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::{PointerLike, 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#[cfg(not(bootstrap))]
241#[rustc_intrinsic]
242#[rustc_intrinsic_must_be_overridden]
243#[unstable(feature = "liballoc_internals", issue = "none")]
244pub fn box_new<T>(_x: T) -> Box<T> {
245    unreachable!()
246}
247
248/// Transition function for the next bootstrap bump.
249#[cfg(bootstrap)]
250#[unstable(feature = "liballoc_internals", issue = "none")]
251#[inline(always)]
252pub fn box_new<T>(x: T) -> Box<T> {
253    #[rustc_box]
254    Box::new(x)
255}
256
257impl<T> Box<T> {
258    /// Allocates memory on the heap and then places `x` into it.
259    ///
260    /// This doesn't actually allocate if `T` is zero-sized.
261    ///
262    /// # Examples
263    ///
264    /// ```
265    /// let five = Box::new(5);
266    /// ```
267    #[cfg(not(no_global_oom_handling))]
268    #[inline(always)]
269    #[stable(feature = "rust1", since = "1.0.0")]
270    #[must_use]
271    #[rustc_diagnostic_item = "box_new"]
272    #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
273    pub fn new(x: T) -> Self {
274        return box_new(x);
275    }
276
277    /// Constructs a new box with uninitialized contents.
278    ///
279    /// # Examples
280    ///
281    /// ```
282    /// let mut five = Box::<u32>::new_uninit();
283    /// // Deferred initialization:
284    /// five.write(5);
285    /// let five = unsafe { five.assume_init() };
286    ///
287    /// assert_eq!(*five, 5)
288    /// ```
289    #[cfg(not(no_global_oom_handling))]
290    #[stable(feature = "new_uninit", since = "1.82.0")]
291    #[must_use]
292    #[inline]
293    pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
294        Self::new_uninit_in(Global)
295    }
296
297    /// Constructs a new `Box` with uninitialized contents, with the memory
298    /// being filled with `0` bytes.
299    ///
300    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
301    /// of this method.
302    ///
303    /// # Examples
304    ///
305    /// ```
306    /// #![feature(new_zeroed_alloc)]
307    ///
308    /// let zero = Box::<u32>::new_zeroed();
309    /// let zero = unsafe { zero.assume_init() };
310    ///
311    /// assert_eq!(*zero, 0)
312    /// ```
313    ///
314    /// [zeroed]: mem::MaybeUninit::zeroed
315    #[cfg(not(no_global_oom_handling))]
316    #[inline]
317    #[unstable(feature = "new_zeroed_alloc", issue = "129396")]
318    #[must_use]
319    pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
320        Self::new_zeroed_in(Global)
321    }
322
323    /// Constructs a new `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
324    /// `x` will be pinned in memory and unable to be moved.
325    ///
326    /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin(x)`
327    /// does the same as <code>[Box::into_pin]\([Box::new]\(x))</code>. Consider using
328    /// [`into_pin`](Box::into_pin) if you already have a `Box<T>`, or if you want to
329    /// construct a (pinned) `Box` in a different way than with [`Box::new`].
330    #[cfg(not(no_global_oom_handling))]
331    #[stable(feature = "pin", since = "1.33.0")]
332    #[must_use]
333    #[inline(always)]
334    pub fn pin(x: T) -> Pin<Box<T>> {
335        Box::new(x).into()
336    }
337
338    /// Allocates memory on the heap then places `x` into it,
339    /// returning an error if the allocation fails
340    ///
341    /// This doesn't actually allocate if `T` is zero-sized.
342    ///
343    /// # Examples
344    ///
345    /// ```
346    /// #![feature(allocator_api)]
347    ///
348    /// let five = Box::try_new(5)?;
349    /// # Ok::<(), std::alloc::AllocError>(())
350    /// ```
351    #[unstable(feature = "allocator_api", issue = "32838")]
352    #[inline]
353    pub fn try_new(x: T) -> Result<Self, AllocError> {
354        Self::try_new_in(x, Global)
355    }
356
357    /// Constructs a new box with uninitialized contents on the heap,
358    /// returning an error if the allocation fails
359    ///
360    /// # Examples
361    ///
362    /// ```
363    /// #![feature(allocator_api)]
364    ///
365    /// let mut five = Box::<u32>::try_new_uninit()?;
366    /// // Deferred initialization:
367    /// five.write(5);
368    /// let five = unsafe { five.assume_init() };
369    ///
370    /// assert_eq!(*five, 5);
371    /// # Ok::<(), std::alloc::AllocError>(())
372    /// ```
373    #[unstable(feature = "allocator_api", issue = "32838")]
374    // #[unstable(feature = "new_uninit", issue = "63291")]
375    #[inline]
376    pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
377        Box::try_new_uninit_in(Global)
378    }
379
380    /// Constructs a new `Box` with uninitialized contents, with the memory
381    /// being filled with `0` bytes on the heap
382    ///
383    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
384    /// of this method.
385    ///
386    /// # Examples
387    ///
388    /// ```
389    /// #![feature(allocator_api)]
390    ///
391    /// let zero = Box::<u32>::try_new_zeroed()?;
392    /// let zero = unsafe { zero.assume_init() };
393    ///
394    /// assert_eq!(*zero, 0);
395    /// # Ok::<(), std::alloc::AllocError>(())
396    /// ```
397    ///
398    /// [zeroed]: mem::MaybeUninit::zeroed
399    #[unstable(feature = "allocator_api", issue = "32838")]
400    // #[unstable(feature = "new_uninit", issue = "63291")]
401    #[inline]
402    pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
403        Box::try_new_zeroed_in(Global)
404    }
405}
406
407impl<T, A: Allocator> Box<T, A> {
408    /// Allocates memory in the given allocator then places `x` into it.
409    ///
410    /// This doesn't actually allocate if `T` is zero-sized.
411    ///
412    /// # Examples
413    ///
414    /// ```
415    /// #![feature(allocator_api)]
416    ///
417    /// use std::alloc::System;
418    ///
419    /// let five = Box::new_in(5, System);
420    /// ```
421    #[cfg(not(no_global_oom_handling))]
422    #[unstable(feature = "allocator_api", issue = "32838")]
423    #[must_use]
424    #[inline]
425    pub fn new_in(x: T, alloc: A) -> Self
426    where
427        A: Allocator,
428    {
429        let mut boxed = Self::new_uninit_in(alloc);
430        boxed.write(x);
431        unsafe { boxed.assume_init() }
432    }
433
434    /// Allocates memory in the given allocator then places `x` into it,
435    /// returning an error if the allocation fails
436    ///
437    /// This doesn't actually allocate if `T` is zero-sized.
438    ///
439    /// # Examples
440    ///
441    /// ```
442    /// #![feature(allocator_api)]
443    ///
444    /// use std::alloc::System;
445    ///
446    /// let five = Box::try_new_in(5, System)?;
447    /// # Ok::<(), std::alloc::AllocError>(())
448    /// ```
449    #[unstable(feature = "allocator_api", issue = "32838")]
450    #[inline]
451    pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
452    where
453        A: Allocator,
454    {
455        let mut boxed = Self::try_new_uninit_in(alloc)?;
456        boxed.write(x);
457        unsafe { Ok(boxed.assume_init()) }
458    }
459
460    /// Constructs a new box with uninitialized contents in the provided allocator.
461    ///
462    /// # Examples
463    ///
464    /// ```
465    /// #![feature(allocator_api)]
466    ///
467    /// use std::alloc::System;
468    ///
469    /// let mut five = Box::<u32, _>::new_uninit_in(System);
470    /// // Deferred initialization:
471    /// five.write(5);
472    /// let five = unsafe { five.assume_init() };
473    ///
474    /// assert_eq!(*five, 5)
475    /// ```
476    #[unstable(feature = "allocator_api", issue = "32838")]
477    #[cfg(not(no_global_oom_handling))]
478    #[must_use]
479    // #[unstable(feature = "new_uninit", issue = "63291")]
480    pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
481    where
482        A: Allocator,
483    {
484        let layout = Layout::new::<mem::MaybeUninit<T>>();
485        // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
486        // That would make code size bigger.
487        match Box::try_new_uninit_in(alloc) {
488            Ok(m) => m,
489            Err(_) => handle_alloc_error(layout),
490        }
491    }
492
493    /// Constructs a new box with uninitialized contents in the provided allocator,
494    /// returning an error if the allocation fails
495    ///
496    /// # Examples
497    ///
498    /// ```
499    /// #![feature(allocator_api)]
500    ///
501    /// use std::alloc::System;
502    ///
503    /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
504    /// // Deferred initialization:
505    /// five.write(5);
506    /// let five = unsafe { five.assume_init() };
507    ///
508    /// assert_eq!(*five, 5);
509    /// # Ok::<(), std::alloc::AllocError>(())
510    /// ```
511    #[unstable(feature = "allocator_api", issue = "32838")]
512    // #[unstable(feature = "new_uninit", issue = "63291")]
513    pub fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
514    where
515        A: Allocator,
516    {
517        let ptr = if T::IS_ZST {
518            NonNull::dangling()
519        } else {
520            let layout = Layout::new::<mem::MaybeUninit<T>>();
521            alloc.allocate(layout)?.cast()
522        };
523        unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
524    }
525
526    /// Constructs a new `Box` with uninitialized contents, with the memory
527    /// being filled with `0` bytes in the provided allocator.
528    ///
529    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
530    /// of this method.
531    ///
532    /// # Examples
533    ///
534    /// ```
535    /// #![feature(allocator_api)]
536    ///
537    /// use std::alloc::System;
538    ///
539    /// let zero = Box::<u32, _>::new_zeroed_in(System);
540    /// let zero = unsafe { zero.assume_init() };
541    ///
542    /// assert_eq!(*zero, 0)
543    /// ```
544    ///
545    /// [zeroed]: mem::MaybeUninit::zeroed
546    #[unstable(feature = "allocator_api", issue = "32838")]
547    #[cfg(not(no_global_oom_handling))]
548    // #[unstable(feature = "new_uninit", issue = "63291")]
549    #[must_use]
550    pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
551    where
552        A: Allocator,
553    {
554        let layout = Layout::new::<mem::MaybeUninit<T>>();
555        // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
556        // That would make code size bigger.
557        match Box::try_new_zeroed_in(alloc) {
558            Ok(m) => m,
559            Err(_) => handle_alloc_error(layout),
560        }
561    }
562
563    /// Constructs a new `Box` with uninitialized contents, with the memory
564    /// being filled with `0` bytes in the provided allocator,
565    /// returning an error if the allocation fails,
566    ///
567    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
568    /// of this method.
569    ///
570    /// # Examples
571    ///
572    /// ```
573    /// #![feature(allocator_api)]
574    ///
575    /// use std::alloc::System;
576    ///
577    /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
578    /// let zero = unsafe { zero.assume_init() };
579    ///
580    /// assert_eq!(*zero, 0);
581    /// # Ok::<(), std::alloc::AllocError>(())
582    /// ```
583    ///
584    /// [zeroed]: mem::MaybeUninit::zeroed
585    #[unstable(feature = "allocator_api", issue = "32838")]
586    // #[unstable(feature = "new_uninit", issue = "63291")]
587    pub fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
588    where
589        A: Allocator,
590    {
591        let ptr = if T::IS_ZST {
592            NonNull::dangling()
593        } else {
594            let layout = Layout::new::<mem::MaybeUninit<T>>();
595            alloc.allocate_zeroed(layout)?.cast()
596        };
597        unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
598    }
599
600    /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then
601    /// `x` will be pinned in memory and unable to be moved.
602    ///
603    /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin_in(x, alloc)`
604    /// does the same as <code>[Box::into_pin]\([Box::new_in]\(x, alloc))</code>. Consider using
605    /// [`into_pin`](Box::into_pin) if you already have a `Box<T, A>`, or if you want to
606    /// construct a (pinned) `Box` in a different way than with [`Box::new_in`].
607    #[cfg(not(no_global_oom_handling))]
608    #[unstable(feature = "allocator_api", issue = "32838")]
609    #[must_use]
610    #[inline(always)]
611    pub fn pin_in(x: T, alloc: A) -> Pin<Self>
612    where
613        A: 'static + Allocator,
614    {
615        Self::into_pin(Self::new_in(x, alloc))
616    }
617
618    /// Converts a `Box<T>` into a `Box<[T]>`
619    ///
620    /// This conversion does not allocate on the heap and happens in place.
621    #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
622    pub fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
623        let (raw, alloc) = Box::into_raw_with_allocator(boxed);
624        unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
625    }
626
627    /// Consumes the `Box`, returning the wrapped value.
628    ///
629    /// # Examples
630    ///
631    /// ```
632    /// #![feature(box_into_inner)]
633    ///
634    /// let c = Box::new(5);
635    ///
636    /// assert_eq!(Box::into_inner(c), 5);
637    /// ```
638    #[unstable(feature = "box_into_inner", issue = "80437")]
639    #[inline]
640    pub fn into_inner(boxed: Self) -> T {
641        *boxed
642    }
643}
644
645impl<T> Box<[T]> {
646    /// Constructs a new boxed slice with uninitialized contents.
647    ///
648    /// # Examples
649    ///
650    /// ```
651    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
652    /// // Deferred initialization:
653    /// values[0].write(1);
654    /// values[1].write(2);
655    /// values[2].write(3);
656    /// let values = unsafe {values.assume_init() };
657    ///
658    /// assert_eq!(*values, [1, 2, 3])
659    /// ```
660    #[cfg(not(no_global_oom_handling))]
661    #[stable(feature = "new_uninit", since = "1.82.0")]
662    #[must_use]
663    pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
664        unsafe { RawVec::with_capacity(len).into_box(len) }
665    }
666
667    /// Constructs a new boxed slice with uninitialized contents, with the memory
668    /// being filled with `0` bytes.
669    ///
670    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
671    /// of this method.
672    ///
673    /// # Examples
674    ///
675    /// ```
676    /// #![feature(new_zeroed_alloc)]
677    ///
678    /// let values = Box::<[u32]>::new_zeroed_slice(3);
679    /// let values = unsafe { values.assume_init() };
680    ///
681    /// assert_eq!(*values, [0, 0, 0])
682    /// ```
683    ///
684    /// [zeroed]: mem::MaybeUninit::zeroed
685    #[cfg(not(no_global_oom_handling))]
686    #[unstable(feature = "new_zeroed_alloc", issue = "129396")]
687    #[must_use]
688    pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
689        unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
690    }
691
692    /// Constructs a new boxed slice with uninitialized contents. Returns an error if
693    /// the allocation fails.
694    ///
695    /// # Examples
696    ///
697    /// ```
698    /// #![feature(allocator_api)]
699    ///
700    /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
701    /// // Deferred initialization:
702    /// values[0].write(1);
703    /// values[1].write(2);
704    /// values[2].write(3);
705    /// let values = unsafe { values.assume_init() };
706    ///
707    /// assert_eq!(*values, [1, 2, 3]);
708    /// # Ok::<(), std::alloc::AllocError>(())
709    /// ```
710    #[unstable(feature = "allocator_api", issue = "32838")]
711    #[inline]
712    pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
713        let ptr = if T::IS_ZST || len == 0 {
714            NonNull::dangling()
715        } else {
716            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
717                Ok(l) => l,
718                Err(_) => return Err(AllocError),
719            };
720            Global.allocate(layout)?.cast()
721        };
722        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
723    }
724
725    /// Constructs a new boxed slice with uninitialized contents, with the memory
726    /// being filled with `0` bytes. Returns an error if the allocation fails.
727    ///
728    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
729    /// of this method.
730    ///
731    /// # Examples
732    ///
733    /// ```
734    /// #![feature(allocator_api)]
735    ///
736    /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
737    /// let values = unsafe { values.assume_init() };
738    ///
739    /// assert_eq!(*values, [0, 0, 0]);
740    /// # Ok::<(), std::alloc::AllocError>(())
741    /// ```
742    ///
743    /// [zeroed]: mem::MaybeUninit::zeroed
744    #[unstable(feature = "allocator_api", issue = "32838")]
745    #[inline]
746    pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
747        let ptr = if T::IS_ZST || len == 0 {
748            NonNull::dangling()
749        } else {
750            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
751                Ok(l) => l,
752                Err(_) => return Err(AllocError),
753            };
754            Global.allocate_zeroed(layout)?.cast()
755        };
756        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
757    }
758
759    /// Converts the boxed slice into a boxed array.
760    ///
761    /// This operation does not reallocate; the underlying array of the slice is simply reinterpreted as an array type.
762    ///
763    /// If `N` is not exactly equal to the length of `self`, then this method returns `None`.
764    #[unstable(feature = "slice_as_array", issue = "133508")]
765    #[inline]
766    #[must_use]
767    pub fn into_array<const N: usize>(self) -> Option<Box<[T; N]>> {
768        if self.len() == N {
769            let ptr = Self::into_raw(self) as *mut [T; N];
770
771            // 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.
772            let me = unsafe { Box::from_raw(ptr) };
773            Some(me)
774        } else {
775            None
776        }
777    }
778}
779
780impl<T, A: Allocator> Box<[T], A> {
781    /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
782    ///
783    /// # Examples
784    ///
785    /// ```
786    /// #![feature(allocator_api)]
787    ///
788    /// use std::alloc::System;
789    ///
790    /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
791    /// // Deferred initialization:
792    /// values[0].write(1);
793    /// values[1].write(2);
794    /// values[2].write(3);
795    /// let values = unsafe { values.assume_init() };
796    ///
797    /// assert_eq!(*values, [1, 2, 3])
798    /// ```
799    #[cfg(not(no_global_oom_handling))]
800    #[unstable(feature = "allocator_api", issue = "32838")]
801    // #[unstable(feature = "new_uninit", issue = "63291")]
802    #[must_use]
803    pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
804        unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
805    }
806
807    /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
808    /// with the memory being filled with `0` bytes.
809    ///
810    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
811    /// of this method.
812    ///
813    /// # Examples
814    ///
815    /// ```
816    /// #![feature(allocator_api)]
817    ///
818    /// use std::alloc::System;
819    ///
820    /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
821    /// let values = unsafe { values.assume_init() };
822    ///
823    /// assert_eq!(*values, [0, 0, 0])
824    /// ```
825    ///
826    /// [zeroed]: mem::MaybeUninit::zeroed
827    #[cfg(not(no_global_oom_handling))]
828    #[unstable(feature = "allocator_api", issue = "32838")]
829    // #[unstable(feature = "new_uninit", issue = "63291")]
830    #[must_use]
831    pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
832        unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
833    }
834
835    /// Constructs a new boxed slice with uninitialized contents in the provided allocator. Returns an error if
836    /// the allocation fails.
837    ///
838    /// # Examples
839    ///
840    /// ```
841    /// #![feature(allocator_api)]
842    ///
843    /// use std::alloc::System;
844    ///
845    /// let mut values = Box::<[u32], _>::try_new_uninit_slice_in(3, System)?;
846    /// // Deferred initialization:
847    /// values[0].write(1);
848    /// values[1].write(2);
849    /// values[2].write(3);
850    /// let values = unsafe { values.assume_init() };
851    ///
852    /// assert_eq!(*values, [1, 2, 3]);
853    /// # Ok::<(), std::alloc::AllocError>(())
854    /// ```
855    #[unstable(feature = "allocator_api", issue = "32838")]
856    #[inline]
857    pub fn try_new_uninit_slice_in(
858        len: usize,
859        alloc: A,
860    ) -> Result<Box<[mem::MaybeUninit<T>], A>, AllocError> {
861        let ptr = if T::IS_ZST || len == 0 {
862            NonNull::dangling()
863        } else {
864            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
865                Ok(l) => l,
866                Err(_) => return Err(AllocError),
867            };
868            alloc.allocate(layout)?.cast()
869        };
870        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, alloc).into_box(len)) }
871    }
872
873    /// Constructs a new boxed slice with uninitialized contents in the provided allocator, with the memory
874    /// being filled with `0` bytes. Returns an error if the allocation fails.
875    ///
876    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
877    /// of this method.
878    ///
879    /// # Examples
880    ///
881    /// ```
882    /// #![feature(allocator_api)]
883    ///
884    /// use std::alloc::System;
885    ///
886    /// let values = Box::<[u32], _>::try_new_zeroed_slice_in(3, System)?;
887    /// let values = unsafe { values.assume_init() };
888    ///
889    /// assert_eq!(*values, [0, 0, 0]);
890    /// # Ok::<(), std::alloc::AllocError>(())
891    /// ```
892    ///
893    /// [zeroed]: mem::MaybeUninit::zeroed
894    #[unstable(feature = "allocator_api", issue = "32838")]
895    #[inline]
896    pub fn try_new_zeroed_slice_in(
897        len: usize,
898        alloc: A,
899    ) -> Result<Box<[mem::MaybeUninit<T>], A>, AllocError> {
900        let ptr = if T::IS_ZST || len == 0 {
901            NonNull::dangling()
902        } else {
903            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
904                Ok(l) => l,
905                Err(_) => return Err(AllocError),
906            };
907            alloc.allocate_zeroed(layout)?.cast()
908        };
909        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, alloc).into_box(len)) }
910    }
911}
912
913impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
914    /// Converts to `Box<T, A>`.
915    ///
916    /// # Safety
917    ///
918    /// As with [`MaybeUninit::assume_init`],
919    /// it is up to the caller to guarantee that the value
920    /// really is in an initialized state.
921    /// Calling this when the content is not yet fully initialized
922    /// causes immediate undefined behavior.
923    ///
924    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
925    ///
926    /// # Examples
927    ///
928    /// ```
929    /// let mut five = Box::<u32>::new_uninit();
930    /// // Deferred initialization:
931    /// five.write(5);
932    /// let five: Box<u32> = unsafe { five.assume_init() };
933    ///
934    /// assert_eq!(*five, 5)
935    /// ```
936    #[stable(feature = "new_uninit", since = "1.82.0")]
937    #[inline]
938    pub unsafe fn assume_init(self) -> Box<T, A> {
939        let (raw, alloc) = Box::into_raw_with_allocator(self);
940        unsafe { Box::from_raw_in(raw as *mut T, alloc) }
941    }
942
943    /// Writes the value and converts to `Box<T, A>`.
944    ///
945    /// This method converts the box similarly to [`Box::assume_init`] but
946    /// writes `value` into it before conversion thus guaranteeing safety.
947    /// In some scenarios use of this method may improve performance because
948    /// the compiler may be able to optimize copying from stack.
949    ///
950    /// # Examples
951    ///
952    /// ```
953    /// #![feature(box_uninit_write)]
954    ///
955    /// let big_box = Box::<[usize; 1024]>::new_uninit();
956    ///
957    /// let mut array = [0; 1024];
958    /// for (i, place) in array.iter_mut().enumerate() {
959    ///     *place = i;
960    /// }
961    ///
962    /// // The optimizer may be able to elide this copy, so previous code writes
963    /// // to heap directly.
964    /// let big_box = Box::write(big_box, array);
965    ///
966    /// for (i, x) in big_box.iter().enumerate() {
967    ///     assert_eq!(*x, i);
968    /// }
969    /// ```
970    #[unstable(feature = "box_uninit_write", issue = "129397")]
971    #[inline]
972    pub fn write(mut boxed: Self, value: T) -> Box<T, A> {
973        unsafe {
974            (*boxed).write(value);
975            boxed.assume_init()
976        }
977    }
978}
979
980impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
981    /// Converts to `Box<[T], A>`.
982    ///
983    /// # Safety
984    ///
985    /// As with [`MaybeUninit::assume_init`],
986    /// it is up to the caller to guarantee that the values
987    /// really are in an initialized state.
988    /// Calling this when the content is not yet fully initialized
989    /// causes immediate undefined behavior.
990    ///
991    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
992    ///
993    /// # Examples
994    ///
995    /// ```
996    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
997    /// // Deferred initialization:
998    /// values[0].write(1);
999    /// values[1].write(2);
1000    /// values[2].write(3);
1001    /// let values = unsafe { values.assume_init() };
1002    ///
1003    /// assert_eq!(*values, [1, 2, 3])
1004    /// ```
1005    #[stable(feature = "new_uninit", since = "1.82.0")]
1006    #[inline]
1007    pub unsafe fn assume_init(self) -> Box<[T], A> {
1008        let (raw, alloc) = Box::into_raw_with_allocator(self);
1009        unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
1010    }
1011}
1012
1013impl<T: ?Sized> Box<T> {
1014    /// Constructs a box from a raw pointer.
1015    ///
1016    /// After calling this function, the raw pointer is owned by the
1017    /// resulting `Box`. Specifically, the `Box` destructor will call
1018    /// the destructor of `T` and free the allocated memory. For this
1019    /// to be safe, the memory must have been allocated in accordance
1020    /// with the [memory layout] used by `Box` .
1021    ///
1022    /// # Safety
1023    ///
1024    /// This function is unsafe because improper use may lead to
1025    /// memory problems. For example, a double-free may occur if the
1026    /// function is called twice on the same raw pointer.
1027    ///
1028    /// The raw pointer must point to a block of memory allocated by the global allocator.
1029    ///
1030    /// The safety conditions are described in the [memory layout] section.
1031    ///
1032    /// # Examples
1033    ///
1034    /// Recreate a `Box` which was previously converted to a raw pointer
1035    /// using [`Box::into_raw`]:
1036    /// ```
1037    /// let x = Box::new(5);
1038    /// let ptr = Box::into_raw(x);
1039    /// let x = unsafe { Box::from_raw(ptr) };
1040    /// ```
1041    /// Manually create a `Box` from scratch by using the global allocator:
1042    /// ```
1043    /// use std::alloc::{alloc, Layout};
1044    ///
1045    /// unsafe {
1046    ///     let ptr = alloc(Layout::new::<i32>()) as *mut i32;
1047    ///     // In general .write is required to avoid attempting to destruct
1048    ///     // the (uninitialized) previous contents of `ptr`, though for this
1049    ///     // simple example `*ptr = 5` would have worked as well.
1050    ///     ptr.write(5);
1051    ///     let x = Box::from_raw(ptr);
1052    /// }
1053    /// ```
1054    ///
1055    /// [memory layout]: self#memory-layout
1056    #[stable(feature = "box_raw", since = "1.4.0")]
1057    #[inline]
1058    #[must_use = "call `drop(Box::from_raw(ptr))` if you intend to drop the `Box`"]
1059    pub unsafe fn from_raw(raw: *mut T) -> Self {
1060        unsafe { Self::from_raw_in(raw, Global) }
1061    }
1062
1063    /// Constructs a box from a `NonNull` pointer.
1064    ///
1065    /// After calling this function, the `NonNull` pointer is owned by
1066    /// the resulting `Box`. Specifically, the `Box` destructor will call
1067    /// the destructor of `T` and free the allocated memory. For this
1068    /// to be safe, the memory must have been allocated in accordance
1069    /// with the [memory layout] used by `Box` .
1070    ///
1071    /// # Safety
1072    ///
1073    /// This function is unsafe because improper use may lead to
1074    /// memory problems. For example, a double-free may occur if the
1075    /// function is called twice on the same `NonNull` pointer.
1076    ///
1077    /// The non-null pointer must point to a block of memory allocated by the global allocator.
1078    ///
1079    /// The safety conditions are described in the [memory layout] section.
1080    ///
1081    /// # Examples
1082    ///
1083    /// Recreate a `Box` which was previously converted to a `NonNull`
1084    /// pointer using [`Box::into_non_null`]:
1085    /// ```
1086    /// #![feature(box_vec_non_null)]
1087    ///
1088    /// let x = Box::new(5);
1089    /// let non_null = Box::into_non_null(x);
1090    /// let x = unsafe { Box::from_non_null(non_null) };
1091    /// ```
1092    /// Manually create a `Box` from scratch by using the global allocator:
1093    /// ```
1094    /// #![feature(box_vec_non_null)]
1095    ///
1096    /// use std::alloc::{alloc, Layout};
1097    /// use std::ptr::NonNull;
1098    ///
1099    /// unsafe {
1100    ///     let non_null = NonNull::new(alloc(Layout::new::<i32>()).cast::<i32>())
1101    ///         .expect("allocation failed");
1102    ///     // In general .write is required to avoid attempting to destruct
1103    ///     // the (uninitialized) previous contents of `non_null`.
1104    ///     non_null.write(5);
1105    ///     let x = Box::from_non_null(non_null);
1106    /// }
1107    /// ```
1108    ///
1109    /// [memory layout]: self#memory-layout
1110    #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1111    #[inline]
1112    #[must_use = "call `drop(Box::from_non_null(ptr))` if you intend to drop the `Box`"]
1113    pub unsafe fn from_non_null(ptr: NonNull<T>) -> Self {
1114        unsafe { Self::from_raw(ptr.as_ptr()) }
1115    }
1116}
1117
1118impl<T: ?Sized, A: Allocator> Box<T, A> {
1119    /// Constructs a box from a raw pointer in the given allocator.
1120    ///
1121    /// After calling this function, the raw pointer is owned by the
1122    /// resulting `Box`. Specifically, the `Box` destructor will call
1123    /// the destructor of `T` and free the allocated memory. For this
1124    /// to be safe, the memory must have been allocated in accordance
1125    /// with the [memory layout] used by `Box` .
1126    ///
1127    /// # Safety
1128    ///
1129    /// This function is unsafe because improper use may lead to
1130    /// memory problems. For example, a double-free may occur if the
1131    /// function is called twice on the same raw pointer.
1132    ///
1133    /// The raw pointer must point to a block of memory allocated by `alloc`.
1134    ///
1135    /// # Examples
1136    ///
1137    /// Recreate a `Box` which was previously converted to a raw pointer
1138    /// using [`Box::into_raw_with_allocator`]:
1139    /// ```
1140    /// #![feature(allocator_api)]
1141    ///
1142    /// use std::alloc::System;
1143    ///
1144    /// let x = Box::new_in(5, System);
1145    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1146    /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
1147    /// ```
1148    /// Manually create a `Box` from scratch by using the system allocator:
1149    /// ```
1150    /// #![feature(allocator_api, slice_ptr_get)]
1151    ///
1152    /// use std::alloc::{Allocator, Layout, System};
1153    ///
1154    /// unsafe {
1155    ///     let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
1156    ///     // In general .write is required to avoid attempting to destruct
1157    ///     // the (uninitialized) previous contents of `ptr`, though for this
1158    ///     // simple example `*ptr = 5` would have worked as well.
1159    ///     ptr.write(5);
1160    ///     let x = Box::from_raw_in(ptr, System);
1161    /// }
1162    /// # Ok::<(), std::alloc::AllocError>(())
1163    /// ```
1164    ///
1165    /// [memory layout]: self#memory-layout
1166    #[unstable(feature = "allocator_api", issue = "32838")]
1167    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1168    #[inline]
1169    pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
1170        Box(unsafe { Unique::new_unchecked(raw) }, alloc)
1171    }
1172
1173    /// Constructs a box from a `NonNull` pointer in the given allocator.
1174    ///
1175    /// After calling this function, the `NonNull` pointer is owned by
1176    /// the resulting `Box`. Specifically, the `Box` destructor will call
1177    /// the destructor of `T` and free the allocated memory. For this
1178    /// to be safe, the memory must have been allocated in accordance
1179    /// with the [memory layout] used by `Box` .
1180    ///
1181    /// # Safety
1182    ///
1183    /// This function is unsafe because improper use may lead to
1184    /// memory problems. For example, a double-free may occur if the
1185    /// function is called twice on the same raw pointer.
1186    ///
1187    /// The non-null pointer must point to a block of memory allocated by `alloc`.
1188    ///
1189    /// # Examples
1190    ///
1191    /// Recreate a `Box` which was previously converted to a `NonNull` pointer
1192    /// using [`Box::into_non_null_with_allocator`]:
1193    /// ```
1194    /// #![feature(allocator_api, box_vec_non_null)]
1195    ///
1196    /// use std::alloc::System;
1197    ///
1198    /// let x = Box::new_in(5, System);
1199    /// let (non_null, alloc) = Box::into_non_null_with_allocator(x);
1200    /// let x = unsafe { Box::from_non_null_in(non_null, alloc) };
1201    /// ```
1202    /// Manually create a `Box` from scratch by using the system allocator:
1203    /// ```
1204    /// #![feature(allocator_api, box_vec_non_null, slice_ptr_get)]
1205    ///
1206    /// use std::alloc::{Allocator, Layout, System};
1207    ///
1208    /// unsafe {
1209    ///     let non_null = System.allocate(Layout::new::<i32>())?.cast::<i32>();
1210    ///     // In general .write is required to avoid attempting to destruct
1211    ///     // the (uninitialized) previous contents of `non_null`.
1212    ///     non_null.write(5);
1213    ///     let x = Box::from_non_null_in(non_null, System);
1214    /// }
1215    /// # Ok::<(), std::alloc::AllocError>(())
1216    /// ```
1217    ///
1218    /// [memory layout]: self#memory-layout
1219    #[unstable(feature = "allocator_api", issue = "32838")]
1220    // #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1221    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1222    #[inline]
1223    pub const unsafe fn from_non_null_in(raw: NonNull<T>, alloc: A) -> Self {
1224        // SAFETY: guaranteed by the caller.
1225        unsafe { Box::from_raw_in(raw.as_ptr(), alloc) }
1226    }
1227
1228    /// Consumes the `Box`, returning a wrapped raw pointer.
1229    ///
1230    /// The pointer will be properly aligned and non-null.
1231    ///
1232    /// After calling this function, the caller is responsible for the
1233    /// memory previously managed by the `Box`. In particular, the
1234    /// caller should properly destroy `T` and release the memory, taking
1235    /// into account the [memory layout] used by `Box`. The easiest way to
1236    /// do this is to convert the raw pointer back into a `Box` with the
1237    /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
1238    /// the cleanup.
1239    ///
1240    /// Note: this is an associated function, which means that you have
1241    /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
1242    /// is so that there is no conflict with a method on the inner type.
1243    ///
1244    /// # Examples
1245    /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
1246    /// for automatic cleanup:
1247    /// ```
1248    /// let x = Box::new(String::from("Hello"));
1249    /// let ptr = Box::into_raw(x);
1250    /// let x = unsafe { Box::from_raw(ptr) };
1251    /// ```
1252    /// Manual cleanup by explicitly running the destructor and deallocating
1253    /// the memory:
1254    /// ```
1255    /// use std::alloc::{dealloc, Layout};
1256    /// use std::ptr;
1257    ///
1258    /// let x = Box::new(String::from("Hello"));
1259    /// let ptr = Box::into_raw(x);
1260    /// unsafe {
1261    ///     ptr::drop_in_place(ptr);
1262    ///     dealloc(ptr as *mut u8, Layout::new::<String>());
1263    /// }
1264    /// ```
1265    /// Note: This is equivalent to the following:
1266    /// ```
1267    /// let x = Box::new(String::from("Hello"));
1268    /// let ptr = Box::into_raw(x);
1269    /// unsafe {
1270    ///     drop(Box::from_raw(ptr));
1271    /// }
1272    /// ```
1273    ///
1274    /// [memory layout]: self#memory-layout
1275    #[must_use = "losing the pointer will leak memory"]
1276    #[stable(feature = "box_raw", since = "1.4.0")]
1277    #[inline]
1278    pub fn into_raw(b: Self) -> *mut T {
1279        // Make sure Miri realizes that we transition from a noalias pointer to a raw pointer here.
1280        unsafe { &raw mut *&mut *Self::into_raw_with_allocator(b).0 }
1281    }
1282
1283    /// Consumes the `Box`, returning a wrapped `NonNull` pointer.
1284    ///
1285    /// The pointer will be properly aligned.
1286    ///
1287    /// After calling this function, the caller is responsible for the
1288    /// memory previously managed by the `Box`. In particular, the
1289    /// caller should properly destroy `T` and release the memory, taking
1290    /// into account the [memory layout] used by `Box`. The easiest way to
1291    /// do this is to convert the `NonNull` pointer back into a `Box` with the
1292    /// [`Box::from_non_null`] function, allowing the `Box` destructor to
1293    /// perform the cleanup.
1294    ///
1295    /// Note: this is an associated function, which means that you have
1296    /// to call it as `Box::into_non_null(b)` instead of `b.into_non_null()`.
1297    /// This is so that there is no conflict with a method on the inner type.
1298    ///
1299    /// # Examples
1300    /// Converting the `NonNull` pointer back into a `Box` with [`Box::from_non_null`]
1301    /// for automatic cleanup:
1302    /// ```
1303    /// #![feature(box_vec_non_null)]
1304    ///
1305    /// let x = Box::new(String::from("Hello"));
1306    /// let non_null = Box::into_non_null(x);
1307    /// let x = unsafe { Box::from_non_null(non_null) };
1308    /// ```
1309    /// Manual cleanup by explicitly running the destructor and deallocating
1310    /// the memory:
1311    /// ```
1312    /// #![feature(box_vec_non_null)]
1313    ///
1314    /// use std::alloc::{dealloc, Layout};
1315    ///
1316    /// let x = Box::new(String::from("Hello"));
1317    /// let non_null = Box::into_non_null(x);
1318    /// unsafe {
1319    ///     non_null.drop_in_place();
1320    ///     dealloc(non_null.as_ptr().cast::<u8>(), Layout::new::<String>());
1321    /// }
1322    /// ```
1323    /// Note: This is equivalent to the following:
1324    /// ```
1325    /// #![feature(box_vec_non_null)]
1326    ///
1327    /// let x = Box::new(String::from("Hello"));
1328    /// let non_null = Box::into_non_null(x);
1329    /// unsafe {
1330    ///     drop(Box::from_non_null(non_null));
1331    /// }
1332    /// ```
1333    ///
1334    /// [memory layout]: self#memory-layout
1335    #[must_use = "losing the pointer will leak memory"]
1336    #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1337    #[inline]
1338    pub fn into_non_null(b: Self) -> NonNull<T> {
1339        // SAFETY: `Box` is guaranteed to be non-null.
1340        unsafe { NonNull::new_unchecked(Self::into_raw(b)) }
1341    }
1342
1343    /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
1344    ///
1345    /// The pointer will be properly aligned and non-null.
1346    ///
1347    /// After calling this function, the caller is responsible for the
1348    /// memory previously managed by the `Box`. In particular, the
1349    /// caller should properly destroy `T` and release the memory, taking
1350    /// into account the [memory layout] used by `Box`. The easiest way to
1351    /// do this is to convert the raw pointer back into a `Box` with the
1352    /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
1353    /// the cleanup.
1354    ///
1355    /// Note: this is an associated function, which means that you have
1356    /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
1357    /// is so that there is no conflict with a method on the inner type.
1358    ///
1359    /// # Examples
1360    /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
1361    /// for automatic cleanup:
1362    /// ```
1363    /// #![feature(allocator_api)]
1364    ///
1365    /// use std::alloc::System;
1366    ///
1367    /// let x = Box::new_in(String::from("Hello"), System);
1368    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1369    /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
1370    /// ```
1371    /// Manual cleanup by explicitly running the destructor and deallocating
1372    /// the memory:
1373    /// ```
1374    /// #![feature(allocator_api)]
1375    ///
1376    /// use std::alloc::{Allocator, Layout, System};
1377    /// use std::ptr::{self, NonNull};
1378    ///
1379    /// let x = Box::new_in(String::from("Hello"), System);
1380    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1381    /// unsafe {
1382    ///     ptr::drop_in_place(ptr);
1383    ///     let non_null = NonNull::new_unchecked(ptr);
1384    ///     alloc.deallocate(non_null.cast(), Layout::new::<String>());
1385    /// }
1386    /// ```
1387    ///
1388    /// [memory layout]: self#memory-layout
1389    #[must_use = "losing the pointer will leak memory"]
1390    #[unstable(feature = "allocator_api", issue = "32838")]
1391    #[inline]
1392    pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
1393        let mut b = mem::ManuallyDrop::new(b);
1394        // We carefully get the raw pointer out in a way that Miri's aliasing model understands what
1395        // is happening: using the primitive "deref" of `Box`. In case `A` is *not* `Global`, we
1396        // want *no* aliasing requirements here!
1397        // In case `A` *is* `Global`, this does not quite have the right behavior; `into_raw`
1398        // works around that.
1399        let ptr = &raw mut **b;
1400        let alloc = unsafe { ptr::read(&b.1) };
1401        (ptr, alloc)
1402    }
1403
1404    /// Consumes the `Box`, returning a wrapped `NonNull` pointer and the allocator.
1405    ///
1406    /// The pointer will be properly aligned.
1407    ///
1408    /// After calling this function, the caller is responsible for the
1409    /// memory previously managed by the `Box`. In particular, the
1410    /// caller should properly destroy `T` and release the memory, taking
1411    /// into account the [memory layout] used by `Box`. The easiest way to
1412    /// do this is to convert the `NonNull` pointer back into a `Box` with the
1413    /// [`Box::from_non_null_in`] function, allowing the `Box` destructor to
1414    /// perform the cleanup.
1415    ///
1416    /// Note: this is an associated function, which means that you have
1417    /// to call it as `Box::into_non_null_with_allocator(b)` instead of
1418    /// `b.into_non_null_with_allocator()`. This is so that there is no
1419    /// conflict with a method on the inner type.
1420    ///
1421    /// # Examples
1422    /// Converting the `NonNull` pointer back into a `Box` with
1423    /// [`Box::from_non_null_in`] for automatic cleanup:
1424    /// ```
1425    /// #![feature(allocator_api, box_vec_non_null)]
1426    ///
1427    /// use std::alloc::System;
1428    ///
1429    /// let x = Box::new_in(String::from("Hello"), System);
1430    /// let (non_null, alloc) = Box::into_non_null_with_allocator(x);
1431    /// let x = unsafe { Box::from_non_null_in(non_null, alloc) };
1432    /// ```
1433    /// Manual cleanup by explicitly running the destructor and deallocating
1434    /// the memory:
1435    /// ```
1436    /// #![feature(allocator_api, box_vec_non_null)]
1437    ///
1438    /// use std::alloc::{Allocator, Layout, System};
1439    ///
1440    /// let x = Box::new_in(String::from("Hello"), System);
1441    /// let (non_null, alloc) = Box::into_non_null_with_allocator(x);
1442    /// unsafe {
1443    ///     non_null.drop_in_place();
1444    ///     alloc.deallocate(non_null.cast::<u8>(), Layout::new::<String>());
1445    /// }
1446    /// ```
1447    ///
1448    /// [memory layout]: self#memory-layout
1449    #[must_use = "losing the pointer will leak memory"]
1450    #[unstable(feature = "allocator_api", issue = "32838")]
1451    // #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1452    #[inline]
1453    pub fn into_non_null_with_allocator(b: Self) -> (NonNull<T>, A) {
1454        let (ptr, alloc) = Box::into_raw_with_allocator(b);
1455        // SAFETY: `Box` is guaranteed to be non-null.
1456        unsafe { (NonNull::new_unchecked(ptr), alloc) }
1457    }
1458
1459    #[unstable(
1460        feature = "ptr_internals",
1461        issue = "none",
1462        reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
1463    )]
1464    #[inline]
1465    #[doc(hidden)]
1466    pub fn into_unique(b: Self) -> (Unique<T>, A) {
1467        let (ptr, alloc) = Box::into_raw_with_allocator(b);
1468        unsafe { (Unique::from(&mut *ptr), alloc) }
1469    }
1470
1471    /// Returns a raw mutable pointer to the `Box`'s contents.
1472    ///
1473    /// The caller must ensure that the `Box` outlives the pointer this
1474    /// function returns, or else it will end up dangling.
1475    ///
1476    /// This method guarantees that for the purpose of the aliasing model, this method
1477    /// does not materialize a reference to the underlying memory, and thus the returned pointer
1478    /// will remain valid when mixed with other calls to [`as_ptr`] and [`as_mut_ptr`].
1479    /// Note that calling other methods that materialize references to the memory
1480    /// may still invalidate this pointer.
1481    /// See the example below for how this guarantee can be used.
1482    ///
1483    /// # Examples
1484    ///
1485    /// Due to the aliasing guarantee, the following code is legal:
1486    ///
1487    /// ```rust
1488    /// #![feature(box_as_ptr)]
1489    ///
1490    /// unsafe {
1491    ///     let mut b = Box::new(0);
1492    ///     let ptr1 = Box::as_mut_ptr(&mut b);
1493    ///     ptr1.write(1);
1494    ///     let ptr2 = Box::as_mut_ptr(&mut b);
1495    ///     ptr2.write(2);
1496    ///     // Notably, the write to `ptr2` did *not* invalidate `ptr1`:
1497    ///     ptr1.write(3);
1498    /// }
1499    /// ```
1500    ///
1501    /// [`as_mut_ptr`]: Self::as_mut_ptr
1502    /// [`as_ptr`]: Self::as_ptr
1503    #[unstable(feature = "box_as_ptr", issue = "129090")]
1504    #[rustc_never_returns_null_ptr]
1505    #[rustc_as_ptr]
1506    #[inline]
1507    pub fn as_mut_ptr(b: &mut Self) -> *mut T {
1508        // This is a primitive deref, not going through `DerefMut`, and therefore not materializing
1509        // any references.
1510        &raw mut **b
1511    }
1512
1513    /// Returns a raw pointer to the `Box`'s contents.
1514    ///
1515    /// The caller must ensure that the `Box` outlives the pointer this
1516    /// function returns, or else it will end up dangling.
1517    ///
1518    /// The caller must also ensure that the memory the pointer (non-transitively) points to
1519    /// is never written to (except inside an `UnsafeCell`) using this pointer or any pointer
1520    /// derived from it. If you need to mutate the contents of the `Box`, use [`as_mut_ptr`].
1521    ///
1522    /// This method guarantees that for the purpose of the aliasing model, this method
1523    /// does not materialize a reference to the underlying memory, and thus the returned pointer
1524    /// will remain valid when mixed with other calls to [`as_ptr`] and [`as_mut_ptr`].
1525    /// Note that calling other methods that materialize mutable references to the memory,
1526    /// as well as writing to this memory, may still invalidate this pointer.
1527    /// See the example below for how this guarantee can be used.
1528    ///
1529    /// # Examples
1530    ///
1531    /// Due to the aliasing guarantee, the following code is legal:
1532    ///
1533    /// ```rust
1534    /// #![feature(box_as_ptr)]
1535    ///
1536    /// unsafe {
1537    ///     let mut v = Box::new(0);
1538    ///     let ptr1 = Box::as_ptr(&v);
1539    ///     let ptr2 = Box::as_mut_ptr(&mut v);
1540    ///     let _val = ptr2.read();
1541    ///     // No write to this memory has happened yet, so `ptr1` is still valid.
1542    ///     let _val = ptr1.read();
1543    ///     // However, once we do a write...
1544    ///     ptr2.write(1);
1545    ///     // ... `ptr1` is no longer valid.
1546    ///     // This would be UB: let _val = ptr1.read();
1547    /// }
1548    /// ```
1549    ///
1550    /// [`as_mut_ptr`]: Self::as_mut_ptr
1551    /// [`as_ptr`]: Self::as_ptr
1552    #[unstable(feature = "box_as_ptr", issue = "129090")]
1553    #[rustc_never_returns_null_ptr]
1554    #[rustc_as_ptr]
1555    #[inline]
1556    pub fn as_ptr(b: &Self) -> *const T {
1557        // This is a primitive deref, not going through `DerefMut`, and therefore not materializing
1558        // any references.
1559        &raw const **b
1560    }
1561
1562    /// Returns a reference to the underlying allocator.
1563    ///
1564    /// Note: this is an associated function, which means that you have
1565    /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
1566    /// is so that there is no conflict with a method on the inner type.
1567    #[unstable(feature = "allocator_api", issue = "32838")]
1568    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1569    #[inline]
1570    pub const fn allocator(b: &Self) -> &A {
1571        &b.1
1572    }
1573
1574    /// Consumes and leaks the `Box`, returning a mutable reference,
1575    /// `&'a mut T`.
1576    ///
1577    /// Note that the type `T` must outlive the chosen lifetime `'a`. If the type
1578    /// has only static references, or none at all, then this may be chosen to be
1579    /// `'static`.
1580    ///
1581    /// This function is mainly useful for data that lives for the remainder of
1582    /// the program's life. Dropping the returned reference will cause a memory
1583    /// leak. If this is not acceptable, the reference should first be wrapped
1584    /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
1585    /// then be dropped which will properly destroy `T` and release the
1586    /// allocated memory.
1587    ///
1588    /// Note: this is an associated function, which means that you have
1589    /// to call it as `Box::leak(b)` instead of `b.leak()`. This
1590    /// is so that there is no conflict with a method on the inner type.
1591    ///
1592    /// # Examples
1593    ///
1594    /// Simple usage:
1595    ///
1596    /// ```
1597    /// let x = Box::new(41);
1598    /// let static_ref: &'static mut usize = Box::leak(x);
1599    /// *static_ref += 1;
1600    /// assert_eq!(*static_ref, 42);
1601    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
1602    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
1603    /// # drop(unsafe { Box::from_raw(static_ref) });
1604    /// ```
1605    ///
1606    /// Unsized data:
1607    ///
1608    /// ```
1609    /// let x = vec![1, 2, 3].into_boxed_slice();
1610    /// let static_ref = Box::leak(x);
1611    /// static_ref[0] = 4;
1612    /// assert_eq!(*static_ref, [4, 2, 3]);
1613    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
1614    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
1615    /// # drop(unsafe { Box::from_raw(static_ref) });
1616    /// ```
1617    #[stable(feature = "box_leak", since = "1.26.0")]
1618    #[inline]
1619    pub fn leak<'a>(b: Self) -> &'a mut T
1620    where
1621        A: 'a,
1622    {
1623        unsafe { &mut *Box::into_raw(b) }
1624    }
1625
1626    /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
1627    /// `*boxed` will be pinned in memory and unable to be moved.
1628    ///
1629    /// This conversion does not allocate on the heap and happens in place.
1630    ///
1631    /// This is also available via [`From`].
1632    ///
1633    /// Constructing and pinning a `Box` with <code>Box::into_pin([Box::new]\(x))</code>
1634    /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
1635    /// This `into_pin` method is useful if you already have a `Box<T>`, or you are
1636    /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
1637    ///
1638    /// # Notes
1639    ///
1640    /// It's not recommended that crates add an impl like `From<Box<T>> for Pin<T>`,
1641    /// as it'll introduce an ambiguity when calling `Pin::from`.
1642    /// A demonstration of such a poor impl is shown below.
1643    ///
1644    /// ```compile_fail
1645    /// # use std::pin::Pin;
1646    /// struct Foo; // A type defined in this crate.
1647    /// impl From<Box<()>> for Pin<Foo> {
1648    ///     fn from(_: Box<()>) -> Pin<Foo> {
1649    ///         Pin::new(Foo)
1650    ///     }
1651    /// }
1652    ///
1653    /// let foo = Box::new(());
1654    /// let bar = Pin::from(foo);
1655    /// ```
1656    #[stable(feature = "box_into_pin", since = "1.63.0")]
1657    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1658    pub const fn into_pin(boxed: Self) -> Pin<Self>
1659    where
1660        A: 'static,
1661    {
1662        // It's not possible to move or replace the insides of a `Pin<Box<T>>`
1663        // when `T: !Unpin`, so it's safe to pin it directly without any
1664        // additional requirements.
1665        unsafe { Pin::new_unchecked(boxed) }
1666    }
1667}
1668
1669#[stable(feature = "rust1", since = "1.0.0")]
1670unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
1671    #[inline]
1672    fn drop(&mut self) {
1673        // the T in the Box is dropped by the compiler before the destructor is run
1674
1675        let ptr = self.0;
1676
1677        unsafe {
1678            let layout = Layout::for_value_raw(ptr.as_ptr());
1679            if layout.size() != 0 {
1680                self.1.deallocate(From::from(ptr.cast()), layout);
1681            }
1682        }
1683    }
1684}
1685
1686#[cfg(not(no_global_oom_handling))]
1687#[stable(feature = "rust1", since = "1.0.0")]
1688impl<T: Default> Default for Box<T> {
1689    /// Creates a `Box<T>`, with the `Default` value for T.
1690    #[inline]
1691    fn default() -> Self {
1692        Box::write(Box::new_uninit(), T::default())
1693    }
1694}
1695
1696#[cfg(not(no_global_oom_handling))]
1697#[stable(feature = "rust1", since = "1.0.0")]
1698impl<T> Default for Box<[T]> {
1699    #[inline]
1700    fn default() -> Self {
1701        let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
1702        Box(ptr, Global)
1703    }
1704}
1705
1706#[cfg(not(no_global_oom_handling))]
1707#[stable(feature = "default_box_extra", since = "1.17.0")]
1708impl Default for Box<str> {
1709    #[inline]
1710    fn default() -> Self {
1711        // SAFETY: This is the same as `Unique::cast<U>` but with an unsized `U = str`.
1712        let ptr: Unique<str> = unsafe {
1713            let bytes: Unique<[u8]> = Unique::<[u8; 0]>::dangling();
1714            Unique::new_unchecked(bytes.as_ptr() as *mut str)
1715        };
1716        Box(ptr, Global)
1717    }
1718}
1719
1720#[cfg(not(no_global_oom_handling))]
1721#[stable(feature = "rust1", since = "1.0.0")]
1722impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
1723    /// Returns a new box with a `clone()` of this box's contents.
1724    ///
1725    /// # Examples
1726    ///
1727    /// ```
1728    /// let x = Box::new(5);
1729    /// let y = x.clone();
1730    ///
1731    /// // The value is the same
1732    /// assert_eq!(x, y);
1733    ///
1734    /// // But they are unique objects
1735    /// assert_ne!(&*x as *const i32, &*y as *const i32);
1736    /// ```
1737    #[inline]
1738    fn clone(&self) -> Self {
1739        // Pre-allocate memory to allow writing the cloned value directly.
1740        let mut boxed = Self::new_uninit_in(self.1.clone());
1741        unsafe {
1742            (**self).clone_to_uninit(boxed.as_mut_ptr().cast());
1743            boxed.assume_init()
1744        }
1745    }
1746
1747    /// Copies `source`'s contents into `self` without creating a new allocation.
1748    ///
1749    /// # Examples
1750    ///
1751    /// ```
1752    /// let x = Box::new(5);
1753    /// let mut y = Box::new(10);
1754    /// let yp: *const i32 = &*y;
1755    ///
1756    /// y.clone_from(&x);
1757    ///
1758    /// // The value is the same
1759    /// assert_eq!(x, y);
1760    ///
1761    /// // And no allocation occurred
1762    /// assert_eq!(yp, &*y);
1763    /// ```
1764    #[inline]
1765    fn clone_from(&mut self, source: &Self) {
1766        (**self).clone_from(&(**source));
1767    }
1768}
1769
1770#[cfg(not(no_global_oom_handling))]
1771#[stable(feature = "box_slice_clone", since = "1.3.0")]
1772impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
1773    fn clone(&self) -> Self {
1774        let alloc = Box::allocator(self).clone();
1775        self.to_vec_in(alloc).into_boxed_slice()
1776    }
1777
1778    /// Copies `source`'s contents into `self` without creating a new allocation,
1779    /// so long as the two are of the same length.
1780    ///
1781    /// # Examples
1782    ///
1783    /// ```
1784    /// let x = Box::new([5, 6, 7]);
1785    /// let mut y = Box::new([8, 9, 10]);
1786    /// let yp: *const [i32] = &*y;
1787    ///
1788    /// y.clone_from(&x);
1789    ///
1790    /// // The value is the same
1791    /// assert_eq!(x, y);
1792    ///
1793    /// // And no allocation occurred
1794    /// assert_eq!(yp, &*y);
1795    /// ```
1796    fn clone_from(&mut self, source: &Self) {
1797        if self.len() == source.len() {
1798            self.clone_from_slice(&source);
1799        } else {
1800            *self = source.clone();
1801        }
1802    }
1803}
1804
1805#[cfg(not(no_global_oom_handling))]
1806#[stable(feature = "box_slice_clone", since = "1.3.0")]
1807impl Clone for Box<str> {
1808    fn clone(&self) -> Self {
1809        // this makes a copy of the data
1810        let buf: Box<[u8]> = self.as_bytes().into();
1811        unsafe { from_boxed_utf8_unchecked(buf) }
1812    }
1813}
1814
1815#[stable(feature = "rust1", since = "1.0.0")]
1816impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
1817    #[inline]
1818    fn eq(&self, other: &Self) -> bool {
1819        PartialEq::eq(&**self, &**other)
1820    }
1821    #[inline]
1822    fn ne(&self, other: &Self) -> bool {
1823        PartialEq::ne(&**self, &**other)
1824    }
1825}
1826
1827#[stable(feature = "rust1", since = "1.0.0")]
1828impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
1829    #[inline]
1830    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1831        PartialOrd::partial_cmp(&**self, &**other)
1832    }
1833    #[inline]
1834    fn lt(&self, other: &Self) -> bool {
1835        PartialOrd::lt(&**self, &**other)
1836    }
1837    #[inline]
1838    fn le(&self, other: &Self) -> bool {
1839        PartialOrd::le(&**self, &**other)
1840    }
1841    #[inline]
1842    fn ge(&self, other: &Self) -> bool {
1843        PartialOrd::ge(&**self, &**other)
1844    }
1845    #[inline]
1846    fn gt(&self, other: &Self) -> bool {
1847        PartialOrd::gt(&**self, &**other)
1848    }
1849}
1850
1851#[stable(feature = "rust1", since = "1.0.0")]
1852impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
1853    #[inline]
1854    fn cmp(&self, other: &Self) -> Ordering {
1855        Ord::cmp(&**self, &**other)
1856    }
1857}
1858
1859#[stable(feature = "rust1", since = "1.0.0")]
1860impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}
1861
1862#[stable(feature = "rust1", since = "1.0.0")]
1863impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
1864    fn hash<H: Hasher>(&self, state: &mut H) {
1865        (**self).hash(state);
1866    }
1867}
1868
1869#[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
1870impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
1871    fn finish(&self) -> u64 {
1872        (**self).finish()
1873    }
1874    fn write(&mut self, bytes: &[u8]) {
1875        (**self).write(bytes)
1876    }
1877    fn write_u8(&mut self, i: u8) {
1878        (**self).write_u8(i)
1879    }
1880    fn write_u16(&mut self, i: u16) {
1881        (**self).write_u16(i)
1882    }
1883    fn write_u32(&mut self, i: u32) {
1884        (**self).write_u32(i)
1885    }
1886    fn write_u64(&mut self, i: u64) {
1887        (**self).write_u64(i)
1888    }
1889    fn write_u128(&mut self, i: u128) {
1890        (**self).write_u128(i)
1891    }
1892    fn write_usize(&mut self, i: usize) {
1893        (**self).write_usize(i)
1894    }
1895    fn write_i8(&mut self, i: i8) {
1896        (**self).write_i8(i)
1897    }
1898    fn write_i16(&mut self, i: i16) {
1899        (**self).write_i16(i)
1900    }
1901    fn write_i32(&mut self, i: i32) {
1902        (**self).write_i32(i)
1903    }
1904    fn write_i64(&mut self, i: i64) {
1905        (**self).write_i64(i)
1906    }
1907    fn write_i128(&mut self, i: i128) {
1908        (**self).write_i128(i)
1909    }
1910    fn write_isize(&mut self, i: isize) {
1911        (**self).write_isize(i)
1912    }
1913    fn write_length_prefix(&mut self, len: usize) {
1914        (**self).write_length_prefix(len)
1915    }
1916    fn write_str(&mut self, s: &str) {
1917        (**self).write_str(s)
1918    }
1919}
1920
1921#[stable(feature = "rust1", since = "1.0.0")]
1922impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
1923    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1924        fmt::Display::fmt(&**self, f)
1925    }
1926}
1927
1928#[stable(feature = "rust1", since = "1.0.0")]
1929impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
1930    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1931        fmt::Debug::fmt(&**self, f)
1932    }
1933}
1934
1935#[stable(feature = "rust1", since = "1.0.0")]
1936impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
1937    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1938        // It's not possible to extract the inner Uniq directly from the Box,
1939        // instead we cast it to a *const which aliases the Unique
1940        let ptr: *const T = &**self;
1941        fmt::Pointer::fmt(&ptr, f)
1942    }
1943}
1944
1945#[stable(feature = "rust1", since = "1.0.0")]
1946impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
1947    type Target = T;
1948
1949    fn deref(&self) -> &T {
1950        &**self
1951    }
1952}
1953
1954#[stable(feature = "rust1", since = "1.0.0")]
1955impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
1956    fn deref_mut(&mut self) -> &mut T {
1957        &mut **self
1958    }
1959}
1960
1961#[unstable(feature = "deref_pure_trait", issue = "87121")]
1962unsafe impl<T: ?Sized, A: Allocator> DerefPure for Box<T, A> {}
1963
1964#[unstable(feature = "legacy_receiver_trait", issue = "none")]
1965impl<T: ?Sized, A: Allocator> LegacyReceiver for Box<T, A> {}
1966
1967#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1968impl<Args: Tuple, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
1969    type Output = <F as FnOnce<Args>>::Output;
1970
1971    extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
1972        <F as FnOnce<Args>>::call_once(*self, args)
1973    }
1974}
1975
1976#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1977impl<Args: Tuple, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
1978    extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
1979        <F as FnMut<Args>>::call_mut(self, args)
1980    }
1981}
1982
1983#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1984impl<Args: Tuple, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
1985    extern "rust-call" fn call(&self, args: Args) -> Self::Output {
1986        <F as Fn<Args>>::call(self, args)
1987    }
1988}
1989
1990#[stable(feature = "async_closure", since = "1.85.0")]
1991impl<Args: Tuple, F: AsyncFnOnce<Args> + ?Sized, A: Allocator> AsyncFnOnce<Args> for Box<F, A> {
1992    type Output = F::Output;
1993    type CallOnceFuture = F::CallOnceFuture;
1994
1995    extern "rust-call" fn async_call_once(self, args: Args) -> Self::CallOnceFuture {
1996        F::async_call_once(*self, args)
1997    }
1998}
1999
2000#[stable(feature = "async_closure", since = "1.85.0")]
2001impl<Args: Tuple, F: AsyncFnMut<Args> + ?Sized, A: Allocator> AsyncFnMut<Args> for Box<F, A> {
2002    type CallRefFuture<'a>
2003        = F::CallRefFuture<'a>
2004    where
2005        Self: 'a;
2006
2007    extern "rust-call" fn async_call_mut(&mut self, args: Args) -> Self::CallRefFuture<'_> {
2008        F::async_call_mut(self, args)
2009    }
2010}
2011
2012#[stable(feature = "async_closure", since = "1.85.0")]
2013impl<Args: Tuple, F: AsyncFn<Args> + ?Sized, A: Allocator> AsyncFn<Args> for Box<F, A> {
2014    extern "rust-call" fn async_call(&self, args: Args) -> Self::CallRefFuture<'_> {
2015        F::async_call(self, args)
2016    }
2017}
2018
2019#[unstable(feature = "coerce_unsized", issue = "18598")]
2020impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
2021
2022#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2023unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Box<T, A> {}
2024
2025// It is quite crucial that we only allow the `Global` allocator here.
2026// Handling arbitrary custom allocators (which can affect the `Box` layout heavily!)
2027// would need a lot of codegen and interpreter adjustments.
2028#[unstable(feature = "dispatch_from_dyn", issue = "none")]
2029impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
2030
2031#[stable(feature = "box_borrow", since = "1.1.0")]
2032impl<T: ?Sized, A: Allocator> Borrow<T> for Box<T, A> {
2033    fn borrow(&self) -> &T {
2034        &**self
2035    }
2036}
2037
2038#[stable(feature = "box_borrow", since = "1.1.0")]
2039impl<T: ?Sized, A: Allocator> BorrowMut<T> for Box<T, A> {
2040    fn borrow_mut(&mut self) -> &mut T {
2041        &mut **self
2042    }
2043}
2044
2045#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2046impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
2047    fn as_ref(&self) -> &T {
2048        &**self
2049    }
2050}
2051
2052#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2053impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
2054    fn as_mut(&mut self) -> &mut T {
2055        &mut **self
2056    }
2057}
2058
2059/* Nota bene
2060 *
2061 *  We could have chosen not to add this impl, and instead have written a
2062 *  function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
2063 *  because Box<T> implements Unpin even when T does not, as a result of
2064 *  this impl.
2065 *
2066 *  We chose this API instead of the alternative for a few reasons:
2067 *      - Logically, it is helpful to understand pinning in regard to the
2068 *        memory region being pointed to. For this reason none of the
2069 *        standard library pointer types support projecting through a pin
2070 *        (Box<T> is the only pointer type in std for which this would be
2071 *        safe.)
2072 *      - It is in practice very useful to have Box<T> be unconditionally
2073 *        Unpin because of trait objects, for which the structural auto
2074 *        trait functionality does not apply (e.g., Box<dyn Foo> would
2075 *        otherwise not be Unpin).
2076 *
2077 *  Another type with the same semantics as Box but only a conditional
2078 *  implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
2079 *  could have a method to project a Pin<T> from it.
2080 */
2081#[stable(feature = "pin", since = "1.33.0")]
2082impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> {}
2083
2084#[unstable(feature = "coroutine_trait", issue = "43122")]
2085impl<G: ?Sized + Coroutine<R> + Unpin, R, A: Allocator> Coroutine<R> for Box<G, A> {
2086    type Yield = G::Yield;
2087    type Return = G::Return;
2088
2089    fn resume(mut self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return> {
2090        G::resume(Pin::new(&mut *self), arg)
2091    }
2092}
2093
2094#[unstable(feature = "coroutine_trait", issue = "43122")]
2095impl<G: ?Sized + Coroutine<R>, R, A: Allocator> Coroutine<R> for Pin<Box<G, A>>
2096where
2097    A: 'static,
2098{
2099    type Yield = G::Yield;
2100    type Return = G::Return;
2101
2102    fn resume(mut self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return> {
2103        G::resume((*self).as_mut(), arg)
2104    }
2105}
2106
2107#[stable(feature = "futures_api", since = "1.36.0")]
2108impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A> {
2109    type Output = F::Output;
2110
2111    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
2112        F::poll(Pin::new(&mut *self), cx)
2113    }
2114}
2115
2116#[stable(feature = "box_error", since = "1.8.0")]
2117impl<E: Error> Error for Box<E> {
2118    #[allow(deprecated, deprecated_in_future)]
2119    fn description(&self) -> &str {
2120        Error::description(&**self)
2121    }
2122
2123    #[allow(deprecated)]
2124    fn cause(&self) -> Option<&dyn Error> {
2125        Error::cause(&**self)
2126    }
2127
2128    fn source(&self) -> Option<&(dyn Error + 'static)> {
2129        Error::source(&**self)
2130    }
2131
2132    fn provide<'b>(&'b self, request: &mut error::Request<'b>) {
2133        Error::provide(&**self, request);
2134    }
2135}
2136
2137#[unstable(feature = "pointer_like_trait", issue = "none")]
2138impl<T> PointerLike for Box<T> {}