
 676 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
 677
 678 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 679 impl<T: Clone> Clone for Box<[T]> {
 680     fn clone(&self) -> Self {
 681         let mut new = BoxBuilder {
 682             data: RawVec::with_capacity(self.len()),
 683             len: 0,
 684         };
 685
 686         let mut target = new.data.ptr();
 687
 688         for item in self.iter() {
 689             unsafe {
 690                 ptr::write(target, item.clone());
 691                 target = target.offset(1);
 692             };
 693
 694             new.len += 1;
 695         }
 696
 697         return unsafe { new.into_box() };
 698
 699         // Helper type for responding to panics correctly.
 700         struct BoxBuilder<T> {
 701             data: RawVec<T>,
 702             len: usize,
 703         }
 704
 705         impl<T> BoxBuilder<T> {
 706             unsafe fn into_box(self) -> Box<[T]> {
 707                 let raw = ptr::read(&self.data);
 708                 mem::forget(self);
 709                 raw.into_box()
 710             }
 711         }
 712
 713         impl<T> Drop for BoxBuilder<T> {
 714             fn drop(&mut self) {
 715                 let mut data = self.data.ptr();
 716                 let max = unsafe { data.offset(self.len as isize) };
 717
 718                 while data != max {
 719                     unsafe {
 720                         ptr::read(data);
 721                         data = data.offset(1);
 722                     }
 723                 }
 724             }
 725         }
 726     }
 727 }
 728
 729 #[stable(feature = "box_borrow", since = "1.1.0")]
 730 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
 731     fn borrow(&self) -> &T {
 732         &**self
 733     }
 734 }
 735
 736 #[stable(feature = "box_borrow", since = "1.1.0")]
 737 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
 738     fn borrow_mut(&mut self) -> &mut T {
 739         &mut **self
 740     }
 741 }
 742
 743 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
 744 impl<T: ?Sized> AsRef<T> for Box<T> {
 745     fn as_ref(&self) -> &T {
 746         &**self
 747     }
 748 }
 749
 750 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
 751 impl<T: ?Sized> AsMut<T> for Box<T> {
 752     fn as_mut(&mut self) -> &mut T {
 753         &mut **self
 754     }
 755 }