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alloc/
boxed.rs

1//! The `Box<T>` type for heap allocation.
2//!
3//! [`Box<T>`], casually referred to as a 'box', provides the simplest form of
4//! heap allocation in Rust. Boxes provide ownership for this allocation, and
5//! drop their contents when they go out of scope. Boxes also ensure that they
6//! never allocate more than `isize::MAX` bytes.
7//!
8//! # Examples
9//!
10//! Move a value from the stack to the heap by creating a [`Box`]:
11//!
12//! ```
13//! let val: u8 = 5;
14//! let boxed: Box<u8> = Box::new(val);
15//! ```
16//!
17//! Move a value from a [`Box`] back to the stack by [dereferencing]:
18//!
19//! ```
20//! let boxed: Box<u8> = Box::new(5);
21//! let val: u8 = *boxed;
22//! ```
23//!
24//! Creating a recursive data structure:
25//!
26//! ```
27//! # #[allow(dead_code)]
28//! #[derive(Debug)]
29//! enum List<T> {
30//!     Cons(T, Box<List<T>>),
31//!     Nil,
32//! }
33//!
34//! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
35//! println!("{list:?}");
36//! ```
37//!
38//! This will print `Cons(1, Cons(2, Nil))`.
39//!
40//! Recursive structures must be boxed, because if the definition of `Cons`
41//! looked like this:
42//!
43//! ```compile_fail,E0072
44//! # enum List<T> {
45//! Cons(T, List<T>),
46//! # }
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<T>`], which has a defined size, we know how
52//! big `Cons` needs to be.
53//!
54//! # Memory layout
55//!
56//! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for its allocation. It is
57//! valid to convert both ways between a [`Box`] and a raw pointer allocated with the [`Global`]
58//! allocator, given that the [`Layout`] used with the allocator is correct for the type and the raw
59//! pointer points to a valid value of the right type. More precisely, a `value: *mut T` that has
60//! been allocated with the [`Global`] allocator with `Layout::for_value(&*value)` may be converted
61//! into a box using [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut T`
62//! obtained from [`Box::<T>::into_raw`] may be deallocated using the [`Global`] allocator with
63//! [`Layout::for_value(&*value)`].
64//!
65//! For zero-sized values, the `Box` pointer has to be non-null and sufficiently aligned. The
66//! recommended way to build a Box to a ZST if `Box::new` cannot be used is to use
67//! [`ptr::NonNull::dangling`].
68//!
69//! On top of these basic layout requirements, a `Box<T>` must point to a valid value of `T`.
70//!
71//! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented
72//! as a single pointer and is also ABI-compatible with C pointers
73//! (i.e. the C type `T*`). This means that if you have extern "C"
74//! Rust functions that will be called from C, you can define those
75//! Rust functions using `Box<T>` types, and use `T*` as corresponding
76//! type on the C side. As an example, consider this C header which
77//! declares functions that create and destroy some kind of `Foo`
78//! value:
79//!
80//! ```c
81//! /* C header */
82//!
83//! /* Returns ownership to the caller */
84//! struct Foo* foo_new(void);
85//!
86//! /* Takes ownership from the caller; no-op when invoked with null */
87//! void foo_delete(struct Foo*);
88//! ```
89//!
90//! These two functions might be implemented in Rust as follows. Here, the
91//! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
92//! the ownership constraints. Note also that the nullable argument to
93//! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
94//! cannot be null.
95//!
96//! ```
97//! #[repr(C)]
98//! pub struct Foo;
99//!
100//! #[unsafe(no_mangle)]
101//! pub extern "C" fn foo_new() -> Box<Foo> {
102//!     Box::new(Foo)
103//! }
104//!
105//! #[unsafe(no_mangle)]
106//! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
107//! ```
108//!
109//! Even though `Box<T>` has the same representation and C ABI as a C pointer,
110//! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
111//! and expect things to work. `Box<T>` values will always be fully aligned,
112//! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
113//! free the value with the global allocator. In general, the best practice
114//! is to only use `Box<T>` for pointers that originated from the global
115//! allocator.
116//!
117//! **Important.** At least at present, you should avoid using
118//! `Box<T>` types for functions that are defined in C but invoked
119//! from Rust. In those cases, you should directly mirror the C types
120//! as closely as possible. Using types like `Box<T>` where the C
121//! definition is just using `T*` can lead to undefined behavior, as
122//! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
123//!
124//! # Considerations for unsafe code
125//!
126//! **Warning: This section is not normative and is subject to change, possibly
127//! being relaxed in the future! It is a simplified summary of the rules
128//! currently implemented in the compiler.**
129//!
130//! The aliasing rules for `Box<T>` are the same as for `&mut T`. `Box<T>`
131//! asserts uniqueness over its content. Using raw pointers derived from a box
132//! after that box has been mutated through, moved or borrowed as `&mut T`
133//! is not allowed. For more guidance on working with box from unsafe code, see
134//! [rust-lang/unsafe-code-guidelines#326][ucg#326].
135//!
136//! # Editions
137//!
138//! A special case exists for the implementation of `IntoIterator` for arrays on the Rust 2021
139//! edition, as documented [here][array]. Unfortunately, it was later found that a similar
140//! workaround should be added for boxed slices, and this was applied in the 2024 edition.
141//!
142//! Specifically, `IntoIterator` is implemented for `Box<[T]>` on all editions, but specific calls
143//! to `into_iter()` for boxed slices will defer to the slice implementation on editions before
144//! 2024:
145//!
146//! ```rust,edition2021
147//! // Rust 2015, 2018, and 2021:
148//!
149//! # #![allow(boxed_slice_into_iter)] // override our `deny(warnings)`
150//! let boxed_slice: Box<[i32]> = vec![0; 3].into_boxed_slice();
151//!
152//! // This creates a slice iterator, producing references to each value.
153//! for item in boxed_slice.into_iter().enumerate() {
154//!     let (i, x): (usize, &i32) = item;
155//!     println!("boxed_slice[{i}] = {x}");
156//! }
157//!
158//! // The `boxed_slice_into_iter` lint suggests this change for future compatibility:
159//! for item in boxed_slice.iter().enumerate() {
160//!     let (i, x): (usize, &i32) = item;
161//!     println!("boxed_slice[{i}] = {x}");
162//! }
163//!
164//! // You can explicitly iterate a boxed slice by value using `IntoIterator::into_iter`
165//! for item in IntoIterator::into_iter(boxed_slice).enumerate() {
166//!     let (i, x): (usize, i32) = item;
167//!     println!("boxed_slice[{i}] = {x}");
168//! }
169//! ```
170//!
171//! Similar to the array implementation, this may be modified in the future to remove this override,
172//! and it's best to avoid relying on this edition-dependent behavior if you wish to preserve
173//! compatibility with future versions of the compiler.
174//!
175//! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
176//! [ucg#326]: https://github.com/rust-lang/unsafe-code-guidelines/issues/326
177//! [dereferencing]: core::ops::Deref
178//! [`Box::<T>::from_raw(value)`]: Box::from_raw
179//! [`Global`]: crate::alloc::Global
180//! [`Layout`]: crate::alloc::Layout
181//! [`Layout::for_value(&*value)`]: crate::alloc::Layout::for_value
182//! [valid]: ptr#safety
183
184#![stable(feature = "rust1", since = "1.0.0")]
185
186use core::borrow::{Borrow, BorrowMut};
187#[cfg(not(no_global_oom_handling))]
188use core::clone::CloneToUninit;
189use core::cmp::Ordering;
190use core::error::{self, Error};
191use core::fmt;
192use core::future::Future;
193use core::hash::{Hash, Hasher};
194use core::marker::{Tuple, Unsize};
195use core::mem::{self, SizedTypeProperties};
196use core::ops::{
197    AsyncFn, AsyncFnMut, AsyncFnOnce, CoerceUnsized, Coroutine, CoroutineState, Deref, DerefMut,
198    DerefPure, DispatchFromDyn, LegacyReceiver,
199};
200use core::pin::{Pin, PinCoerceUnsized};
201use core::ptr::{self, NonNull, Unique};
202use core::task::{Context, Poll};
203
204#[cfg(not(no_global_oom_handling))]
205use crate::alloc::handle_alloc_error;
206use crate::alloc::{AllocError, Allocator, Global, Layout};
207use crate::raw_vec::RawVec;
208#[cfg(not(no_global_oom_handling))]
209use crate::str::from_boxed_utf8_unchecked;
210
211/// Conversion related impls for `Box<_>` (`From`, `downcast`, etc)
212mod convert;
213/// Iterator related impls for `Box<_>`.
214mod iter;
215/// [`ThinBox`] implementation.
216mod thin;
217
218#[unstable(feature = "thin_box", issue = "92791")]
219pub use thin::ThinBox;
220
221/// A pointer type that uniquely owns a heap allocation of type `T`.
222///
223/// See the [module-level documentation](../../std/boxed/index.html) for more.
224#[lang = "owned_box"]
225#[fundamental]
226#[stable(feature = "rust1", since = "1.0.0")]
227#[rustc_insignificant_dtor]
228#[doc(search_unbox)]
229// The declaration of the `Box` struct must be kept in sync with the
230// compiler or ICEs will happen.
231pub struct Box<
232    T: ?Sized,
233    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
234>(Unique<T>, A);
235
236/// Constructs a `Box<T>` by calling the `exchange_malloc` lang item and moving the argument into
237/// the newly allocated memory. This is an intrinsic to avoid unnecessary copies.
238///
239/// This is the surface syntax for `box <expr>` expressions.
240#[rustc_intrinsic]
241#[unstable(feature = "liballoc_internals", issue = "none")]
242pub fn box_new<T>(x: T) -> Box<T>;
243
244impl<T> Box<T> {
245    /// Allocates memory on the heap and then places `x` into it.
246    ///
247    /// This doesn't actually allocate if `T` is zero-sized.
248    ///
249    /// # Examples
250    ///
251    /// ```
252    /// let five = Box::new(5);
253    /// ```
254    #[cfg(not(no_global_oom_handling))]
255    #[inline(always)]
256    #[stable(feature = "rust1", since = "1.0.0")]
257    #[must_use]
258    #[rustc_diagnostic_item = "box_new"]
259    #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
260    pub fn new(x: T) -> Self {
261        return box_new(x);
262    }
263
264    /// Constructs a new box with uninitialized contents.
265    ///
266    /// # Examples
267    ///
268    /// ```
269    /// let mut five = Box::<u32>::new_uninit();
270    /// // Deferred initialization:
271    /// five.write(5);
272    /// let five = unsafe { five.assume_init() };
273    ///
274    /// assert_eq!(*five, 5)
275    /// ```
276    #[cfg(not(no_global_oom_handling))]
277    #[stable(feature = "new_uninit", since = "1.82.0")]
278    #[must_use]
279    #[inline]
280    pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
281        Self::new_uninit_in(Global)
282    }
283
284    /// Constructs a new `Box` with uninitialized contents, with the memory
285    /// being filled with `0` bytes.
286    ///
287    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
288    /// of this method.
289    ///
290    /// # Examples
291    ///
292    /// ```
293    /// let zero = Box::<u32>::new_zeroed();
294    /// let zero = unsafe { zero.assume_init() };
295    ///
296    /// assert_eq!(*zero, 0)
297    /// ```
298    ///
299    /// [zeroed]: mem::MaybeUninit::zeroed
300    #[cfg(not(no_global_oom_handling))]
301    #[inline]
302    #[stable(feature = "new_zeroed_alloc", since = "CURRENT_RUSTC_VERSION")]
303    #[must_use]
304    pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
305        Self::new_zeroed_in(Global)
306    }
307
308    /// Constructs a new `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
309    /// `x` will be pinned in memory and unable to be moved.
310    ///
311    /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin(x)`
312    /// does the same as <code>[Box::into_pin]\([Box::new]\(x))</code>. Consider using
313    /// [`into_pin`](Box::into_pin) if you already have a `Box<T>`, or if you want to
314    /// construct a (pinned) `Box` in a different way than with [`Box::new`].
315    #[cfg(not(no_global_oom_handling))]
316    #[stable(feature = "pin", since = "1.33.0")]
317    #[must_use]
318    #[inline(always)]
319    pub fn pin(x: T) -> Pin<Box<T>> {
320        Box::new(x).into()
321    }
322
323    /// Allocates memory on the heap then places `x` into it,
324    /// returning an error if the allocation fails
325    ///
326    /// This doesn't actually allocate if `T` is zero-sized.
327    ///
328    /// # Examples
329    ///
330    /// ```
331    /// #![feature(allocator_api)]
332    ///
333    /// let five = Box::try_new(5)?;
334    /// # Ok::<(), std::alloc::AllocError>(())
335    /// ```
336    #[unstable(feature = "allocator_api", issue = "32838")]
337    #[inline]
338    pub fn try_new(x: T) -> Result<Self, AllocError> {
339        Self::try_new_in(x, Global)
340    }
341
342    /// Constructs a new box with uninitialized contents on the heap,
343    /// returning an error if the allocation fails
344    ///
345    /// # Examples
346    ///
347    /// ```
348    /// #![feature(allocator_api)]
349    ///
350    /// let mut five = Box::<u32>::try_new_uninit()?;
351    /// // Deferred initialization:
352    /// five.write(5);
353    /// let five = unsafe { five.assume_init() };
354    ///
355    /// assert_eq!(*five, 5);
356    /// # Ok::<(), std::alloc::AllocError>(())
357    /// ```
358    #[unstable(feature = "allocator_api", issue = "32838")]
359    #[inline]
360    pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
361        Box::try_new_uninit_in(Global)
362    }
363
364    /// Constructs a new `Box` with uninitialized contents, with the memory
365    /// being filled with `0` bytes on the heap
366    ///
367    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
368    /// of this method.
369    ///
370    /// # Examples
371    ///
372    /// ```
373    /// #![feature(allocator_api)]
374    ///
375    /// let zero = Box::<u32>::try_new_zeroed()?;
376    /// let zero = unsafe { zero.assume_init() };
377    ///
378    /// assert_eq!(*zero, 0);
379    /// # Ok::<(), std::alloc::AllocError>(())
380    /// ```
381    ///
382    /// [zeroed]: mem::MaybeUninit::zeroed
383    #[unstable(feature = "allocator_api", issue = "32838")]
384    #[inline]
385    pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
386        Box::try_new_zeroed_in(Global)
387    }
388}
389
390impl<T, A: Allocator> Box<T, A> {
391    /// Allocates memory in the given allocator then places `x` into it.
392    ///
393    /// This doesn't actually allocate if `T` is zero-sized.
394    ///
395    /// # Examples
396    ///
397    /// ```
398    /// #![feature(allocator_api)]
399    ///
400    /// use std::alloc::System;
401    ///
402    /// let five = Box::new_in(5, System);
403    /// ```
404    #[cfg(not(no_global_oom_handling))]
405    #[unstable(feature = "allocator_api", issue = "32838")]
406    #[must_use]
407    #[inline]
408    pub fn new_in(x: T, alloc: A) -> Self
409    where
410        A: Allocator,
411    {
412        let mut boxed = Self::new_uninit_in(alloc);
413        boxed.write(x);
414        unsafe { boxed.assume_init() }
415    }
416
417    /// Allocates memory in the given allocator then places `x` into it,
418    /// returning an error if the allocation fails
419    ///
420    /// This doesn't actually allocate if `T` is zero-sized.
421    ///
422    /// # Examples
423    ///
424    /// ```
425    /// #![feature(allocator_api)]
426    ///
427    /// use std::alloc::System;
428    ///
429    /// let five = Box::try_new_in(5, System)?;
430    /// # Ok::<(), std::alloc::AllocError>(())
431    /// ```
432    #[unstable(feature = "allocator_api", issue = "32838")]
433    #[inline]
434    pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
435    where
436        A: Allocator,
437    {
438        let mut boxed = Self::try_new_uninit_in(alloc)?;
439        boxed.write(x);
440        unsafe { Ok(boxed.assume_init()) }
441    }
442
443    /// Constructs a new box with uninitialized contents in the provided allocator.
444    ///
445    /// # Examples
446    ///
447    /// ```
448    /// #![feature(allocator_api)]
449    ///
450    /// use std::alloc::System;
451    ///
452    /// let mut five = Box::<u32, _>::new_uninit_in(System);
453    /// // Deferred initialization:
454    /// five.write(5);
455    /// let five = unsafe { five.assume_init() };
456    ///
457    /// assert_eq!(*five, 5)
458    /// ```
459    #[unstable(feature = "allocator_api", issue = "32838")]
460    #[cfg(not(no_global_oom_handling))]
461    #[must_use]
462    pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
463    where
464        A: Allocator,
465    {
466        let layout = Layout::new::<mem::MaybeUninit<T>>();
467        // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
468        // That would make code size bigger.
469        match Box::try_new_uninit_in(alloc) {
470            Ok(m) => m,
471            Err(_) => handle_alloc_error(layout),
472        }
473    }
474
475    /// Constructs a new box with uninitialized contents in the provided allocator,
476    /// returning an error if the allocation fails
477    ///
478    /// # Examples
479    ///
480    /// ```
481    /// #![feature(allocator_api)]
482    ///
483    /// use std::alloc::System;
484    ///
485    /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
486    /// // Deferred initialization:
487    /// five.write(5);
488    /// let five = unsafe { five.assume_init() };
489    ///
490    /// assert_eq!(*five, 5);
491    /// # Ok::<(), std::alloc::AllocError>(())
492    /// ```
493    #[unstable(feature = "allocator_api", issue = "32838")]
494    pub fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
495    where
496        A: Allocator,
497    {
498        let ptr = if T::IS_ZST {
499            NonNull::dangling()
500        } else {
501            let layout = Layout::new::<mem::MaybeUninit<T>>();
502            alloc.allocate(layout)?.cast()
503        };
504        unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
505    }
506
507    /// Constructs a new `Box` with uninitialized contents, with the memory
508    /// being filled with `0` bytes in the provided allocator.
509    ///
510    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
511    /// of this method.
512    ///
513    /// # Examples
514    ///
515    /// ```
516    /// #![feature(allocator_api)]
517    ///
518    /// use std::alloc::System;
519    ///
520    /// let zero = Box::<u32, _>::new_zeroed_in(System);
521    /// let zero = unsafe { zero.assume_init() };
522    ///
523    /// assert_eq!(*zero, 0)
524    /// ```
525    ///
526    /// [zeroed]: mem::MaybeUninit::zeroed
527    #[unstable(feature = "allocator_api", issue = "32838")]
528    #[cfg(not(no_global_oom_handling))]
529    #[must_use]
530    pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
531    where
532        A: Allocator,
533    {
534        let layout = Layout::new::<mem::MaybeUninit<T>>();
535        // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
536        // That would make code size bigger.
537        match Box::try_new_zeroed_in(alloc) {
538            Ok(m) => m,
539            Err(_) => handle_alloc_error(layout),
540        }
541    }
542
543    /// Constructs a new `Box` with uninitialized contents, with the memory
544    /// being filled with `0` bytes in the provided allocator,
545    /// returning an error if the allocation fails,
546    ///
547    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
548    /// of this method.
549    ///
550    /// # Examples
551    ///
552    /// ```
553    /// #![feature(allocator_api)]
554    ///
555    /// use std::alloc::System;
556    ///
557    /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
558    /// let zero = unsafe { zero.assume_init() };
559    ///
560    /// assert_eq!(*zero, 0);
561    /// # Ok::<(), std::alloc::AllocError>(())
562    /// ```
563    ///
564    /// [zeroed]: mem::MaybeUninit::zeroed
565    #[unstable(feature = "allocator_api", issue = "32838")]
566    pub fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
567    where
568        A: Allocator,
569    {
570        let ptr = if T::IS_ZST {
571            NonNull::dangling()
572        } else {
573            let layout = Layout::new::<mem::MaybeUninit<T>>();
574            alloc.allocate_zeroed(layout)?.cast()
575        };
576        unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
577    }
578
579    /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then
580    /// `x` will be pinned in memory and unable to be moved.
581    ///
582    /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin_in(x, alloc)`
583    /// does the same as <code>[Box::into_pin]\([Box::new_in]\(x, alloc))</code>. Consider using
584    /// [`into_pin`](Box::into_pin) if you already have a `Box<T, A>`, or if you want to
585    /// construct a (pinned) `Box` in a different way than with [`Box::new_in`].
586    #[cfg(not(no_global_oom_handling))]
587    #[unstable(feature = "allocator_api", issue = "32838")]
588    #[must_use]
589    #[inline(always)]
590    pub fn pin_in(x: T, alloc: A) -> Pin<Self>
591    where
592        A: 'static + Allocator,
593    {
594        Self::into_pin(Self::new_in(x, alloc))
595    }
596
597    /// Converts a `Box<T>` into a `Box<[T]>`
598    ///
599    /// This conversion does not allocate on the heap and happens in place.
600    #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
601    pub fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
602        let (raw, alloc) = Box::into_raw_with_allocator(boxed);
603        unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
604    }
605
606    /// Consumes the `Box`, returning the wrapped value.
607    ///
608    /// # Examples
609    ///
610    /// ```
611    /// #![feature(box_into_inner)]
612    ///
613    /// let c = Box::new(5);
614    ///
615    /// assert_eq!(Box::into_inner(c), 5);
616    /// ```
617    #[unstable(feature = "box_into_inner", issue = "80437")]
618    #[inline]
619    pub fn into_inner(boxed: Self) -> T {
620        *boxed
621    }
622}
623
624impl<T> Box<[T]> {
625    /// Constructs a new boxed slice with uninitialized contents.
626    ///
627    /// # Examples
628    ///
629    /// ```
630    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
631    /// // Deferred initialization:
632    /// values[0].write(1);
633    /// values[1].write(2);
634    /// values[2].write(3);
635    /// let values = unsafe { values.assume_init() };
636    ///
637    /// assert_eq!(*values, [1, 2, 3])
638    /// ```
639    #[cfg(not(no_global_oom_handling))]
640    #[stable(feature = "new_uninit", since = "1.82.0")]
641    #[must_use]
642    pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
643        unsafe { RawVec::with_capacity(len).into_box(len) }
644    }
645
646    /// Constructs a new boxed slice with uninitialized contents, with the memory
647    /// being filled with `0` bytes.
648    ///
649    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
650    /// of this method.
651    ///
652    /// # Examples
653    ///
654    /// ```
655    /// let values = Box::<[u32]>::new_zeroed_slice(3);
656    /// let values = unsafe { values.assume_init() };
657    ///
658    /// assert_eq!(*values, [0, 0, 0])
659    /// ```
660    ///
661    /// [zeroed]: mem::MaybeUninit::zeroed
662    #[cfg(not(no_global_oom_handling))]
663    #[stable(feature = "new_zeroed_alloc", since = "CURRENT_RUSTC_VERSION")]
664    #[must_use]
665    pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
666        unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
667    }
668
669    /// Constructs a new boxed slice with uninitialized contents. Returns an error if
670    /// the allocation fails.
671    ///
672    /// # Examples
673    ///
674    /// ```
675    /// #![feature(allocator_api)]
676    ///
677    /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
678    /// // Deferred initialization:
679    /// values[0].write(1);
680    /// values[1].write(2);
681    /// values[2].write(3);
682    /// let values = unsafe { values.assume_init() };
683    ///
684    /// assert_eq!(*values, [1, 2, 3]);
685    /// # Ok::<(), std::alloc::AllocError>(())
686    /// ```
687    #[unstable(feature = "allocator_api", issue = "32838")]
688    #[inline]
689    pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
690        let ptr = if T::IS_ZST || len == 0 {
691            NonNull::dangling()
692        } else {
693            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
694                Ok(l) => l,
695                Err(_) => return Err(AllocError),
696            };
697            Global.allocate(layout)?.cast()
698        };
699        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
700    }
701
702    /// Constructs a new boxed slice with uninitialized contents, with the memory
703    /// being filled with `0` bytes. Returns an error if the allocation fails.
704    ///
705    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
706    /// of this method.
707    ///
708    /// # Examples
709    ///
710    /// ```
711    /// #![feature(allocator_api)]
712    ///
713    /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
714    /// let values = unsafe { values.assume_init() };
715    ///
716    /// assert_eq!(*values, [0, 0, 0]);
717    /// # Ok::<(), std::alloc::AllocError>(())
718    /// ```
719    ///
720    /// [zeroed]: mem::MaybeUninit::zeroed
721    #[unstable(feature = "allocator_api", issue = "32838")]
722    #[inline]
723    pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
724        let ptr = if T::IS_ZST || len == 0 {
725            NonNull::dangling()
726        } else {
727            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
728                Ok(l) => l,
729                Err(_) => return Err(AllocError),
730            };
731            Global.allocate_zeroed(layout)?.cast()
732        };
733        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
734    }
735
736    /// Converts the boxed slice into a boxed array.
737    ///
738    /// This operation does not reallocate; the underlying array of the slice is simply reinterpreted as an array type.
739    ///
740    /// If `N` is not exactly equal to the length of `self`, then this method returns `None`.
741    #[unstable(feature = "slice_as_array", issue = "133508")]
742    #[inline]
743    #[must_use]
744    pub fn into_array<const N: usize>(self) -> Option<Box<[T; N]>> {
745        if self.len() == N {
746            let ptr = Self::into_raw(self) as *mut [T; N];
747
748            // SAFETY: The underlying array of a slice has the exact same layout as an actual array `[T; N]` if `N` is equal to the slice's length.
749            let me = unsafe { Box::from_raw(ptr) };
750            Some(me)
751        } else {
752            None
753        }
754    }
755}
756
757impl<T, A: Allocator> Box<[T], A> {
758    /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
759    ///
760    /// # Examples
761    ///
762    /// ```
763    /// #![feature(allocator_api)]
764    ///
765    /// use std::alloc::System;
766    ///
767    /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
768    /// // Deferred initialization:
769    /// values[0].write(1);
770    /// values[1].write(2);
771    /// values[2].write(3);
772    /// let values = unsafe { values.assume_init() };
773    ///
774    /// assert_eq!(*values, [1, 2, 3])
775    /// ```
776    #[cfg(not(no_global_oom_handling))]
777    #[unstable(feature = "allocator_api", issue = "32838")]
778    #[must_use]
779    pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
780        unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
781    }
782
783    /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
784    /// with the memory being filled with `0` bytes.
785    ///
786    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
787    /// of this method.
788    ///
789    /// # Examples
790    ///
791    /// ```
792    /// #![feature(allocator_api)]
793    ///
794    /// use std::alloc::System;
795    ///
796    /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
797    /// let values = unsafe { values.assume_init() };
798    ///
799    /// assert_eq!(*values, [0, 0, 0])
800    /// ```
801    ///
802    /// [zeroed]: mem::MaybeUninit::zeroed
803    #[cfg(not(no_global_oom_handling))]
804    #[unstable(feature = "allocator_api", issue = "32838")]
805    #[must_use]
806    pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
807        unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
808    }
809
810    /// Constructs a new boxed slice with uninitialized contents in the provided allocator. Returns an error if
811    /// the allocation fails.
812    ///
813    /// # Examples
814    ///
815    /// ```
816    /// #![feature(allocator_api)]
817    ///
818    /// use std::alloc::System;
819    ///
820    /// let mut values = Box::<[u32], _>::try_new_uninit_slice_in(3, System)?;
821    /// // Deferred initialization:
822    /// values[0].write(1);
823    /// values[1].write(2);
824    /// values[2].write(3);
825    /// let values = unsafe { values.assume_init() };
826    ///
827    /// assert_eq!(*values, [1, 2, 3]);
828    /// # Ok::<(), std::alloc::AllocError>(())
829    /// ```
830    #[unstable(feature = "allocator_api", issue = "32838")]
831    #[inline]
832    pub fn try_new_uninit_slice_in(
833        len: usize,
834        alloc: A,
835    ) -> Result<Box<[mem::MaybeUninit<T>], A>, AllocError> {
836        let ptr = if T::IS_ZST || len == 0 {
837            NonNull::dangling()
838        } else {
839            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
840                Ok(l) => l,
841                Err(_) => return Err(AllocError),
842            };
843            alloc.allocate(layout)?.cast()
844        };
845        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, alloc).into_box(len)) }
846    }
847
848    /// Constructs a new boxed slice with uninitialized contents in the provided allocator, with the memory
849    /// being filled with `0` bytes. Returns an error if the allocation fails.
850    ///
851    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
852    /// of this method.
853    ///
854    /// # Examples
855    ///
856    /// ```
857    /// #![feature(allocator_api)]
858    ///
859    /// use std::alloc::System;
860    ///
861    /// let values = Box::<[u32], _>::try_new_zeroed_slice_in(3, System)?;
862    /// let values = unsafe { values.assume_init() };
863    ///
864    /// assert_eq!(*values, [0, 0, 0]);
865    /// # Ok::<(), std::alloc::AllocError>(())
866    /// ```
867    ///
868    /// [zeroed]: mem::MaybeUninit::zeroed
869    #[unstable(feature = "allocator_api", issue = "32838")]
870    #[inline]
871    pub fn try_new_zeroed_slice_in(
872        len: usize,
873        alloc: A,
874    ) -> Result<Box<[mem::MaybeUninit<T>], A>, AllocError> {
875        let ptr = if T::IS_ZST || len == 0 {
876            NonNull::dangling()
877        } else {
878            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
879                Ok(l) => l,
880                Err(_) => return Err(AllocError),
881            };
882            alloc.allocate_zeroed(layout)?.cast()
883        };
884        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, alloc).into_box(len)) }
885    }
886}
887
888impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
889    /// Converts to `Box<T, A>`.
890    ///
891    /// # Safety
892    ///
893    /// As with [`MaybeUninit::assume_init`],
894    /// it is up to the caller to guarantee that the value
895    /// really is in an initialized state.
896    /// Calling this when the content is not yet fully initialized
897    /// causes immediate undefined behavior.
898    ///
899    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
900    ///
901    /// # Examples
902    ///
903    /// ```
904    /// let mut five = Box::<u32>::new_uninit();
905    /// // Deferred initialization:
906    /// five.write(5);
907    /// let five: Box<u32> = unsafe { five.assume_init() };
908    ///
909    /// assert_eq!(*five, 5)
910    /// ```
911    #[stable(feature = "new_uninit", since = "1.82.0")]
912    #[inline]
913    pub unsafe fn assume_init(self) -> Box<T, A> {
914        let (raw, alloc) = Box::into_raw_with_allocator(self);
915        unsafe { Box::from_raw_in(raw as *mut T, alloc) }
916    }
917
918    /// Writes the value and converts to `Box<T, A>`.
919    ///
920    /// This method converts the box similarly to [`Box::assume_init`] but
921    /// writes `value` into it before conversion thus guaranteeing safety.
922    /// In some scenarios use of this method may improve performance because
923    /// the compiler may be able to optimize copying from stack.
924    ///
925    /// # Examples
926    ///
927    /// ```
928    /// let big_box = Box::<[usize; 1024]>::new_uninit();
929    ///
930    /// let mut array = [0; 1024];
931    /// for (i, place) in array.iter_mut().enumerate() {
932    ///     *place = i;
933    /// }
934    ///
935    /// // The optimizer may be able to elide this copy, so previous code writes
936    /// // to heap directly.
937    /// let big_box = Box::write(big_box, array);
938    ///
939    /// for (i, x) in big_box.iter().enumerate() {
940    ///     assert_eq!(*x, i);
941    /// }
942    /// ```
943    #[stable(feature = "box_uninit_write", since = "1.87.0")]
944    #[inline]
945    pub fn write(mut boxed: Self, value: T) -> Box<T, A> {
946        unsafe {
947            (*boxed).write(value);
948            boxed.assume_init()
949        }
950    }
951}
952
953impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
954    /// Converts to `Box<[T], A>`.
955    ///
956    /// # Safety
957    ///
958    /// As with [`MaybeUninit::assume_init`],
959    /// it is up to the caller to guarantee that the values
960    /// really are in an initialized state.
961    /// Calling this when the content is not yet fully initialized
962    /// causes immediate undefined behavior.
963    ///
964    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
965    ///
966    /// # Examples
967    ///
968    /// ```
969    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
970    /// // Deferred initialization:
971    /// values[0].write(1);
972    /// values[1].write(2);
973    /// values[2].write(3);
974    /// let values = unsafe { values.assume_init() };
975    ///
976    /// assert_eq!(*values, [1, 2, 3])
977    /// ```
978    #[stable(feature = "new_uninit", since = "1.82.0")]
979    #[inline]
980    pub unsafe fn assume_init(self) -> Box<[T], A> {
981        let (raw, alloc) = Box::into_raw_with_allocator(self);
982        unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
983    }
984}
985
986impl<T: ?Sized> Box<T> {
987    /// Constructs a box from a raw pointer.
988    ///
989    /// After calling this function, the raw pointer is owned by the
990    /// resulting `Box`. Specifically, the `Box` destructor will call
991    /// the destructor of `T` and free the allocated memory. For this
992    /// to be safe, the memory must have been allocated in accordance
993    /// with the [memory layout] used by `Box` .
994    ///
995    /// # Safety
996    ///
997    /// This function is unsafe because improper use may lead to
998    /// memory problems. For example, a double-free may occur if the
999    /// function is called twice on the same raw pointer.
1000    ///
1001    /// The raw pointer must point to a block of memory allocated by the global allocator.
1002    ///
1003    /// The safety conditions are described in the [memory layout] section.
1004    ///
1005    /// # Examples
1006    ///
1007    /// Recreate a `Box` which was previously converted to a raw pointer
1008    /// using [`Box::into_raw`]:
1009    /// ```
1010    /// let x = Box::new(5);
1011    /// let ptr = Box::into_raw(x);
1012    /// let x = unsafe { Box::from_raw(ptr) };
1013    /// ```
1014    /// Manually create a `Box` from scratch by using the global allocator:
1015    /// ```
1016    /// use std::alloc::{alloc, Layout};
1017    ///
1018    /// unsafe {
1019    ///     let ptr = alloc(Layout::new::<i32>()) as *mut i32;
1020    ///     // In general .write is required to avoid attempting to destruct
1021    ///     // the (uninitialized) previous contents of `ptr`, though for this
1022    ///     // simple example `*ptr = 5` would have worked as well.
1023    ///     ptr.write(5);
1024    ///     let x = Box::from_raw(ptr);
1025    /// }
1026    /// ```
1027    ///
1028    /// [memory layout]: self#memory-layout
1029    #[stable(feature = "box_raw", since = "1.4.0")]
1030    #[inline]
1031    #[must_use = "call `drop(Box::from_raw(ptr))` if you intend to drop the `Box`"]
1032    pub unsafe fn from_raw(raw: *mut T) -> Self {
1033        unsafe { Self::from_raw_in(raw, Global) }
1034    }
1035
1036    /// Constructs a box from a `NonNull` pointer.
1037    ///
1038    /// After calling this function, the `NonNull` pointer is owned by
1039    /// the resulting `Box`. Specifically, the `Box` destructor will call
1040    /// the destructor of `T` and free the allocated memory. For this
1041    /// to be safe, the memory must have been allocated in accordance
1042    /// with the [memory layout] used by `Box` .
1043    ///
1044    /// # Safety
1045    ///
1046    /// This function is unsafe because improper use may lead to
1047    /// memory problems. For example, a double-free may occur if the
1048    /// function is called twice on the same `NonNull` pointer.
1049    ///
1050    /// The non-null pointer must point to a block of memory allocated by the global allocator.
1051    ///
1052    /// The safety conditions are described in the [memory layout] section.
1053    ///
1054    /// # Examples
1055    ///
1056    /// Recreate a `Box` which was previously converted to a `NonNull`
1057    /// pointer using [`Box::into_non_null`]:
1058    /// ```
1059    /// #![feature(box_vec_non_null)]
1060    ///
1061    /// let x = Box::new(5);
1062    /// let non_null = Box::into_non_null(x);
1063    /// let x = unsafe { Box::from_non_null(non_null) };
1064    /// ```
1065    /// Manually create a `Box` from scratch by using the global allocator:
1066    /// ```
1067    /// #![feature(box_vec_non_null)]
1068    ///
1069    /// use std::alloc::{alloc, Layout};
1070    /// use std::ptr::NonNull;
1071    ///
1072    /// unsafe {
1073    ///     let non_null = NonNull::new(alloc(Layout::new::<i32>()).cast::<i32>())
1074    ///         .expect("allocation failed");
1075    ///     // In general .write is required to avoid attempting to destruct
1076    ///     // the (uninitialized) previous contents of `non_null`.
1077    ///     non_null.write(5);
1078    ///     let x = Box::from_non_null(non_null);
1079    /// }
1080    /// ```
1081    ///
1082    /// [memory layout]: self#memory-layout
1083    #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1084    #[inline]
1085    #[must_use = "call `drop(Box::from_non_null(ptr))` if you intend to drop the `Box`"]
1086    pub unsafe fn from_non_null(ptr: NonNull<T>) -> Self {
1087        unsafe { Self::from_raw(ptr.as_ptr()) }
1088    }
1089
1090    /// Consumes the `Box`, returning a wrapped raw pointer.
1091    ///
1092    /// The pointer will be properly aligned and non-null.
1093    ///
1094    /// After calling this function, the caller is responsible for the
1095    /// memory previously managed by the `Box`. In particular, the
1096    /// caller should properly destroy `T` and release the memory, taking
1097    /// into account the [memory layout] used by `Box`. The easiest way to
1098    /// do this is to convert the raw pointer back into a `Box` with the
1099    /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
1100    /// the cleanup.
1101    ///
1102    /// Note: this is an associated function, which means that you have
1103    /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
1104    /// is so that there is no conflict with a method on the inner type.
1105    ///
1106    /// # Examples
1107    /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
1108    /// for automatic cleanup:
1109    /// ```
1110    /// let x = Box::new(String::from("Hello"));
1111    /// let ptr = Box::into_raw(x);
1112    /// let x = unsafe { Box::from_raw(ptr) };
1113    /// ```
1114    /// Manual cleanup by explicitly running the destructor and deallocating
1115    /// the memory:
1116    /// ```
1117    /// use std::alloc::{dealloc, Layout};
1118    /// use std::ptr;
1119    ///
1120    /// let x = Box::new(String::from("Hello"));
1121    /// let ptr = Box::into_raw(x);
1122    /// unsafe {
1123    ///     ptr::drop_in_place(ptr);
1124    ///     dealloc(ptr as *mut u8, Layout::new::<String>());
1125    /// }
1126    /// ```
1127    /// Note: This is equivalent to the following:
1128    /// ```
1129    /// let x = Box::new(String::from("Hello"));
1130    /// let ptr = Box::into_raw(x);
1131    /// unsafe {
1132    ///     drop(Box::from_raw(ptr));
1133    /// }
1134    /// ```
1135    ///
1136    /// [memory layout]: self#memory-layout
1137    #[must_use = "losing the pointer will leak memory"]
1138    #[stable(feature = "box_raw", since = "1.4.0")]
1139    #[inline]
1140    pub fn into_raw(b: Self) -> *mut T {
1141        // Avoid `into_raw_with_allocator` as that interacts poorly with Miri's Stacked Borrows.
1142        let mut b = mem::ManuallyDrop::new(b);
1143        // We go through the built-in deref for `Box`, which is crucial for Miri to recognize this
1144        // operation for it's alias tracking.
1145        &raw mut **b
1146    }
1147
1148    /// Consumes the `Box`, returning a wrapped `NonNull` pointer.
1149    ///
1150    /// The pointer will be properly aligned.
1151    ///
1152    /// After calling this function, the caller is responsible for the
1153    /// memory previously managed by the `Box`. In particular, the
1154    /// caller should properly destroy `T` and release the memory, taking
1155    /// into account the [memory layout] used by `Box`. The easiest way to
1156    /// do this is to convert the `NonNull` pointer back into a `Box` with the
1157    /// [`Box::from_non_null`] function, allowing the `Box` destructor to
1158    /// perform the cleanup.
1159    ///
1160    /// Note: this is an associated function, which means that you have
1161    /// to call it as `Box::into_non_null(b)` instead of `b.into_non_null()`.
1162    /// This is so that there is no conflict with a method on the inner type.
1163    ///
1164    /// # Examples
1165    /// Converting the `NonNull` pointer back into a `Box` with [`Box::from_non_null`]
1166    /// for automatic cleanup:
1167    /// ```
1168    /// #![feature(box_vec_non_null)]
1169    ///
1170    /// let x = Box::new(String::from("Hello"));
1171    /// let non_null = Box::into_non_null(x);
1172    /// let x = unsafe { Box::from_non_null(non_null) };
1173    /// ```
1174    /// Manual cleanup by explicitly running the destructor and deallocating
1175    /// the memory:
1176    /// ```
1177    /// #![feature(box_vec_non_null)]
1178    ///
1179    /// use std::alloc::{dealloc, Layout};
1180    ///
1181    /// let x = Box::new(String::from("Hello"));
1182    /// let non_null = Box::into_non_null(x);
1183    /// unsafe {
1184    ///     non_null.drop_in_place();
1185    ///     dealloc(non_null.as_ptr().cast::<u8>(), Layout::new::<String>());
1186    /// }
1187    /// ```
1188    /// Note: This is equivalent to the following:
1189    /// ```
1190    /// #![feature(box_vec_non_null)]
1191    ///
1192    /// let x = Box::new(String::from("Hello"));
1193    /// let non_null = Box::into_non_null(x);
1194    /// unsafe {
1195    ///     drop(Box::from_non_null(non_null));
1196    /// }
1197    /// ```
1198    ///
1199    /// [memory layout]: self#memory-layout
1200    #[must_use = "losing the pointer will leak memory"]
1201    #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1202    #[inline]
1203    pub fn into_non_null(b: Self) -> NonNull<T> {
1204        // SAFETY: `Box` is guaranteed to be non-null.
1205        unsafe { NonNull::new_unchecked(Self::into_raw(b)) }
1206    }
1207}
1208
1209impl<T: ?Sized, A: Allocator> Box<T, A> {
1210    /// Constructs a box from a raw pointer in the given allocator.
1211    ///
1212    /// After calling this function, the raw pointer is owned by the
1213    /// resulting `Box`. Specifically, the `Box` destructor will call
1214    /// the destructor of `T` and free the allocated memory. For this
1215    /// to be safe, the memory must have been allocated in accordance
1216    /// with the [memory layout] used by `Box` .
1217    ///
1218    /// # Safety
1219    ///
1220    /// This function is unsafe because improper use may lead to
1221    /// memory problems. For example, a double-free may occur if the
1222    /// function is called twice on the same raw pointer.
1223    ///
1224    /// The raw pointer must point to a block of memory allocated by `alloc`.
1225    ///
1226    /// # Examples
1227    ///
1228    /// Recreate a `Box` which was previously converted to a raw pointer
1229    /// using [`Box::into_raw_with_allocator`]:
1230    /// ```
1231    /// #![feature(allocator_api)]
1232    ///
1233    /// use std::alloc::System;
1234    ///
1235    /// let x = Box::new_in(5, System);
1236    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1237    /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
1238    /// ```
1239    /// Manually create a `Box` from scratch by using the system allocator:
1240    /// ```
1241    /// #![feature(allocator_api, slice_ptr_get)]
1242    ///
1243    /// use std::alloc::{Allocator, Layout, System};
1244    ///
1245    /// unsafe {
1246    ///     let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
1247    ///     // In general .write is required to avoid attempting to destruct
1248    ///     // the (uninitialized) previous contents of `ptr`, though for this
1249    ///     // simple example `*ptr = 5` would have worked as well.
1250    ///     ptr.write(5);
1251    ///     let x = Box::from_raw_in(ptr, System);
1252    /// }
1253    /// # Ok::<(), std::alloc::AllocError>(())
1254    /// ```
1255    ///
1256    /// [memory layout]: self#memory-layout
1257    #[unstable(feature = "allocator_api", issue = "32838")]
1258    #[inline]
1259    pub unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
1260        Box(unsafe { Unique::new_unchecked(raw) }, alloc)
1261    }
1262
1263    /// Constructs a box from a `NonNull` pointer in the given allocator.
1264    ///
1265    /// After calling this function, the `NonNull` pointer is owned by
1266    /// the resulting `Box`. Specifically, the `Box` destructor will call
1267    /// the destructor of `T` and free the allocated memory. For this
1268    /// to be safe, the memory must have been allocated in accordance
1269    /// with the [memory layout] used by `Box` .
1270    ///
1271    /// # Safety
1272    ///
1273    /// This function is unsafe because improper use may lead to
1274    /// memory problems. For example, a double-free may occur if the
1275    /// function is called twice on the same raw pointer.
1276    ///
1277    /// The non-null pointer must point to a block of memory allocated by `alloc`.
1278    ///
1279    /// # Examples
1280    ///
1281    /// Recreate a `Box` which was previously converted to a `NonNull` pointer
1282    /// using [`Box::into_non_null_with_allocator`]:
1283    /// ```
1284    /// #![feature(allocator_api, box_vec_non_null)]
1285    ///
1286    /// use std::alloc::System;
1287    ///
1288    /// let x = Box::new_in(5, System);
1289    /// let (non_null, alloc) = Box::into_non_null_with_allocator(x);
1290    /// let x = unsafe { Box::from_non_null_in(non_null, alloc) };
1291    /// ```
1292    /// Manually create a `Box` from scratch by using the system allocator:
1293    /// ```
1294    /// #![feature(allocator_api, box_vec_non_null, slice_ptr_get)]
1295    ///
1296    /// use std::alloc::{Allocator, Layout, System};
1297    ///
1298    /// unsafe {
1299    ///     let non_null = System.allocate(Layout::new::<i32>())?.cast::<i32>();
1300    ///     // In general .write is required to avoid attempting to destruct
1301    ///     // the (uninitialized) previous contents of `non_null`.
1302    ///     non_null.write(5);
1303    ///     let x = Box::from_non_null_in(non_null, System);
1304    /// }
1305    /// # Ok::<(), std::alloc::AllocError>(())
1306    /// ```
1307    ///
1308    /// [memory layout]: self#memory-layout
1309    #[unstable(feature = "allocator_api", issue = "32838")]
1310    // #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1311    #[inline]
1312    pub unsafe fn from_non_null_in(raw: NonNull<T>, alloc: A) -> Self {
1313        // SAFETY: guaranteed by the caller.
1314        unsafe { Box::from_raw_in(raw.as_ptr(), alloc) }
1315    }
1316
1317    /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
1318    ///
1319    /// The pointer will be properly aligned and non-null.
1320    ///
1321    /// After calling this function, the caller is responsible for the
1322    /// memory previously managed by the `Box`. In particular, the
1323    /// caller should properly destroy `T` and release the memory, taking
1324    /// into account the [memory layout] used by `Box`. The easiest way to
1325    /// do this is to convert the raw pointer back into a `Box` with the
1326    /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
1327    /// the cleanup.
1328    ///
1329    /// Note: this is an associated function, which means that you have
1330    /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
1331    /// is so that there is no conflict with a method on the inner type.
1332    ///
1333    /// # Examples
1334    /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
1335    /// for automatic cleanup:
1336    /// ```
1337    /// #![feature(allocator_api)]
1338    ///
1339    /// use std::alloc::System;
1340    ///
1341    /// let x = Box::new_in(String::from("Hello"), System);
1342    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1343    /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
1344    /// ```
1345    /// Manual cleanup by explicitly running the destructor and deallocating
1346    /// the memory:
1347    /// ```
1348    /// #![feature(allocator_api)]
1349    ///
1350    /// use std::alloc::{Allocator, Layout, System};
1351    /// use std::ptr::{self, NonNull};
1352    ///
1353    /// let x = Box::new_in(String::from("Hello"), System);
1354    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1355    /// unsafe {
1356    ///     ptr::drop_in_place(ptr);
1357    ///     let non_null = NonNull::new_unchecked(ptr);
1358    ///     alloc.deallocate(non_null.cast(), Layout::new::<String>());
1359    /// }
1360    /// ```
1361    ///
1362    /// [memory layout]: self#memory-layout
1363    #[must_use = "losing the pointer will leak memory"]
1364    #[unstable(feature = "allocator_api", issue = "32838")]
1365    #[inline]
1366    pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
1367        let mut b = mem::ManuallyDrop::new(b);
1368        // We carefully get the raw pointer out in a way that Miri's aliasing model understands what
1369        // is happening: using the primitive "deref" of `Box`. In case `A` is *not* `Global`, we
1370        // want *no* aliasing requirements here!
1371        // In case `A` *is* `Global`, this does not quite have the right behavior; `into_raw`
1372        // works around that.
1373        let ptr = &raw mut **b;
1374        let alloc = unsafe { ptr::read(&b.1) };
1375        (ptr, alloc)
1376    }
1377
1378    /// Consumes the `Box`, returning a wrapped `NonNull` pointer and the allocator.
1379    ///
1380    /// The pointer will be properly aligned.
1381    ///
1382    /// After calling this function, the caller is responsible for the
1383    /// memory previously managed by the `Box`. In particular, the
1384    /// caller should properly destroy `T` and release the memory, taking
1385    /// into account the [memory layout] used by `Box`. The easiest way to
1386    /// do this is to convert the `NonNull` pointer back into a `Box` with the
1387    /// [`Box::from_non_null_in`] function, allowing the `Box` destructor to
1388    /// perform the cleanup.
1389    ///
1390    /// Note: this is an associated function, which means that you have
1391    /// to call it as `Box::into_non_null_with_allocator(b)` instead of
1392    /// `b.into_non_null_with_allocator()`. This is so that there is no
1393    /// conflict with a method on the inner type.
1394    ///
1395    /// # Examples
1396    /// Converting the `NonNull` pointer back into a `Box` with
1397    /// [`Box::from_non_null_in`] for automatic cleanup:
1398    /// ```
1399    /// #![feature(allocator_api, box_vec_non_null)]
1400    ///
1401    /// use std::alloc::System;
1402    ///
1403    /// let x = Box::new_in(String::from("Hello"), System);
1404    /// let (non_null, alloc) = Box::into_non_null_with_allocator(x);
1405    /// let x = unsafe { Box::from_non_null_in(non_null, alloc) };
1406    /// ```
1407    /// Manual cleanup by explicitly running the destructor and deallocating
1408    /// the memory:
1409    /// ```
1410    /// #![feature(allocator_api, box_vec_non_null)]
1411    ///
1412    /// use std::alloc::{Allocator, Layout, System};
1413    ///
1414    /// let x = Box::new_in(String::from("Hello"), System);
1415    /// let (non_null, alloc) = Box::into_non_null_with_allocator(x);
1416    /// unsafe {
1417    ///     non_null.drop_in_place();
1418    ///     alloc.deallocate(non_null.cast::<u8>(), Layout::new::<String>());
1419    /// }
1420    /// ```
1421    ///
1422    /// [memory layout]: self#memory-layout
1423    #[must_use = "losing the pointer will leak memory"]
1424    #[unstable(feature = "allocator_api", issue = "32838")]
1425    // #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1426    #[inline]
1427    pub fn into_non_null_with_allocator(b: Self) -> (NonNull<T>, A) {
1428        let (ptr, alloc) = Box::into_raw_with_allocator(b);
1429        // SAFETY: `Box` is guaranteed to be non-null.
1430        unsafe { (NonNull::new_unchecked(ptr), alloc) }
1431    }
1432
1433    #[unstable(
1434        feature = "ptr_internals",
1435        issue = "none",
1436        reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
1437    )]
1438    #[inline]
1439    #[doc(hidden)]
1440    pub fn into_unique(b: Self) -> (Unique<T>, A) {
1441        let (ptr, alloc) = Box::into_raw_with_allocator(b);
1442        unsafe { (Unique::from(&mut *ptr), alloc) }
1443    }
1444
1445    /// Returns a raw mutable pointer to the `Box`'s contents.
1446    ///
1447    /// The caller must ensure that the `Box` outlives the pointer this
1448    /// function returns, or else it will end up dangling.
1449    ///
1450    /// This method guarantees that for the purpose of the aliasing model, this method
1451    /// does not materialize a reference to the underlying memory, and thus the returned pointer
1452    /// will remain valid when mixed with other calls to [`as_ptr`] and [`as_mut_ptr`].
1453    /// Note that calling other methods that materialize references to the memory
1454    /// may still invalidate this pointer.
1455    /// See the example below for how this guarantee can be used.
1456    ///
1457    /// # Examples
1458    ///
1459    /// Due to the aliasing guarantee, the following code is legal:
1460    ///
1461    /// ```rust
1462    /// #![feature(box_as_ptr)]
1463    ///
1464    /// unsafe {
1465    ///     let mut b = Box::new(0);
1466    ///     let ptr1 = Box::as_mut_ptr(&mut b);
1467    ///     ptr1.write(1);
1468    ///     let ptr2 = Box::as_mut_ptr(&mut b);
1469    ///     ptr2.write(2);
1470    ///     // Notably, the write to `ptr2` did *not* invalidate `ptr1`:
1471    ///     ptr1.write(3);
1472    /// }
1473    /// ```
1474    ///
1475    /// [`as_mut_ptr`]: Self::as_mut_ptr
1476    /// [`as_ptr`]: Self::as_ptr
1477    #[unstable(feature = "box_as_ptr", issue = "129090")]
1478    #[rustc_never_returns_null_ptr]
1479    #[rustc_as_ptr]
1480    #[inline]
1481    pub fn as_mut_ptr(b: &mut Self) -> *mut T {
1482        // This is a primitive deref, not going through `DerefMut`, and therefore not materializing
1483        // any references.
1484        &raw mut **b
1485    }
1486
1487    /// Returns a raw pointer to the `Box`'s contents.
1488    ///
1489    /// The caller must ensure that the `Box` outlives the pointer this
1490    /// function returns, or else it will end up dangling.
1491    ///
1492    /// The caller must also ensure that the memory the pointer (non-transitively) points to
1493    /// is never written to (except inside an `UnsafeCell`) using this pointer or any pointer
1494    /// derived from it. If you need to mutate the contents of the `Box`, use [`as_mut_ptr`].
1495    ///
1496    /// This method guarantees that for the purpose of the aliasing model, this method
1497    /// does not materialize a reference to the underlying memory, and thus the returned pointer
1498    /// will remain valid when mixed with other calls to [`as_ptr`] and [`as_mut_ptr`].
1499    /// Note that calling other methods that materialize mutable references to the memory,
1500    /// as well as writing to this memory, may still invalidate this pointer.
1501    /// See the example below for how this guarantee can be used.
1502    ///
1503    /// # Examples
1504    ///
1505    /// Due to the aliasing guarantee, the following code is legal:
1506    ///
1507    /// ```rust
1508    /// #![feature(box_as_ptr)]
1509    ///
1510    /// unsafe {
1511    ///     let mut v = Box::new(0);
1512    ///     let ptr1 = Box::as_ptr(&v);
1513    ///     let ptr2 = Box::as_mut_ptr(&mut v);
1514    ///     let _val = ptr2.read();
1515    ///     // No write to this memory has happened yet, so `ptr1` is still valid.
1516    ///     let _val = ptr1.read();
1517    ///     // However, once we do a write...
1518    ///     ptr2.write(1);
1519    ///     // ... `ptr1` is no longer valid.
1520    ///     // This would be UB: let _val = ptr1.read();
1521    /// }
1522    /// ```
1523    ///
1524    /// [`as_mut_ptr`]: Self::as_mut_ptr
1525    /// [`as_ptr`]: Self::as_ptr
1526    #[unstable(feature = "box_as_ptr", issue = "129090")]
1527    #[rustc_never_returns_null_ptr]
1528    #[rustc_as_ptr]
1529    #[inline]
1530    pub fn as_ptr(b: &Self) -> *const T {
1531        // This is a primitive deref, not going through `DerefMut`, and therefore not materializing
1532        // any references.
1533        &raw const **b
1534    }
1535
1536    /// Returns a reference to the underlying allocator.
1537    ///
1538    /// Note: this is an associated function, which means that you have
1539    /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
1540    /// is so that there is no conflict with a method on the inner type.
1541    #[unstable(feature = "allocator_api", issue = "32838")]
1542    #[inline]
1543    pub fn allocator(b: &Self) -> &A {
1544        &b.1
1545    }
1546
1547    /// Consumes and leaks the `Box`, returning a mutable reference,
1548    /// `&'a mut T`.
1549    ///
1550    /// Note that the type `T` must outlive the chosen lifetime `'a`. If the type
1551    /// has only static references, or none at all, then this may be chosen to be
1552    /// `'static`.
1553    ///
1554    /// This function is mainly useful for data that lives for the remainder of
1555    /// the program's life. Dropping the returned reference will cause a memory
1556    /// leak. If this is not acceptable, the reference should first be wrapped
1557    /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
1558    /// then be dropped which will properly destroy `T` and release the
1559    /// allocated memory.
1560    ///
1561    /// Note: this is an associated function, which means that you have
1562    /// to call it as `Box::leak(b)` instead of `b.leak()`. This
1563    /// is so that there is no conflict with a method on the inner type.
1564    ///
1565    /// # Examples
1566    ///
1567    /// Simple usage:
1568    ///
1569    /// ```
1570    /// let x = Box::new(41);
1571    /// let static_ref: &'static mut usize = Box::leak(x);
1572    /// *static_ref += 1;
1573    /// assert_eq!(*static_ref, 42);
1574    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
1575    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
1576    /// # drop(unsafe { Box::from_raw(static_ref) });
1577    /// ```
1578    ///
1579    /// Unsized data:
1580    ///
1581    /// ```
1582    /// let x = vec![1, 2, 3].into_boxed_slice();
1583    /// let static_ref = Box::leak(x);
1584    /// static_ref[0] = 4;
1585    /// assert_eq!(*static_ref, [4, 2, 3]);
1586    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
1587    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
1588    /// # drop(unsafe { Box::from_raw(static_ref) });
1589    /// ```
1590    #[stable(feature = "box_leak", since = "1.26.0")]
1591    #[inline]
1592    pub fn leak<'a>(b: Self) -> &'a mut T
1593    where
1594        A: 'a,
1595    {
1596        let (ptr, alloc) = Box::into_raw_with_allocator(b);
1597        mem::forget(alloc);
1598        unsafe { &mut *ptr }
1599    }
1600
1601    /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
1602    /// `*boxed` will be pinned in memory and unable to be moved.
1603    ///
1604    /// This conversion does not allocate on the heap and happens in place.
1605    ///
1606    /// This is also available via [`From`].
1607    ///
1608    /// Constructing and pinning a `Box` with <code>Box::into_pin([Box::new]\(x))</code>
1609    /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
1610    /// This `into_pin` method is useful if you already have a `Box<T>`, or you are
1611    /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
1612    ///
1613    /// # Notes
1614    ///
1615    /// It's not recommended that crates add an impl like `From<Box<T>> for Pin<T>`,
1616    /// as it'll introduce an ambiguity when calling `Pin::from`.
1617    /// A demonstration of such a poor impl is shown below.
1618    ///
1619    /// ```compile_fail
1620    /// # use std::pin::Pin;
1621    /// struct Foo; // A type defined in this crate.
1622    /// impl From<Box<()>> for Pin<Foo> {
1623    ///     fn from(_: Box<()>) -> Pin<Foo> {
1624    ///         Pin::new(Foo)
1625    ///     }
1626    /// }
1627    ///
1628    /// let foo = Box::new(());
1629    /// let bar = Pin::from(foo);
1630    /// ```
1631    #[stable(feature = "box_into_pin", since = "1.63.0")]
1632    pub fn into_pin(boxed: Self) -> Pin<Self>
1633    where
1634        A: 'static,
1635    {
1636        // It's not possible to move or replace the insides of a `Pin<Box<T>>`
1637        // when `T: !Unpin`, so it's safe to pin it directly without any
1638        // additional requirements.
1639        unsafe { Pin::new_unchecked(boxed) }
1640    }
1641}
1642
1643#[stable(feature = "rust1", since = "1.0.0")]
1644unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
1645    #[inline]
1646    fn drop(&mut self) {
1647        // the T in the Box is dropped by the compiler before the destructor is run
1648
1649        let ptr = self.0;
1650
1651        unsafe {
1652            let layout = Layout::for_value_raw(ptr.as_ptr());
1653            if layout.size() != 0 {
1654                self.1.deallocate(From::from(ptr.cast()), layout);
1655            }
1656        }
1657    }
1658}
1659
1660#[cfg(not(no_global_oom_handling))]
1661#[stable(feature = "rust1", since = "1.0.0")]
1662impl<T: Default> Default for Box<T> {
1663    /// Creates a `Box<T>`, with the `Default` value for `T`.
1664    #[inline]
1665    fn default() -> Self {
1666        let mut x: Box<mem::MaybeUninit<T>> = Box::new_uninit();
1667        unsafe {
1668            // SAFETY: `x` is valid for writing and has the same layout as `T`.
1669            // If `T::default()` panics, dropping `x` will just deallocate the Box as `MaybeUninit<T>`
1670            // does not have a destructor.
1671            //
1672            // We use `ptr::write` as `MaybeUninit::write` creates
1673            // extra stack copies of `T` in debug mode.
1674            //
1675            // See https://github.com/rust-lang/rust/issues/136043 for more context.
1676            ptr::write(&raw mut *x as *mut T, T::default());
1677            // SAFETY: `x` was just initialized above.
1678            x.assume_init()
1679        }
1680    }
1681}
1682
1683#[cfg(not(no_global_oom_handling))]
1684#[stable(feature = "rust1", since = "1.0.0")]
1685impl<T> Default for Box<[T]> {
1686    /// Creates an empty `[T]` inside a `Box`.
1687    #[inline]
1688    fn default() -> Self {
1689        let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
1690        Box(ptr, Global)
1691    }
1692}
1693
1694#[cfg(not(no_global_oom_handling))]
1695#[stable(feature = "default_box_extra", since = "1.17.0")]
1696impl Default for Box<str> {
1697    #[inline]
1698    fn default() -> Self {
1699        // SAFETY: This is the same as `Unique::cast<U>` but with an unsized `U = str`.
1700        let ptr: Unique<str> = unsafe {
1701            let bytes: Unique<[u8]> = Unique::<[u8; 0]>::dangling();
1702            Unique::new_unchecked(bytes.as_ptr() as *mut str)
1703        };
1704        Box(ptr, Global)
1705    }
1706}
1707
1708#[cfg(not(no_global_oom_handling))]
1709#[stable(feature = "pin_default_impls", since = "CURRENT_RUSTC_VERSION")]
1710impl<T> Default for Pin<Box<T>>
1711where
1712    T: ?Sized,
1713    Box<T>: Default,
1714{
1715    #[inline]
1716    fn default() -> Self {
1717        Box::into_pin(Box::<T>::default())
1718    }
1719}
1720
1721#[cfg(not(no_global_oom_handling))]
1722#[stable(feature = "rust1", since = "1.0.0")]
1723impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
1724    /// Returns a new box with a `clone()` of this box's contents.
1725    ///
1726    /// # Examples
1727    ///
1728    /// ```
1729    /// let x = Box::new(5);
1730    /// let y = x.clone();
1731    ///
1732    /// // The value is the same
1733    /// assert_eq!(x, y);
1734    ///
1735    /// // But they are unique objects
1736    /// assert_ne!(&*x as *const i32, &*y as *const i32);
1737    /// ```
1738    #[inline]
1739    fn clone(&self) -> Self {
1740        // Pre-allocate memory to allow writing the cloned value directly.
1741        let mut boxed = Self::new_uninit_in(self.1.clone());
1742        unsafe {
1743            (**self).clone_to_uninit(boxed.as_mut_ptr().cast());
1744            boxed.assume_init()
1745        }
1746    }
1747
1748    /// Copies `source`'s contents into `self` without creating a new allocation.
1749    ///
1750    /// # Examples
1751    ///
1752    /// ```
1753    /// let x = Box::new(5);
1754    /// let mut y = Box::new(10);
1755    /// let yp: *const i32 = &*y;
1756    ///
1757    /// y.clone_from(&x);
1758    ///
1759    /// // The value is the same
1760    /// assert_eq!(x, y);
1761    ///
1762    /// // And no allocation occurred
1763    /// assert_eq!(yp, &*y);
1764    /// ```
1765    #[inline]
1766    fn clone_from(&mut self, source: &Self) {
1767        (**self).clone_from(&(**source));
1768    }
1769}
1770
1771#[cfg(not(no_global_oom_handling))]
1772#[stable(feature = "box_slice_clone", since = "1.3.0")]
1773impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
1774    fn clone(&self) -> Self {
1775        let alloc = Box::allocator(self).clone();
1776        self.to_vec_in(alloc).into_boxed_slice()
1777    }
1778
1779    /// Copies `source`'s contents into `self` without creating a new allocation,
1780    /// so long as the two are of the same length.
1781    ///
1782    /// # Examples
1783    ///
1784    /// ```
1785    /// let x = Box::new([5, 6, 7]);
1786    /// let mut y = Box::new([8, 9, 10]);
1787    /// let yp: *const [i32] = &*y;
1788    ///
1789    /// y.clone_from(&x);
1790    ///
1791    /// // The value is the same
1792    /// assert_eq!(x, y);
1793    ///
1794    /// // And no allocation occurred
1795    /// assert_eq!(yp, &*y);
1796    /// ```
1797    fn clone_from(&mut self, source: &Self) {
1798        if self.len() == source.len() {
1799            self.clone_from_slice(&source);
1800        } else {
1801            *self = source.clone();
1802        }
1803    }
1804}
1805
1806#[cfg(not(no_global_oom_handling))]
1807#[stable(feature = "box_slice_clone", since = "1.3.0")]
1808impl Clone for Box<str> {
1809    fn clone(&self) -> Self {
1810        // this makes a copy of the data
1811        let buf: Box<[u8]> = self.as_bytes().into();
1812        unsafe { from_boxed_utf8_unchecked(buf) }
1813    }
1814}
1815
1816#[stable(feature = "rust1", since = "1.0.0")]
1817impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
1818    #[inline]
1819    fn eq(&self, other: &Self) -> bool {
1820        PartialEq::eq(&**self, &**other)
1821    }
1822    #[inline]
1823    fn ne(&self, other: &Self) -> bool {
1824        PartialEq::ne(&**self, &**other)
1825    }
1826}
1827
1828#[stable(feature = "rust1", since = "1.0.0")]
1829impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
1830    #[inline]
1831    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1832        PartialOrd::partial_cmp(&**self, &**other)
1833    }
1834    #[inline]
1835    fn lt(&self, other: &Self) -> bool {
1836        PartialOrd::lt(&**self, &**other)
1837    }
1838    #[inline]
1839    fn le(&self, other: &Self) -> bool {
1840        PartialOrd::le(&**self, &**other)
1841    }
1842    #[inline]
1843    fn ge(&self, other: &Self) -> bool {
1844        PartialOrd::ge(&**self, &**other)
1845    }
1846    #[inline]
1847    fn gt(&self, other: &Self) -> bool {
1848        PartialOrd::gt(&**self, &**other)
1849    }
1850}
1851
1852#[stable(feature = "rust1", since = "1.0.0")]
1853impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
1854    #[inline]
1855    fn cmp(&self, other: &Self) -> Ordering {
1856        Ord::cmp(&**self, &**other)
1857    }
1858}
1859
1860#[stable(feature = "rust1", since = "1.0.0")]
1861impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}
1862
1863#[stable(feature = "rust1", since = "1.0.0")]
1864impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
1865    fn hash<H: Hasher>(&self, state: &mut H) {
1866        (**self).hash(state);
1867    }
1868}
1869
1870#[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
1871impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
1872    fn finish(&self) -> u64 {
1873        (**self).finish()
1874    }
1875    fn write(&mut self, bytes: &[u8]) {
1876        (**self).write(bytes)
1877    }
1878    fn write_u8(&mut self, i: u8) {
1879        (**self).write_u8(i)
1880    }
1881    fn write_u16(&mut self, i: u16) {
1882        (**self).write_u16(i)
1883    }
1884    fn write_u32(&mut self, i: u32) {
1885        (**self).write_u32(i)
1886    }
1887    fn write_u64(&mut self, i: u64) {
1888        (**self).write_u64(i)
1889    }
1890    fn write_u128(&mut self, i: u128) {
1891        (**self).write_u128(i)
1892    }
1893    fn write_usize(&mut self, i: usize) {
1894        (**self).write_usize(i)
1895    }
1896    fn write_i8(&mut self, i: i8) {
1897        (**self).write_i8(i)
1898    }
1899    fn write_i16(&mut self, i: i16) {
1900        (**self).write_i16(i)
1901    }
1902    fn write_i32(&mut self, i: i32) {
1903        (**self).write_i32(i)
1904    }
1905    fn write_i64(&mut self, i: i64) {
1906        (**self).write_i64(i)
1907    }
1908    fn write_i128(&mut self, i: i128) {
1909        (**self).write_i128(i)
1910    }
1911    fn write_isize(&mut self, i: isize) {
1912        (**self).write_isize(i)
1913    }
1914    fn write_length_prefix(&mut self, len: usize) {
1915        (**self).write_length_prefix(len)
1916    }
1917    fn write_str(&mut self, s: &str) {
1918        (**self).write_str(s)
1919    }
1920}
1921
1922#[stable(feature = "rust1", since = "1.0.0")]
1923impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
1924    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1925        fmt::Display::fmt(&**self, f)
1926    }
1927}
1928
1929#[stable(feature = "rust1", since = "1.0.0")]
1930impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
1931    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1932        fmt::Debug::fmt(&**self, f)
1933    }
1934}
1935
1936#[stable(feature = "rust1", since = "1.0.0")]
1937impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
1938    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1939        // It's not possible to extract the inner Uniq directly from the Box,
1940        // instead we cast it to a *const which aliases the Unique
1941        let ptr: *const T = &**self;
1942        fmt::Pointer::fmt(&ptr, f)
1943    }
1944}
1945
1946#[stable(feature = "rust1", since = "1.0.0")]
1947impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
1948    type Target = T;
1949
1950    fn deref(&self) -> &T {
1951        &**self
1952    }
1953}
1954
1955#[stable(feature = "rust1", since = "1.0.0")]
1956impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
1957    fn deref_mut(&mut self) -> &mut T {
1958        &mut **self
1959    }
1960}
1961
1962#[unstable(feature = "deref_pure_trait", issue = "87121")]
1963unsafe impl<T: ?Sized, A: Allocator> DerefPure for Box<T, A> {}
1964
1965#[unstable(feature = "legacy_receiver_trait", issue = "none")]
1966impl<T: ?Sized, A: Allocator> LegacyReceiver for Box<T, A> {}
1967
1968#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1969impl<Args: Tuple, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
1970    type Output = <F as FnOnce<Args>>::Output;
1971
1972    extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
1973        <F as FnOnce<Args>>::call_once(*self, args)
1974    }
1975}
1976
1977#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1978impl<Args: Tuple, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
1979    extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
1980        <F as FnMut<Args>>::call_mut(self, args)
1981    }
1982}
1983
1984#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1985impl<Args: Tuple, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
1986    extern "rust-call" fn call(&self, args: Args) -> Self::Output {
1987        <F as Fn<Args>>::call(self, args)
1988    }
1989}
1990
1991#[stable(feature = "async_closure", since = "1.85.0")]
1992impl<Args: Tuple, F: AsyncFnOnce<Args> + ?Sized, A: Allocator> AsyncFnOnce<Args> for Box<F, A> {
1993    type Output = F::Output;
1994    type CallOnceFuture = F::CallOnceFuture;
1995
1996    extern "rust-call" fn async_call_once(self, args: Args) -> Self::CallOnceFuture {
1997        F::async_call_once(*self, args)
1998    }
1999}
2000
2001#[stable(feature = "async_closure", since = "1.85.0")]
2002impl<Args: Tuple, F: AsyncFnMut<Args> + ?Sized, A: Allocator> AsyncFnMut<Args> for Box<F, A> {
2003    type CallRefFuture<'a>
2004        = F::CallRefFuture<'a>
2005    where
2006        Self: 'a;
2007
2008    extern "rust-call" fn async_call_mut(&mut self, args: Args) -> Self::CallRefFuture<'_> {
2009        F::async_call_mut(self, args)
2010    }
2011}
2012
2013#[stable(feature = "async_closure", since = "1.85.0")]
2014impl<Args: Tuple, F: AsyncFn<Args> + ?Sized, A: Allocator> AsyncFn<Args> for Box<F, A> {
2015    extern "rust-call" fn async_call(&self, args: Args) -> Self::CallRefFuture<'_> {
2016        F::async_call(self, args)
2017    }
2018}
2019
2020#[unstable(feature = "coerce_unsized", issue = "18598")]
2021impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
2022
2023#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2024unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Box<T, A> {}
2025
2026// It is quite crucial that we only allow the `Global` allocator here.
2027// Handling arbitrary custom allocators (which can affect the `Box` layout heavily!)
2028// would need a lot of codegen and interpreter adjustments.
2029#[unstable(feature = "dispatch_from_dyn", issue = "none")]
2030impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
2031
2032#[stable(feature = "box_borrow", since = "1.1.0")]
2033impl<T: ?Sized, A: Allocator> Borrow<T> for Box<T, A> {
2034    fn borrow(&self) -> &T {
2035        &**self
2036    }
2037}
2038
2039#[stable(feature = "box_borrow", since = "1.1.0")]
2040impl<T: ?Sized, A: Allocator> BorrowMut<T> for Box<T, A> {
2041    fn borrow_mut(&mut self) -> &mut T {
2042        &mut **self
2043    }
2044}
2045
2046#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2047impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
2048    fn as_ref(&self) -> &T {
2049        &**self
2050    }
2051}
2052
2053#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2054impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
2055    fn as_mut(&mut self) -> &mut T {
2056        &mut **self
2057    }
2058}
2059
2060/* Nota bene
2061 *
2062 *  We could have chosen not to add this impl, and instead have written a
2063 *  function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
2064 *  because Box<T> implements Unpin even when T does not, as a result of
2065 *  this impl.
2066 *
2067 *  We chose this API instead of the alternative for a few reasons:
2068 *      - Logically, it is helpful to understand pinning in regard to the
2069 *        memory region being pointed to. For this reason none of the
2070 *        standard library pointer types support projecting through a pin
2071 *        (Box<T> is the only pointer type in std for which this would be
2072 *        safe.)
2073 *      - It is in practice very useful to have Box<T> be unconditionally
2074 *        Unpin because of trait objects, for which the structural auto
2075 *        trait functionality does not apply (e.g., Box<dyn Foo> would
2076 *        otherwise not be Unpin).
2077 *
2078 *  Another type with the same semantics as Box but only a conditional
2079 *  implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
2080 *  could have a method to project a Pin<T> from it.
2081 */
2082#[stable(feature = "pin", since = "1.33.0")]
2083impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> {}
2084
2085#[unstable(feature = "coroutine_trait", issue = "43122")]
2086impl<G: ?Sized + Coroutine<R> + Unpin, R, A: Allocator> Coroutine<R> for Box<G, A> {
2087    type Yield = G::Yield;
2088    type Return = G::Return;
2089
2090    fn resume(mut self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return> {
2091        G::resume(Pin::new(&mut *self), arg)
2092    }
2093}
2094
2095#[unstable(feature = "coroutine_trait", issue = "43122")]
2096impl<G: ?Sized + Coroutine<R>, R, A: Allocator> Coroutine<R> for Pin<Box<G, A>>
2097where
2098    A: 'static,
2099{
2100    type Yield = G::Yield;
2101    type Return = G::Return;
2102
2103    fn resume(mut self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return> {
2104        G::resume((*self).as_mut(), arg)
2105    }
2106}
2107
2108#[stable(feature = "futures_api", since = "1.36.0")]
2109impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A> {
2110    type Output = F::Output;
2111
2112    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
2113        F::poll(Pin::new(&mut *self), cx)
2114    }
2115}
2116
2117#[stable(feature = "box_error", since = "1.8.0")]
2118impl<E: Error> Error for Box<E> {
2119    #[allow(deprecated)]
2120    fn cause(&self) -> Option<&dyn Error> {
2121        Error::cause(&**self)
2122    }
2123
2124    fn source(&self) -> Option<&(dyn Error + 'static)> {
2125        Error::source(&**self)
2126    }
2127
2128    fn provide<'b>(&'b self, request: &mut error::Request<'b>) {
2129        Error::provide(&**self, request);
2130    }
2131}