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