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, Hasher};
  66 use core::iter::FusedIterator;
  67 use core::marker::{self, Unsize};
  68 use core::mem;
  69 use core::ops::{CoerceUnsized, Deref, DerefMut, Generator, GeneratorState};
  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,
 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         Box::from_unique(Unique::new_unchecked(raw))
 273     }
 274
 275     /// Constructs a `Box` from a `Unique<T>` pointer.
 276     ///
 277     /// After calling this function, the memory is owned by a `Box` and `T` can
 278     /// then be destroyed and released upon drop.
 279     ///
 280     /// # Safety
 281     ///
 282     /// A `Unique<T>` can be safely created via [`Unique::new`] and thus doesn't
 283     /// necessarily own the data pointed to nor is the data guaranteed to live
 284     /// as long as the pointer.
 285     ///
 286     /// [`Unique::new`]: ../../core/ptr/struct.Unique.html#method.new
 287     ///
 288     /// # Examples
 289     ///
 290     /// ```
 291     /// #![feature(unique)]
 292     ///
 293     /// fn main() {
 294     ///     let x = Box::new(5);
 295     ///     let ptr = Box::into_unique(x);
 296     ///     let x = unsafe { Box::from_unique(ptr) };
 297     /// }
 298     /// ```
 299     #[unstable(feature = "unique", reason = "needs an RFC to flesh out design",
 300                issue = "27730")]
 301     #[inline]
 302     pub unsafe fn from_unique(u: Unique<T>) -> Self {
 303         mem::transmute(u)
 304     }
 305
 306     /// Consumes the `Box`, returning the wrapped raw pointer.
 307     ///
 308     /// After calling this function, the caller is responsible for the
 309     /// memory previously managed by the `Box`. In particular, the
 310     /// caller should properly destroy `T` and release the memory. The
 311     /// proper way to do so is to convert the raw pointer back into a
 312     /// `Box` with the [`Box::from_raw`] function.
 313     ///
 314     /// Note: this is an associated function, which means that you have
 315     /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
 316     /// is so that there is no conflict with a method on the inner type.
 317     ///
 318     /// [`Box::from_raw`]: struct.Box.html#method.from_raw
 319     ///
 320     /// # Examples
 321     ///
 322     /// ```
 323     /// let x = Box::new(5);
 324     /// let ptr = Box::into_raw(x);
 325     /// ```
 326     #[stable(feature = "box_raw", since = "1.4.0")]
 327     #[inline]
 328     pub fn into_raw(b: Box<T>) -> *mut T {
 329         Box::into_unique(b).as_ptr()
 330     }
 331
 332     /// Consumes the `Box`, returning the wrapped pointer as `Unique<T>`.
 333     ///
 334     /// After calling this function, the caller is responsible for the
 335     /// memory previously managed by the `Box`. In particular, the
 336     /// caller should properly destroy `T` and release the memory. The
 337     /// proper way to do so is to either convert the `Unique<T>` pointer:
 338     ///
 339     /// - Into a `Box` with the [`Box::from_unique`] function.
 340     ///
 341     /// - Into a raw pointer and back into a `Box` with the [`Box::from_raw`]
 342     ///   function.
 343     ///
 344     /// Note: this is an associated function, which means that you have
 345     /// to call it as `Box::into_unique(b)` instead of `b.into_unique()`. This
 346     /// is so that there is no conflict with a method on the inner type.
 347     ///
 348     /// [`Box::from_unique`]: struct.Box.html#method.from_unique
 349     /// [`Box::from_raw`]: struct.Box.html#method.from_raw
 350     ///
 351     /// # Examples
 352     ///
 353     /// ```
 354     /// #![feature(unique)]
 355     ///
 356     /// fn main() {
 357     ///     let x = Box::new(5);
 358     ///     let ptr = Box::into_unique(x);
 359     /// }
 360     /// ```
 361     #[unstable(feature = "unique", reason = "needs an RFC to flesh out design",
 362                issue = "27730")]
 363     #[inline]
 364     pub fn into_unique(b: Box<T>) -> Unique<T> {
 365         unsafe { mem::transmute(b) }
 366     }
 367 }
 368
 369 #[stable(feature = "rust1", since = "1.0.0")]
 370 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
 371     fn drop(&mut self) {
 372         // FIXME: Do nothing, drop is currently performed by compiler.
 373     }
 374 }
 375
 376 #[stable(feature = "rust1", since = "1.0.0")]
 377 impl<T: Default> Default for Box<T> {
 378     /// Creates a `Box<T>`, with the `Default` value for T.
 379     fn default() -> Box<T> {
 380         box Default::default()
 381     }
 382 }
 383
 384 #[stable(feature = "rust1", since = "1.0.0")]
 385 impl<T> Default for Box<[T]> {
 386     fn default() -> Box<[T]> {
 387         Box::<[T; 0]>::new([])
 388     }
 389 }
 390
 391 #[stable(feature = "default_box_extra", since = "1.17.0")]
 392 impl Default for Box<str> {
 393     fn default() -> Box<str> {
 394         unsafe { from_boxed_utf8_unchecked(Default::default()) }
 395     }
 396 }
 397
 398 #[stable(feature = "rust1", since = "1.0.0")]
 399 impl<T: Clone> Clone for Box<T> {
 400     /// Returns a new box with a `clone()` of this box's contents.
 401     ///
 402     /// # Examples
 403     ///
 404     /// ```
 405     /// let x = Box::new(5);
 406     /// let y = x.clone();
 407     /// ```
 408     #[rustfmt_skip]
 409     #[inline]
 410     fn clone(&self) -> Box<T> {
 411         box { (**self).clone() }
 412     }
 413     /// Copies `source`'s contents into `self` without creating a new allocation.
 414     ///
 415     /// # Examples
 416     ///
 417     /// ```
 418     /// let x = Box::new(5);
 419     /// let mut y = Box::new(10);
 420     ///
 421     /// y.clone_from(&x);
 422     ///
 423     /// assert_eq!(*y, 5);
 424     /// ```
 425     #[inline]
 426     fn clone_from(&mut self, source: &Box<T>) {
 427         (**self).clone_from(&(**source));
 428     }
 429 }
 430
 431
 432 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 433 impl Clone for Box<str> {
 434     fn clone(&self) -> Self {
 435         let len = self.len();
 436         let buf = RawVec::with_capacity(len);
 437         unsafe {
 438             ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
 439             from_boxed_utf8_unchecked(buf.into_box())
 440         }
 441     }
 442 }
 443
 444 #[stable(feature = "rust1", since = "1.0.0")]
 445 impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
 446     #[inline]
 447     fn eq(&self, other: &Box<T>) -> bool {
 448         PartialEq::eq(&**self, &**other)
 449     }
 450     #[inline]
 451     fn ne(&self, other: &Box<T>) -> bool {
 452         PartialEq::ne(&**self, &**other)
 453     }
 454 }
 455 #[stable(feature = "rust1", since = "1.0.0")]
 456 impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
 457     #[inline]
 458     fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
 459         PartialOrd::partial_cmp(&**self, &**other)
 460     }
 461     #[inline]
 462     fn lt(&self, other: &Box<T>) -> bool {
 463         PartialOrd::lt(&**self, &**other)
 464     }
 465     #[inline]
 466     fn le(&self, other: &Box<T>) -> bool {
 467         PartialOrd::le(&**self, &**other)
 468     }
 469     #[inline]
 470     fn ge(&self, other: &Box<T>) -> bool {
 471         PartialOrd::ge(&**self, &**other)
 472     }
 473     #[inline]
 474     fn gt(&self, other: &Box<T>) -> bool {
 475         PartialOrd::gt(&**self, &**other)
 476     }
 477 }
 478 #[stable(feature = "rust1", since = "1.0.0")]
 479 impl<T: ?Sized + Ord> Ord for Box<T> {
 480     #[inline]
 481     fn cmp(&self, other: &Box<T>) -> Ordering {
 482         Ord::cmp(&**self, &**other)
 483     }
 484 }
 485 #[stable(feature = "rust1", since = "1.0.0")]
 486 impl<T: ?Sized + Eq> Eq for Box<T> {}
 487
 488 #[stable(feature = "rust1", since = "1.0.0")]
 489 impl<T: ?Sized + Hash> Hash for Box<T> {
 490     fn hash<H: hash::Hasher>(&self, state: &mut H) {
 491         (**self).hash(state);
 492     }
 493 }
 494
 495 #[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
 496 impl<T: ?Sized + Hasher> Hasher for Box<T> {
 497     fn finish(&self) -> u64 {
 498         (**self).finish()
 499     }
 500     fn write(&mut self, bytes: &[u8]) {
 501         (**self).write(bytes)
 502     }
 503     fn write_u8(&mut self, i: u8) {
 504         (**self).write_u8(i)
 505     }
 506     fn write_u16(&mut self, i: u16) {
 507         (**self).write_u16(i)
 508     }
 509     fn write_u32(&mut self, i: u32) {
 510         (**self).write_u32(i)
 511     }
 512     fn write_u64(&mut self, i: u64) {
 513         (**self).write_u64(i)
 514     }
 515     fn write_u128(&mut self, i: u128) {
 516         (**self).write_u128(i)
 517     }
 518     fn write_usize(&mut self, i: usize) {
 519         (**self).write_usize(i)
 520     }
 521     fn write_i8(&mut self, i: i8) {
 522         (**self).write_i8(i)
 523     }
 524     fn write_i16(&mut self, i: i16) {
 525         (**self).write_i16(i)
 526     }
 527     fn write_i32(&mut self, i: i32) {
 528         (**self).write_i32(i)
 529     }
 530     fn write_i64(&mut self, i: i64) {
 531         (**self).write_i64(i)
 532     }
 533     fn write_i128(&mut self, i: i128) {
 534         (**self).write_i128(i)
 535     }
 536     fn write_isize(&mut self, i: isize) {
 537         (**self).write_isize(i)
 538     }
 539 }
 540
 541 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
 542 impl<T> From<T> for Box<T> {
 543     fn from(t: T) -> Self {
 544         Box::new(t)
 545     }
 546 }
 547
 548 #[stable(feature = "box_from_slice", since = "1.17.0")]
 549 impl<'a, T: Copy> From<&'a [T]> for Box<[T]> {
 550     fn from(slice: &'a [T]) -> Box<[T]> {
 551         let mut boxed = unsafe { RawVec::with_capacity(slice.len()).into_box() };
 552         boxed.copy_from_slice(slice);
 553         boxed
 554     }
 555 }
 556
 557 #[stable(feature = "box_from_slice", since = "1.17.0")]
 558 impl<'a> From<&'a str> for Box<str> {
 559     fn from(s: &'a str) -> Box<str> {
 560         unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
 561     }
 562 }
 563
 564 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
 565 impl From<Box<str>> for Box<[u8]> {
 566     fn from(s: Box<str>) -> Self {
 567         unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
 568     }
 569 }
 570
 571 impl Box<Any> {
 572     #[inline]
 573     #[stable(feature = "rust1", since = "1.0.0")]
 574     /// Attempt to downcast the box to a concrete type.
 575     ///
 576     /// # Examples
 577     ///
 578     /// ```
 579     /// use std::any::Any;
 580     ///
 581     /// fn print_if_string(value: Box<Any>) {
 582     ///     if let Ok(string) = value.downcast::<String>() {
 583     ///         println!("String ({}): {}", string.len(), string);
 584     ///     }
 585     /// }
 586     ///
 587     /// fn main() {
 588     ///     let my_string = "Hello World".to_string();
 589     ///     print_if_string(Box::new(my_string));
 590     ///     print_if_string(Box::new(0i8));
 591     /// }
 592     /// ```
 593     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
 594         if self.is::<T>() {
 595             unsafe {
 596                 let raw: *mut Any = Box::into_raw(self);
 597                 Ok(Box::from_raw(raw as *mut T))
 598             }
 599         } else {
 600             Err(self)
 601         }
 602     }
 603 }
 604
 605 impl Box<Any + Send> {
 606     #[inline]
 607     #[stable(feature = "rust1", since = "1.0.0")]
 608     /// Attempt to downcast the box to a concrete type.
 609     ///
 610     /// # Examples
 611     ///
 612     /// ```
 613     /// use std::any::Any;
 614     ///
 615     /// fn print_if_string(value: Box<Any + Send>) {
 616     ///     if let Ok(string) = value.downcast::<String>() {
 617     ///         println!("String ({}): {}", string.len(), string);
 618     ///     }
 619     /// }
 620     ///
 621     /// fn main() {
 622     ///     let my_string = "Hello World".to_string();
 623     ///     print_if_string(Box::new(my_string));
 624     ///     print_if_string(Box::new(0i8));
 625     /// }
 626     /// ```
 627     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
 628         <Box<Any>>::downcast(self).map_err(|s| unsafe {
 629             // reapply the Send marker
 630             mem::transmute::<Box<Any>, Box<Any + Send>>(s)
 631         })
 632     }
 633 }
 634
 635 #[stable(feature = "rust1", since = "1.0.0")]
 636 impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
 637     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 638         fmt::Display::fmt(&**self, f)
 639     }
 640 }
 641
 642 #[stable(feature = "rust1", since = "1.0.0")]
 643 impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
 644     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 645         fmt::Debug::fmt(&**self, f)
 646     }
 647 }
 648
 649 #[stable(feature = "rust1", since = "1.0.0")]
 650 impl<T: ?Sized> fmt::Pointer for Box<T> {
 651     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 652         // It's not possible to extract the inner Uniq directly from the Box,
 653         // instead we cast it to a *const which aliases the Unique
 654         let ptr: *const T = &**self;
 655         fmt::Pointer::fmt(&ptr, f)
 656     }
 657 }
 658
 659 #[stable(feature = "rust1", since = "1.0.0")]
 660 impl<T: ?Sized> Deref for Box<T> {
 661     type Target = T;
 662
 663     fn deref(&self) -> &T {
 664         &**self
 665     }
 666 }
 667
 668 #[stable(feature = "rust1", since = "1.0.0")]
 669 impl<T: ?Sized> DerefMut for Box<T> {
 670     fn deref_mut(&mut self) -> &mut T {
 671         &mut **self
 672     }
 673 }
 674
 675 #[stable(feature = "rust1", since = "1.0.0")]
 676 impl<I: Iterator + ?Sized> Iterator for Box<I> {
 677     type Item = I::Item;
 678     fn next(&mut self) -> Option<I::Item> {
 679         (**self).next()
 680     }
 681     fn size_hint(&self) -> (usize, Option<usize>) {
 682         (**self).size_hint()
 683     }
 684     fn nth(&mut self, n: usize) -> Option<I::Item> {
 685         (**self).nth(n)
 686     }
 687 }
 688 #[stable(feature = "rust1", since = "1.0.0")]
 689 impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
 690     fn next_back(&mut self) -> Option<I::Item> {
 691         (**self).next_back()
 692     }
 693 }
 694 #[stable(feature = "rust1", since = "1.0.0")]
 695 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
 696     fn len(&self) -> usize {
 697         (**self).len()
 698     }
 699     fn is_empty(&self) -> bool {
 700         (**self).is_empty()
 701     }
 702 }
 703
 704 #[unstable(feature = "fused", issue = "35602")]
 705 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
 706
 707
 708 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
 709 /// closure objects. The idea is that where one would normally store a
 710 /// `Box<FnOnce()>` in a data structure, you should use
 711 /// `Box<FnBox()>`. The two traits behave essentially the same, except
 712 /// that a `FnBox` closure can only be called if it is boxed. (Note
 713 /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
 714 /// closures become directly usable.)
 715 ///
 716 /// # Examples
 717 ///
 718 /// Here is a snippet of code which creates a hashmap full of boxed
 719 /// once closures and then removes them one by one, calling each
 720 /// closure as it is removed. Note that the type of the closures
 721 /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
 722 /// -> i32>`.
 723 ///
 724 /// ```
 725 /// #![feature(fnbox)]
 726 ///
 727 /// use std::boxed::FnBox;
 728 /// use std::collections::HashMap;
 729 ///
 730 /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
 731 ///     let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
 732 ///     map.insert(1, Box::new(|| 22));
 733 ///     map.insert(2, Box::new(|| 44));
 734 ///     map
 735 /// }
 736 ///
 737 /// fn main() {
 738 ///     let mut map = make_map();
 739 ///     for i in &[1, 2] {
 740 ///         let f = map.remove(&i).unwrap();
 741 ///         assert_eq!(f(), i * 22);
 742 ///     }
 743 /// }
 744 /// ```
 745 #[rustc_paren_sugar]
 746 #[unstable(feature = "fnbox",
 747            reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
 748 pub trait FnBox<A> {
 749     type Output;
 750
 751     fn call_box(self: Box<Self>, args: A) -> Self::Output;
 752 }
 753
 754 #[unstable(feature = "fnbox",
 755            reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
 756 impl<A, F> FnBox<A> for F
 757     where F: FnOnce<A>
 758 {
 759     type Output = F::Output;
 760
 761     fn call_box(self: Box<F>, args: A) -> F::Output {
 762         self.call_once(args)
 763     }
 764 }
 765
 766 #[unstable(feature = "fnbox",
 767            reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
 768 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a> {
 769     type Output = R;
 770
 771     extern "rust-call" fn call_once(self, args: A) -> R {
 772         self.call_box(args)
 773     }
 774 }
 775
 776 #[unstable(feature = "fnbox",
 777            reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
 778 impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + Send + 'a> {
 779     type Output = R;
 780
 781     extern "rust-call" fn call_once(self, args: A) -> R {
 782         self.call_box(args)
 783     }
 784 }
 785
 786 #[unstable(feature = "coerce_unsized", issue = "27732")]
 787 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
 788
 789 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 790 impl<T: Clone> Clone for Box<[T]> {
 791     fn clone(&self) -> Self {
 792         let mut new = BoxBuilder {
 793             data: RawVec::with_capacity(self.len()),
 794             len: 0,
 795         };
 796
 797         let mut target = new.data.ptr();
 798
 799         for item in self.iter() {
 800             unsafe {
 801                 ptr::write(target, item.clone());
 802                 target = target.offset(1);
 803             };
 804
 805             new.len += 1;
 806         }
 807
 808         return unsafe { new.into_box() };
 809
 810         // Helper type for responding to panics correctly.
 811         struct BoxBuilder<T> {
 812             data: RawVec<T>,
 813             len: usize,
 814         }
 815
 816         impl<T> BoxBuilder<T> {
 817             unsafe fn into_box(self) -> Box<[T]> {
 818                 let raw = ptr::read(&self.data);
 819                 mem::forget(self);
 820                 raw.into_box()
 821             }
 822         }
 823
 824         impl<T> Drop for BoxBuilder<T> {
 825             fn drop(&mut self) {
 826                 let mut data = self.data.ptr();
 827                 let max = unsafe { data.offset(self.len as isize) };
 828
 829                 while data != max {
 830                     unsafe {
 831                         ptr::read(data);
 832                         data = data.offset(1);
 833                     }
 834                 }
 835             }
 836         }
 837     }
 838 }
 839
 840 #[stable(feature = "box_borrow", since = "1.1.0")]
 841 impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
 842     fn borrow(&self) -> &T {
 843         &**self
 844     }
 845 }
 846
 847 #[stable(feature = "box_borrow", since = "1.1.0")]
 848 impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
 849     fn borrow_mut(&mut self) -> &mut T {
 850         &mut **self
 851     }
 852 }
 853
 854 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
 855 impl<T: ?Sized> AsRef<T> for Box<T> {
 856     fn as_ref(&self) -> &T {
 857         &**self
 858     }
 859 }
 860
 861 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
 862 impl<T: ?Sized> AsMut<T> for Box<T> {
 863     fn as_mut(&mut self) -> &mut T {
 864         &mut **self
 865     }
 866 }
 867
 868 #[unstable(feature = "generator_trait", issue = "43122")]
 869 impl<T> Generator for Box<T>
 870     where T: Generator + ?Sized
 871 {
 872     type Yield = T::Yield;
 873     type Return = T::Return;
 874     fn resume(&mut self) -> GeneratorState<Self::Yield, Self::Return> {
 875         (**self).resume()
 876     }
 877 }