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rust/rust/704858caa7ac7b8564d87defbbca9dcbe7d31b33/./library/alloc/src/boxed.rs
blob: 8afe6806b3541d625b99c5a6ffde6cd8295d7c3f [file]
//! The `Box<T>` type for heap allocation.
//!
//! [`Box<T>`], casually referred to as a 'box', provides the simplest form of
//! heap allocation in Rust. Boxes provide ownership for this allocation, and
//! drop their contents when they go out of scope. Boxes also ensure that they
//! never allocate more than `isize::MAX` bytes.
//!
//! # Examples
//!
//! Move a value from the stack to the heap by creating a [`Box`]:
//!
//! ```
//! let val: u8 = 5;
//! let boxed: Box<u8> = Box::new(val);
//! ```
//!
//! Move a value from a [`Box`] back to the stack by [dereferencing]:
//!
//! ```
//! let boxed: Box<u8> = Box::new(5);
//! let val: u8 = *boxed;
//! ```
//!
//! Creating a recursive data structure:
//!
//! ```
//! # #[allow(dead_code)]
//! #[derive(Debug)]
//! enum List<T> {
//! Cons(T, Box<List<T>>),
//! Nil,
//! }
//!
//! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
//! println!("{list:?}");
//! ```
//!
//! This will print `Cons(1, Cons(2, Nil))`.
//!
//! Recursive structures must be boxed, because if the definition of `Cons`
//! looked like this:
//!
//! ```compile_fail,E0072
//! # enum List<T> {
//! Cons(T, List<T>),
//! # }
//! ```
//!
//! It wouldn't work. This is because the size of a `List` depends on how many
//! elements are in the list, and so we don't know how much memory to allocate
//! for a `Cons`. By introducing a [`Box<T>`], which has a defined size, we know how
//! big `Cons` needs to be.
//!
//! # Memory layout
//!
//! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for its allocation. It is
//! valid to convert both ways between a [`Box`] and a raw pointer allocated with the [`Global`]
//! allocator, given that the [`Layout`] used with the allocator is correct for the type and the raw
//! pointer points to a valid value of the right type. More precisely, a `value: *mut T` that has
//! been allocated with the [`Global`] allocator with `Layout::for_value(&*value)` may be converted
//! into a box using [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut T`
//! obtained from [`Box::<T>::into_raw`] may be deallocated using the [`Global`] allocator with
//! [`Layout::for_value(&*value)`].
//!
//! For zero-sized values, the `Box` pointer has to be non-null and sufficiently aligned. The
//! recommended way to build a Box to a ZST if `Box::new` cannot be used is to use
//! [`ptr::NonNull::dangling`].
//!
//! On top of these basic layout requirements, a `Box<T>` must point to a valid value of `T`.
//!
//! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented
//! as a single pointer and is also ABI-compatible with C pointers
//! (i.e. the C type `T*`). This means that if you have extern "C"
//! Rust functions that will be called from C, you can define those
//! Rust functions using `Box<T>` types, and use `T*` as corresponding
//! type on the C side. As an example, consider this C header which
//! declares functions that create and destroy some kind of `Foo`
//! value:
//!
//! ```c
//! /* C header */
//!
//! /* Returns ownership to the caller */
//! struct Foo* foo_new(void);
//!
//! /* Takes ownership from the caller; no-op when invoked with null */
//! void foo_delete(struct Foo*);
//! ```
//!
//! These two functions might be implemented in Rust as follows. Here, the
//! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
//! the ownership constraints. Note also that the nullable argument to
//! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
//! cannot be null.
//!
//! ```
//! #[repr(C)]
//! pub struct Foo;
//!
//! #[unsafe(no_mangle)]
//! pub extern "C" fn foo_new() -> Box<Foo> {
//! Box::new(Foo)
//! }
//!
//! #[unsafe(no_mangle)]
//! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
//! ```
//!
//! Even though `Box<T>` has the same representation and C ABI as a C pointer,
//! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
//! and expect things to work. `Box<T>` values will always be fully aligned,
//! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
//! free the value with the global allocator. In general, the best practice
//! is to only use `Box<T>` for pointers that originated from the global
//! allocator.
//!
//! **Important.** At least at present, you should avoid using
//! `Box<T>` types for functions that are defined in C but invoked
//! from Rust. In those cases, you should directly mirror the C types
//! as closely as possible. Using types like `Box<T>` where the C
//! definition is just using `T*` can lead to undefined behavior, as
//! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
//!
//! # Considerations for unsafe code
//!
//! **Warning: This section is not normative and is subject to change, possibly
//! being relaxed in the future! It is a simplified summary of the rules
//! currently implemented in the compiler.**
//!
//! The aliasing rules for `Box<T>` are the same as for `&mut T`. `Box<T>`
//! asserts uniqueness over its content. Using raw pointers derived from a box
//! after that box has been mutated through, moved or borrowed as `&mut T`
//! is not allowed. For more guidance on working with box from unsafe code, see
//! [rust-lang/unsafe-code-guidelines#326][ucg#326].
//!
//! # Editions
//!
//! A special case exists for the implementation of `IntoIterator` for arrays on the Rust 2021
//! edition, as documented [here][array]. Unfortunately, it was later found that a similar
//! workaround should be added for boxed slices, and this was applied in the 2024 edition.
//!
//! Specifically, `IntoIterator` is implemented for `Box<[T]>` on all editions, but specific calls
//! to `into_iter()` for boxed slices will defer to the slice implementation on editions before
//! 2024:
//!
//! ```rust,edition2021
//! // Rust 2015, 2018, and 2021:
//!
//! # #![allow(boxed_slice_into_iter)] // override our `deny(warnings)`
//! let boxed_slice: Box<[i32]> = vec![0; 3].into_boxed_slice();
//!
//! // This creates a slice iterator, producing references to each value.
//! for item in boxed_slice.into_iter().enumerate() {
//! let (i, x): (usize, &i32) = item;
//! println!("boxed_slice[{i}] = {x}");
//! }
//!
//! // The `boxed_slice_into_iter` lint suggests this change for future compatibility:
//! for item in boxed_slice.iter().enumerate() {
//! let (i, x): (usize, &i32) = item;
//! println!("boxed_slice[{i}] = {x}");
//! }
//!
//! // You can explicitly iterate a boxed slice by value using `IntoIterator::into_iter`
//! for item in IntoIterator::into_iter(boxed_slice).enumerate() {
//! let (i, x): (usize, i32) = item;
//! println!("boxed_slice[{i}] = {x}");
//! }
//! ```
//!
//! Similar to the array implementation, this may be modified in the future to remove this override,
//! and it's best to avoid relying on this edition-dependent behavior if you wish to preserve
//! compatibility with future versions of the compiler.
//!
//! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
//! [ucg#326]: https://github.com/rust-lang/unsafe-code-guidelines/issues/326
//! [dereferencing]: core::ops::Deref
//! [`Box::<T>::from_raw(value)`]: Box::from_raw
//! [`Global`]: crate::alloc::Global
//! [`Layout`]: crate::alloc::Layout
//! [`Layout::for_value(&*value)`]: crate::alloc::Layout::for_value
//! [valid]: ptr#safety
#![stable(feature = "rust1", since = "1.0.0")]
use core::borrow::{Borrow, BorrowMut};
use core::clone::CloneToUninit;
use core::cmp::Ordering;
use core::error::{self, Error};
use core::fmt;
use core::future::Future;
use core::hash::{Hash, Hasher};
use core::marker::{Tuple, Unsize};
#[cfg(not(no_global_oom_handling))]
use core::mem::MaybeUninit;
use core::mem::{self, SizedTypeProperties};
use core::ops::{
AsyncFn, AsyncFnMut, AsyncFnOnce, CoerceUnsized, Coroutine, CoroutineState, Deref, DerefMut,
DerefPure, DispatchFromDyn, LegacyReceiver,
};
#[cfg(not(no_global_oom_handling))]
use core::ops::{Residual, Try};
use core::pin::{Pin, PinSafePointer};
use core::ptr::{self, NonNull, Unique};
use core::task::{Context, Poll};
#[cfg(not(no_global_oom_handling))]
use crate::alloc::handle_alloc_error;
use crate::alloc::{AllocError, Allocator, Global, Layout, StaticAllocator};
use crate::raw_vec::RawVec;
#[cfg(not(no_global_oom_handling))]
use crate::str::from_boxed_utf8_unchecked_in;
/// Conversion related impls for `Box<_>` (`From`, `downcast`, etc)
mod convert;
/// Iterator related impls for `Box<_>`.
mod iter;
/// [`ThinBox`] implementation.
mod thin;
#[stable(feature = "boxed_array_value_iter", since = "1.99.0")]
pub use iter::BoxedArrayIntoIter;
#[unstable(feature = "thin_box", issue = "92791")]
pub use thin::ThinBox;
/// A pointer type that uniquely owns a heap allocation of type `T`.
///
/// See the [module-level documentation](../../std/boxed/index.html) for more.
#[lang = "owned_box"]
#[fundamental]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_insignificant_dtor]
#[doc(search_unbox)]
// The declaration of the `Box` struct must be kept in sync with the
// compiler or ICEs will happen.
pub struct Box<
T: ?Sized,
#[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
>(Unique<T>, A);
/// Monomorphic function for allocating an uninit `Box`.
#[inline]
// The is a separate function to avoid doing it in every generic version, but it
// looks small to the mir inliner (particularly in panic=abort) so leave it to
// the backend to decide whether pulling it in everywhere is worth doing.
#[rustc_no_mir_inline]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
#[cfg(not(no_global_oom_handling))]
#[rustc_const_unstable(feature = "const_heap", issue = "79597")]
const fn box_new_uninit(layout: Layout) -> *mut u8 {
match Global.allocate(layout) {
Ok(ptr) => ptr.as_mut_ptr(),
Err(_) => handle_alloc_error(layout),
}
}
/// Helper for `vec!`.
///
/// This is unsafe, but has to be marked as safe or else we couldn't use it in `vec!`.
#[doc(hidden)]
#[unstable(feature = "liballoc_internals", issue = "none")]
#[rustc_const_unstable(feature = "const_heap", issue = "79597")]
#[inline(always)]
#[cfg(not(no_global_oom_handling))]
#[rustc_diagnostic_item = "box_assume_init_into_vec_unsafe"]
pub const fn box_assume_init_into_vec_unsafe<T, const N: usize>(
b: Box<MaybeUninit<[T; N]>>,
) -> crate::vec::Vec<T> {
// SAFETY: Technically not, but this can't be
// called stably except in ways we control.
unsafe { (b.assume_init() as Box<[T]>).into_vec() }
}
impl<T> Box<T> {
/// Allocates memory on the heap and then places `x` into it.
///
/// This doesn't actually allocate if `T` is zero-sized.
///
/// # Examples
///
/// ```
/// let five = Box::new(5);
/// ```
#[cfg(not(no_global_oom_handling))]
#[inline(always)]
#[stable(feature = "rust1", since = "1.0.0")]
#[must_use]
#[rustc_diagnostic_item = "box_new"]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub fn new(x: T) -> Self {
// This is `Box::new_uninit` but inlined to avoid build time regressions.
let ptr = box_new_uninit(<T as SizedTypeProperties>::LAYOUT) as *mut T;
// Nothing below can panic so we do not have to worry about deallocating `ptr`.
// SAFETY: we just allocated the box to store `x`.
unsafe { core::intrinsics::write_via_move(ptr, x) };
// SAFETY: we just initialized the memory `ptr` points to.
unsafe { mem::transmute(ptr) }
}
/// Constructs a new box with uninitialized contents.
///
/// # Examples
///
/// ```
/// let mut five = Box::<u32>::new_uninit();
/// // Deferred initialization:
/// five.write(5);
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5)
/// ```
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "new_uninit", since = "1.82.0")]
#[rustc_const_unstable(feature = "const_heap", issue = "79597")]
#[must_use]
#[inline(always)]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub const fn new_uninit() -> Box<mem::MaybeUninit<T>> {
// This is the same as `Self::new_uninit_in(Global)`, but manually inlined (just like
// `Box::new`).
// SAFETY:
// - If `allocate` succeeds, the returned pointer exactly matches what `Box` needs.
unsafe { mem::transmute(box_new_uninit(<T as SizedTypeProperties>::LAYOUT)) }
}
/// Constructs a new `Box` with uninitialized contents, with the memory
/// being filled with `0` bytes.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// let zero = Box::<u32>::new_zeroed();
/// let zero = unsafe { zero.assume_init() };
///
/// assert_eq!(*zero, 0)
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[cfg(not(no_global_oom_handling))]
#[inline]
#[stable(feature = "new_zeroed_alloc", since = "1.92.0")]
#[must_use]
pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
Self::new_zeroed_in(Global)
}
/// Constructs a new `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
/// `x` will be pinned in memory and unable to be moved.
///
/// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin(x)`
/// does the same as <code>[Box::into_pin]\([Box::new]\(x))</code>. Consider using
/// [`into_pin`](Box::into_pin) if you already have a `Box<T>`, or if you want to
/// construct a (pinned) `Box` in a different way than with [`Box::new`].
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "pin", since = "1.33.0")]
#[must_use]
#[inline(always)]
pub fn pin(x: T) -> Pin<Box<T>> {
Box::new(x).into()
}
/// Allocates memory on the heap then places `x` into it,
/// returning an error if the allocation fails
///
/// This doesn't actually allocate if `T` is zero-sized.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// let five = Box::try_new(5)?;
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new(x: T) -> Result<Self, AllocError> {
Self::try_new_in(x, Global)
}
/// Constructs a new box with uninitialized contents on the heap,
/// returning an error if the allocation fails
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// let mut five = Box::<u32>::try_new_uninit()?;
/// // Deferred initialization:
/// five.write(5);
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
Box::try_new_uninit_in(Global)
}
/// Constructs a new `Box` with uninitialized contents, with the memory
/// being filled with `0` bytes on the heap
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// let zero = Box::<u32>::try_new_zeroed()?;
/// let zero = unsafe { zero.assume_init() };
///
/// assert_eq!(*zero, 0);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
Box::try_new_zeroed_in(Global)
}
}
impl<T, A: Allocator> Box<T, A> {
/// Allocates memory in the given allocator then places `x` into it.
///
/// This doesn't actually allocate if `T` is zero-sized.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let five = Box::new_in(5, System);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "allocator_api", issue = "32838")]
#[must_use]
#[inline]
pub fn new_in(x: T, alloc: A) -> Self
where
A: Allocator,
{
let mut boxed = Self::new_uninit_in(alloc);
boxed.write(x);
// SAFETY: Initialised by the above.
unsafe { boxed.assume_init() }
}
/// Allocates memory in the given allocator then places `x` into it,
/// returning an error if the allocation fails
///
/// This doesn't actually allocate if `T` is zero-sized.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let five = Box::try_new_in(5, System)?;
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
where
A: Allocator,
{
let mut boxed = Self::try_new_uninit_in(alloc)?;
boxed.write(x);
// SAFETY: Initialised by the above.
unsafe { Ok(boxed.assume_init()) }
}
/// Constructs a new box with uninitialized contents in the provided allocator.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let mut five = Box::<u32, _>::new_uninit_in(System);
/// // Deferred initialization:
/// five.write(5);
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5)
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
#[cfg(not(no_global_oom_handling))]
#[must_use]
pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
where
A: Allocator,
{
let layout = Layout::new::<mem::MaybeUninit<T>>();
// NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
// That would make code size bigger.
match Box::try_new_uninit_in(alloc) {
Ok(m) => m,
Err(_) => handle_alloc_error(layout),
}
}
/// Constructs a new box with uninitialized contents in the provided allocator,
/// returning an error if the allocation fails
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
/// // Deferred initialization:
/// five.write(5);
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
pub fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
where
A: Allocator,
{
let ptr = if T::IS_ZST {
NonNull::dangling()
} else {
let layout = Layout::new::<mem::MaybeUninit<T>>();
alloc.allocate(layout)?.cast()
};
// SAFETY: Pointer is nonnull and matches the allocator.
unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
}
/// Constructs a new `Box` with uninitialized contents, with the memory
/// being filled with `0` bytes in the provided allocator.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let zero = Box::<u32, _>::new_zeroed_in(System);
/// let zero = unsafe { zero.assume_init() };
///
/// assert_eq!(*zero, 0)
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[unstable(feature = "allocator_api", issue = "32838")]
#[cfg(not(no_global_oom_handling))]
#[must_use]
pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
where
A: Allocator,
{
let layout = Layout::new::<mem::MaybeUninit<T>>();
// NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
// That would make code size bigger.
match Box::try_new_zeroed_in(alloc) {
Ok(m) => m,
Err(_) => handle_alloc_error(layout),
}
}
/// Constructs a new `Box` with uninitialized contents, with the memory
/// being filled with `0` bytes in the provided allocator,
/// returning an error if the allocation fails,
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
/// let zero = unsafe { zero.assume_init() };
///
/// assert_eq!(*zero, 0);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[unstable(feature = "allocator_api", issue = "32838")]
pub fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
where
A: Allocator,
{
let ptr = if T::IS_ZST {
NonNull::dangling()
} else {
let layout = Layout::new::<mem::MaybeUninit<T>>();
alloc.allocate_zeroed(layout)?.cast()
};
// SAFETY: Pointer is nonnull and matches the allocator.
unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
}
/// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then
/// `x` will be pinned in memory and unable to be moved.
///
/// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin_in(x, alloc)`
/// does the same as <code>[Box::into_pin]\([Box::new_in]\(x, alloc))</code>. Consider using
/// [`into_pin`](Box::into_pin) if you already have a `Box<T, A>`, or if you want to
/// construct a (pinned) `Box` in a different way than with [`Box::new_in`].
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
/// use std::alloc::System;
///
/// let x = Box::pin_in(1, System);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "allocator_api", issue = "32838")]
#[must_use]
#[inline(always)]
pub fn pin_in(x: T, alloc: A) -> Pin<Self>
where
A: StaticAllocator,
{
Self::into_pin(Self::new_in(x, alloc))
}
/// Converts a `Box<T>` into a `Box<[T]>`
///
/// This conversion does not allocate on the heap and happens in place.
#[unstable(feature = "box_into_boxed_slice", issue = "71582")]
pub fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
let (raw, alloc) = Box::into_raw_with_allocator(boxed);
// SAFETY: A pointer to T is also a valid pointer to [T; 1].
unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
}
/// Consumes the `Box`, returning the wrapped value.
///
/// # Examples
///
/// ```
/// #![feature(box_into_inner)]
///
/// let c = Box::new(5);
///
/// assert_eq!(Box::into_inner(c), 5);
/// ```
#[unstable(feature = "box_into_inner", issue = "80437")]
#[inline]
pub fn into_inner(boxed: Self) -> T {
*boxed
}
/// Consumes the `Box` without consuming its allocation, returning the wrapped value and a `Box`
/// to the uninitialized memory where the wrapped value used to live.
///
/// This can be used together with [`write`](Box::write) to reuse the allocation for multiple
/// boxed values.
///
/// # Examples
///
/// ```
/// #![feature(box_take)]
///
/// let c = Box::new(5);
///
/// // take the value out of the box
/// let (value, uninit) = Box::take(c);
/// assert_eq!(value, 5);
///
/// // reuse the box for a second value
/// let c = Box::write(uninit, 6);
/// assert_eq!(*c, 6);
/// ```
#[unstable(feature = "box_take", issue = "147212")]
pub fn take(boxed: Self) -> (T, Box<mem::MaybeUninit<T>, A>) {
// SAFETY: Reading out an initialised value & leaving behind a
// box with uninit contents.
unsafe {
let (raw, alloc) = Box::into_non_null_with_allocator(boxed);
let value = raw.read();
let uninit = Box::from_non_null_in(raw.cast_uninit(), alloc);
(value, uninit)
}
}
/// Maps the value in a box, reusing the allocation if possible.
///
/// `f` is called on the value in the box, and the result is returned, also boxed.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Box::map(b, f)` instead of `b.map(f)`. This
/// is so that there is no conflict with a method on the inner type.
///
/// # Examples
///
/// ```
/// let b = Box::new(7);
/// let new = Box::map(b, |i| i + 7);
/// assert_eq!(*new, 14);
/// ```
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "smart_pointer_map", since = "CURRENT_RUSTC_VERSION")]
pub fn map<U>(this: Self, f: impl FnOnce(T) -> U) -> Box<U, A> {
let (value, allocation) = Box::take(this);
let (raw, alloc) = Box::into_non_null_with_allocator(allocation);
if size_of::<T>() == size_of::<U>() && align_of::<T>() == align_of::<U>() {
// SAFETY: We checked that the memory requirements are the same for both types
// and `raw` is already a valid pointer for the requisite memory.
let allocation = unsafe { Box::from_non_null_in(raw.cast::<MaybeUninit<U>>(), alloc) };
Box::write(allocation, f(value))
} else {
if size_of::<T>() != 0 {
// SAFETY: `raw` isn't dangling since it points to a non-zero-sized
// allocation and is never used again after this point.
unsafe { alloc.deallocate(raw.cast(), Layout::for_value(&value)) }
}
Box::new_in(f(value), alloc)
}
}
/// Attempts to map the value in a box, reusing the allocation if possible.
///
/// `f` is called on the value in the box, and if the operation succeeds, the result is
/// returned, also boxed.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Box::try_map(b, f)` instead of `b.try_map(f)`. This
/// is so that there is no conflict with a method on the inner type.
///
/// # Examples
///
/// ```
/// #![feature(smart_pointer_try_map)]
///
/// let b = Box::new(7);
/// let new = Box::try_map(b, u32::try_from).unwrap();
/// assert_eq!(*new, 7);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "smart_pointer_try_map", issue = "144419")]
pub fn try_map<R>(
this: Self,
f: impl FnOnce(T) -> R,
) -> <R::Residual as Residual<Box<R::Output, A>>>::TryType
where
R: Try,
R::Residual: Residual<Box<R::Output, A>>,
{
let (value, allocation) = Box::take(this);
let (raw, alloc) = Box::into_non_null_with_allocator(allocation);
if size_of::<T>() == size_of::<R::Output>() && align_of::<T>() == align_of::<R::Output>() {
let allocation =
// SAFETY: We checked that the memory requirements are the same for both types
// and `raw` is already a valid pointer for the requisite memory.
unsafe { Box::from_non_null_in(raw.cast::<MaybeUninit<R::Output>>(), alloc) };
try { Box::write(allocation, f(value)?) }
} else {
if size_of::<T>() != 0 {
// SAFETY: `raw` isn't dangling since it points to a non-zero-sized
// allocation and is never used again after this point.
unsafe { alloc.deallocate(raw.cast(), Layout::for_value(&value)) }
}
try { Box::new_in(f(value)?, alloc) }
}
}
}
impl<T: ?Sized + CloneToUninit> Box<T> {
/// Allocates memory on the heap then clones `src` into it.
///
/// This doesn't actually allocate if `src` is zero-sized.
///
/// # Examples
///
/// ```
/// #![feature(clone_from_ref)]
///
/// let hello: Box<str> = Box::clone_from_ref("hello");
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "clone_from_ref", issue = "149075")]
#[must_use]
#[inline]
pub fn clone_from_ref(src: &T) -> Box<T> {
Box::clone_from_ref_in(src, Global)
}
/// Allocates memory on the heap then clones `src` into it, returning an error if allocation fails.
///
/// This doesn't actually allocate if `src` is zero-sized.
///
/// # Examples
///
/// ```
/// #![feature(clone_from_ref)]
/// #![feature(allocator_api)]
///
/// let hello: Box<str> = Box::try_clone_from_ref("hello")?;
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "clone_from_ref", issue = "149075")]
//#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_clone_from_ref(src: &T) -> Result<Box<T>, AllocError> {
Box::try_clone_from_ref_in(src, Global)
}
}
impl<T: ?Sized + CloneToUninit, A: Allocator> Box<T, A> {
/// Allocates memory in the given allocator then clones `src` into it.
///
/// This doesn't actually allocate if `src` is zero-sized.
///
/// # Examples
///
/// ```
/// #![feature(clone_from_ref)]
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let hello: Box<str, System> = Box::clone_from_ref_in("hello", System);
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "clone_from_ref", issue = "149075")]
//#[unstable(feature = "allocator_api", issue = "32838")]
#[must_use]
#[inline]
pub fn clone_from_ref_in(src: &T, alloc: A) -> Box<T, A> {
let layout = Layout::for_value::<T>(src);
match Box::try_clone_from_ref_in(src, alloc) {
Ok(bx) => bx,
Err(_) => handle_alloc_error(layout),
}
}
/// Allocates memory in the given allocator then clones `src` into it, returning an error if allocation fails.
///
/// This doesn't actually allocate if `src` is zero-sized.
///
/// # Examples
///
/// ```
/// #![feature(clone_from_ref)]
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let hello: Box<str, System> = Box::try_clone_from_ref_in("hello", System)?;
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "clone_from_ref", issue = "149075")]
//#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_clone_from_ref_in(src: &T, alloc: A) -> Result<Box<T, A>, AllocError> {
struct DeallocDropGuard<'a, A: Allocator>(Layout, &'a A, NonNull<u8>);
impl<'a, A: Allocator> Drop for DeallocDropGuard<'a, A> {
fn drop(&mut self) {
let &mut DeallocDropGuard(layout, alloc, ptr) = self;
// SAFETY: `ptr` was allocated by `*alloc` with layout `layout`
unsafe {
alloc.deallocate(ptr, layout);
}
}
}
let layout = Layout::for_value::<T>(src);
let (ptr, guard) = if layout.size() == 0 {
(layout.dangling_ptr(), None)
} else {
// Safety: layout is non-zero-sized
let ptr = alloc.allocate(layout)?.cast();
(ptr, Some(DeallocDropGuard(layout, &alloc, ptr)))
};
let ptr = ptr.as_ptr();
// SAFETY: `*ptr` is newly allocated (or a ZST), correctly aligned to
// `align_of_val(src)`, and is valid for writes for `size_of_val(src)`.
// If this panics, then `guard` will deallocate for us (if allocation occuured)
unsafe {
<T as CloneToUninit>::clone_to_uninit(src, ptr);
}
// Defuse the deallocate guard
core::mem::forget(guard);
// SAFETY: We just initialized `*ptr` as a clone of `src`
Ok(unsafe { Box::from_raw_in(ptr.with_metadata_of(src), alloc) })
}
}
impl<T> Box<[T]> {
/// Constructs a new boxed slice with uninitialized contents.
///
/// # Examples
///
/// ```
/// let mut values = Box::<[u32]>::new_uninit_slice(3);
/// // Deferred initialization:
/// values[0].write(1);
/// values[1].write(2);
/// values[2].write(3);
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [1, 2, 3])
/// ```
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "new_uninit", since = "1.82.0")]
#[must_use]
pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
// SAFETY: `len` is exactly the capacity of this `RawVec`.
unsafe { RawVec::with_capacity(len).into_box(len) }
}
/// Constructs a new boxed slice with uninitialized contents, with the memory
/// being filled with `0` bytes.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// let values = Box::<[u32]>::new_zeroed_slice(3);
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [0, 0, 0])
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "new_zeroed_alloc", since = "1.92.0")]
#[must_use]
pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
// SAFETY: `len` is exactly the capacity of this `RawVec`.
unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
}
/// Constructs a new boxed slice with uninitialized contents. Returns an error if
/// the allocation fails.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
/// // Deferred initialization:
/// values[0].write(1);
/// values[1].write(2);
/// values[2].write(3);
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [1, 2, 3]);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
let ptr = if T::IS_ZST || len == 0 {
NonNull::dangling()
} else {
let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
Ok(l) => l,
Err(_) => return Err(AllocError),
};
Global.allocate(layout)?.cast()
};
// SAFETY: `ptr` was just allocated with `Global` with the layout for an array of length
// `len`, and the layout creation would have failed if `len` overflowed an isize.
// `into_box` is sound to call since `len` corresponds to the length of the just-created
// `RawVec`.
unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
}
/// Constructs a new boxed slice with uninitialized contents, with the memory
/// being filled with `0` bytes. Returns an error if the allocation fails.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [0, 0, 0]);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
let ptr = if T::IS_ZST || len == 0 {
NonNull::dangling()
} else {
let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
Ok(l) => l,
Err(_) => return Err(AllocError),
};
Global.allocate_zeroed(layout)?.cast()
};
// SAFETY: `ptr` was just allocated with `Global` with the layout for an array of length
// `len`, and the layout creation would have failed if `len` overflowed an isize.
// `into_box` is sound to call since `len` corresponds to the length of the just-created
// `RawVec`.
unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
}
}
impl<T, A: Allocator> Box<[T], A> {
/// Constructs a new boxed slice with uninitialized contents in the provided allocator.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
/// // Deferred initialization:
/// values[0].write(1);
/// values[1].write(2);
/// values[2].write(3);
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [1, 2, 3])
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "allocator_api", issue = "32838")]
#[must_use]
pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
// SAFETY: `len` is exactly the capacity of this `RawVec`.
unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
}
/// Constructs a new boxed slice with uninitialized contents in the provided allocator,
/// with the memory being filled with `0` bytes.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [0, 0, 0])
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "allocator_api", issue = "32838")]
#[must_use]
pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
// SAFETY: `len` is exactly the capacity of this `RawVec`.
unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
}
/// Constructs a new boxed slice with uninitialized contents in the provided allocator. Returns an error if
/// the allocation fails.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let mut values = Box::<[u32], _>::try_new_uninit_slice_in(3, System)?;
/// // Deferred initialization:
/// values[0].write(1);
/// values[1].write(2);
/// values[2].write(3);
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [1, 2, 3]);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new_uninit_slice_in(
len: usize,
alloc: A,
) -> Result<Box<[mem::MaybeUninit<T>], A>, AllocError> {
let ptr = if T::IS_ZST || len == 0 {
NonNull::dangling()
} else {
let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
Ok(l) => l,
Err(_) => return Err(AllocError),
};
alloc.allocate(layout)?.cast()
};
// SAFETY: `ptr` was just allocated with `alloc` with the layout for an array of length
// `len`, and the layout creation would have failed if `len` overflowed an isize.
// `into_box` is sound to call since `len` corresponds to the length of the just-created
// `RawVec`.
unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, alloc).into_box(len)) }
}
/// Constructs a new boxed slice with uninitialized contents in the provided allocator, with the memory
/// being filled with `0` bytes. Returns an error if the allocation fails.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
/// of this method.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let values = Box::<[u32], _>::try_new_zeroed_slice_in(3, System)?;
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [0, 0, 0]);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn try_new_zeroed_slice_in(
len: usize,
alloc: A,
) -> Result<Box<[mem::MaybeUninit<T>], A>, AllocError> {
let ptr = if T::IS_ZST || len == 0 {
NonNull::dangling()
} else {
let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
Ok(l) => l,
Err(_) => return Err(AllocError),
};
alloc.allocate_zeroed(layout)?.cast()
};
// SAFETY: `ptr` was just allocated with `alloc` with the layout for an array of length
// `len`, and the layout creation would have failed if `len` overflowed an isize.
// `into_box` is sound to call since `len` corresponds to the length of the just-created
// `RawVec`.
unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, alloc).into_box(len)) }
}
/// Converts the boxed slice into a boxed array.
///
/// This operation does not reallocate; the underlying array of the slice is simply reinterpreted as an array type.
///
/// # Errors
///
/// Returns the original `Box<[T]>` in the `Err` variant if `self.len()` does not equal `N`.
///
/// # Examples
///
/// ```
/// #![feature(alloc_slice_into_array)]
/// let box_slice: Box<[i32]> = Box::new([1, 2, 3]);
///
/// let box_array: Box<[i32; 3]> = box_slice.into_array().unwrap();
/// ```
#[unstable(feature = "alloc_slice_into_array", issue = "148082")]
#[inline]
pub fn into_array<const N: usize>(self) -> Result<Box<[T; N], A>, Self> {
if self.len() == N {
let (ptr, alloc) = Self::into_raw_with_allocator(self);
let ptr = ptr as *mut [T; N];
// 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.
let me = unsafe { Box::from_raw_in(ptr, alloc) };
Ok(me)
} else {
Err(self)
}
}
}
impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
/// Converts to `Box<T, A>`.
///
/// # Safety
///
/// As with [`MaybeUninit::assume_init`],
/// it is up to the caller to guarantee that the value
/// really is in an initialized state.
/// Calling this when the content is not yet fully initialized
/// causes immediate undefined behavior.
///
/// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
///
/// # Examples
///
/// ```
/// let mut five = Box::<u32>::new_uninit();
/// // Deferred initialization:
/// five.write(5);
/// let five: Box<u32> = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5)
/// ```
#[stable(feature = "new_uninit", since = "1.82.0")]
#[rustc_const_unstable(feature = "const_heap", issue = "79597")]
#[inline(always)]
pub const unsafe fn assume_init(self) -> Box<T, A> {
// This is used in the `vec!` macro, so we optimize for minimal IR generation
// even in debug builds.
// SAFETY: `Box<T>` and `Box<MaybeUninit<T>>` have the same layout.
unsafe { core::intrinsics::transmute_unchecked(self) }
}
/// Writes the value and converts to `Box<T, A>`.
///
/// This method converts the box similarly to [`Box::assume_init`] but
/// writes `value` into it before conversion thus guaranteeing safety.
/// In some scenarios use of this method may improve performance because
/// the compiler may be able to optimize copying from stack.
///
/// # Examples
///
/// ```
/// let big_box = Box::<[usize; 1024]>::new_uninit();
///
/// let mut array = [0; 1024];
/// for (i, place) in array.iter_mut().enumerate() {
/// *place = i;
/// }
///
/// // The optimizer may be able to elide this copy, so previous code writes
/// // to heap directly.
/// let big_box = Box::write(big_box, array);
///
/// for (i, x) in big_box.iter().enumerate() {
/// assert_eq!(*x, i);
/// }
/// ```
#[stable(feature = "box_uninit_write", since = "1.87.0")]
#[inline]
pub fn write(mut boxed: Self, value: T) -> Box<T, A> {
// SAFETY: Writing initialises the boxed value.
unsafe {
(*boxed).write(value);
boxed.assume_init()
}
}
}
impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
/// Converts to `Box<[T], A>`.
///
/// # Safety
///
/// As with [`MaybeUninit::assume_init`],
/// it is up to the caller to guarantee that the values
/// really are in an initialized state.
/// Calling this when the content is not yet fully initialized
/// causes immediate undefined behavior.
///
/// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
///
/// # Examples
///
/// ```
/// let mut values = Box::<[u32]>::new_uninit_slice(3);
/// // Deferred initialization:
/// values[0].write(1);
/// values[1].write(2);
/// values[2].write(3);
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [1, 2, 3])
/// ```
#[stable(feature = "new_uninit", since = "1.82.0")]
#[inline]
pub unsafe fn assume_init(self) -> Box<[T], A> {
let (raw, alloc) = Box::into_raw_with_allocator(self);
// SAFETY: Upheld by caller.
unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
}
}
impl<T: ?Sized> Box<T> {
/// Constructs a box from a raw pointer.
///
/// After calling this function, the raw pointer is owned by the
/// resulting `Box`. Specifically, the `Box` destructor will call
/// the destructor of `T` and free the allocated memory. For this
/// to be safe, the memory must have been allocated in accordance
/// with the [memory layout] used by `Box` .
///
/// # Safety
///
/// This function is unsafe because improper use may lead to
/// memory problems. For example, a double-free may occur if the
/// function is called twice on the same raw pointer.
///
/// The raw pointer must point to a block of memory allocated by the global allocator.
///
/// The safety conditions are described in the [memory layout] section.
/// Note that the [considerations for unsafe code] apply to all `Box<T>` values.
///
/// # Examples
///
/// Recreate a `Box` which was previously converted to a raw pointer
/// using [`Box::into_raw`]:
/// ```
/// let x = Box::new(5);
/// let ptr = Box::into_raw(x);
/// let x = unsafe { Box::from_raw(ptr) };
/// ```
/// Manually create a `Box` from scratch by using the global allocator:
/// ```
/// use std::alloc::{alloc, Layout};
///
/// unsafe {
/// let ptr = alloc(Layout::new::<i32>()) as *mut i32;
/// // In general .write is required to avoid attempting to destruct
/// // the (uninitialized) previous contents of `ptr`, though for this
/// // simple example `*ptr = 5` would have worked as well.
/// ptr.write(5);
/// let x = Box::from_raw(ptr);
/// }
/// ```
///
/// [memory layout]: self#memory-layout
/// [considerations for unsafe code]: self#considerations-for-unsafe-code
#[stable(feature = "box_raw", since = "1.4.0")]
#[inline]
#[must_use = "call `drop(Box::from_raw(ptr))` if you intend to drop the `Box`"]
pub unsafe fn from_raw(raw: *mut T) -> Self {
// SAFETY: Upheld by caller.
unsafe { Self::from_raw_in(raw, Global) }
}
/// Constructs a box from a `NonNull` pointer.
///
/// After calling this function, the `NonNull` pointer is owned by
/// the resulting `Box`. Specifically, the `Box` destructor will call
/// the destructor of `T` and free the allocated memory. For this
/// to be safe, the memory must have been allocated in accordance
/// with the [memory layout] used by `Box` .
///
/// # Safety
///
/// This function is unsafe because improper use may lead to
/// memory problems. For example, a double-free may occur if the
/// function is called twice on the same `NonNull` pointer.
///
/// The non-null pointer must point to a block of memory allocated by the global allocator.
///
/// The safety conditions are described in the [memory layout] section.
/// Note that the [considerations for unsafe code] apply to all `Box<T>` values.
///
/// # Examples
///
/// Recreate a `Box` which was previously converted to a `NonNull`
/// pointer using [`Box::into_non_null`]:
/// ```
/// let x = Box::new(5);
/// let non_null = Box::into_non_null(x);
/// let x = unsafe { Box::from_non_null(non_null) };
/// ```
/// Manually create a `Box` from scratch by using the global allocator:
/// ```
/// use std::alloc::{alloc, Layout};
/// use std::ptr::NonNull;
///
/// unsafe {
/// let non_null = NonNull::new(alloc(Layout::new::<i32>()).cast::<i32>())
/// .expect("alloc should have successfully allocated memory");
/// // In general .write is required to avoid attempting to destruct
/// // the (uninitialized) previous contents of `non_null`.
/// non_null.write(5);
/// let x = Box::from_non_null(non_null);
/// }
/// ```
///
/// [memory layout]: self#memory-layout
/// [considerations for unsafe code]: self#considerations-for-unsafe-code
#[stable(feature = "box_vec_non_null", since = "1.99.0")]
#[inline]
#[must_use = "call `drop(Box::from_non_null(ptr))` if you intend to drop the `Box`"]
pub unsafe fn from_non_null(ptr: NonNull<T>) -> Self {
// SAFETY: Upheld by caller.
unsafe { Self::from_raw(ptr.as_ptr()) }
}
/// Consumes the `Box`, returning a wrapped raw pointer.
///
/// The pointer will be properly aligned and non-null.
///
/// After calling this function, the caller is responsible for the
/// memory previously managed by the `Box`. In particular, the
/// caller should properly destroy `T` and release the memory, taking
/// into account the [memory layout] used by `Box`. The easiest way to
/// do this is to convert the raw pointer back into a `Box` with the
/// [`Box::from_raw`] function, allowing the `Box` destructor to perform
/// the cleanup.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
/// is so that there is no conflict with a method on the inner type.
///
/// # Examples
/// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
/// for automatic cleanup:
/// ```
/// let x = Box::new(String::from("Hello"));
/// let ptr = Box::into_raw(x);
/// let x = unsafe { Box::from_raw(ptr) };
/// ```
/// Manual cleanup by explicitly running the destructor and deallocating
/// the memory:
/// ```
/// use std::alloc::{dealloc, Layout};
/// use std::ptr;
///
/// let x = Box::new(String::from("Hello"));
/// let ptr = Box::into_raw(x);
/// unsafe {
/// ptr::drop_in_place(ptr);
/// dealloc(ptr as *mut u8, Layout::new::<String>());
/// }
/// ```
/// Note: This is equivalent to the following:
/// ```
/// let x = Box::new(String::from("Hello"));
/// let ptr = Box::into_raw(x);
/// unsafe {
/// drop(Box::from_raw(ptr));
/// }
/// ```
///
/// [memory layout]: self#memory-layout
#[must_use = "losing the pointer will leak memory"]
#[stable(feature = "box_raw", since = "1.4.0")]
#[inline]
pub fn into_raw(b: Self) -> *mut T {
// Avoid `into_raw_with_allocator` as that interacts poorly with Miri's Stacked Borrows.
let mut b = mem::ManuallyDrop::new(b);
// We need to give Miri (specifically, Stacked Borrows) a chance to recognize this as a
// safe-to-raw-pointer cast. To achieve this, we first create a mutable reference, and then
// cast that to a raw pointer -- this cast is recognized by the aliasing model and leads to
// a suitable retag.
// It would be wrong for `into_raw_with_allocator` to do the same as that would induce
// uniqueness assumptions (from the `&mut`) that we only want with the default allocator.
(&mut **b) as *mut T
}
/// Consumes the `Box`, returning a wrapped `NonNull` pointer.
///
/// The pointer will be properly aligned.
///
/// After calling this function, the caller is responsible for the
/// memory previously managed by the `Box`. In particular, the
/// caller should properly destroy `T` and release the memory, taking
/// into account the [memory layout] used by `Box`. The easiest way to
/// do this is to convert the `NonNull` pointer back into a `Box` with the
/// [`Box::from_non_null`] function, allowing the `Box` destructor to
/// perform the cleanup.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Box::into_non_null(b)` instead of `b.into_non_null()`.
/// This is so that there is no conflict with a method on the inner type.
///
/// # Examples
/// Converting the `NonNull` pointer back into a `Box` with [`Box::from_non_null`]
/// for automatic cleanup:
/// ```
/// let x = Box::new(String::from("Hello"));
/// let non_null = Box::into_non_null(x);
/// let x = unsafe { Box::from_non_null(non_null) };
/// ```
/// Manual cleanup by explicitly running the destructor and deallocating
/// the memory:
/// ```
/// use std::alloc::{dealloc, Layout};
///
/// let x = Box::new(String::from("Hello"));
/// let non_null = Box::into_non_null(x);
/// unsafe {
/// non_null.drop_in_place();
/// dealloc(non_null.as_ptr().cast::<u8>(), Layout::new::<String>());
/// }
/// ```
/// Note: This is equivalent to the following:
/// ```
/// let x = Box::new(String::from("Hello"));
/// let non_null = Box::into_non_null(x);
/// unsafe {
/// drop(Box::from_non_null(non_null));
/// }
/// ```
///
/// [memory layout]: self#memory-layout
#[must_use = "losing the pointer will leak memory"]
#[stable(feature = "box_vec_non_null", since = "1.99.0")]
#[inline]
pub fn into_non_null(b: Self) -> NonNull<T> {
// As of August 2026, we cannot utilize `Box::leak`
// because whether or not you can reconstruct the `Box`
// later using `Box::from_raw` or `Box::from_non_null` is
// an open question.
// SAFETY: `Box` is guaranteed to be non-null.
unsafe { NonNull::new_unchecked(Self::into_raw(b)) }
}
}
impl<T: ?Sized, A: Allocator> Box<T, A> {
/// Constructs a box from a raw pointer in the given allocator.
///
/// After calling this function, the raw pointer is owned by the
/// resulting `Box`. Specifically, the `Box` destructor will call
/// the destructor of `T` and free the allocated memory. For this
/// to be safe, the memory must have been allocated in accordance
/// with the [memory layout] used by `Box` .
///
/// # Safety
///
/// This function is unsafe because improper use may lead to
/// memory problems. For example, a double-free may occur if the
/// function is called twice on the same raw pointer.
///
/// The raw pointer must point to a block of memory allocated by `alloc`.
///
/// The safety conditions are described in the [memory layout] section.
/// Note that the [considerations for unsafe code] apply to all `Box<T, A>` values.
///
/// # Examples
///
/// Recreate a `Box` which was previously converted to a raw pointer
/// using [`Box::into_raw_with_allocator`]:
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let x = Box::new_in(5, System);
/// let (ptr, alloc) = Box::into_raw_with_allocator(x);
/// let x = unsafe { Box::from_raw_in(ptr, alloc) };
/// ```
/// Manually create a `Box` from scratch by using the system allocator:
/// ```
/// #![feature(allocator_api, slice_ptr_get)]
///
/// use std::alloc::{Allocator, Layout, System};
///
/// unsafe {
/// let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
/// // In general .write is required to avoid attempting to destruct
/// // the (uninitialized) previous contents of `ptr`, though for this
/// // simple example `*ptr = 5` would have worked as well.
/// ptr.write(5);
/// let x = Box::from_raw_in(ptr, System);
/// }
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
///
/// [memory layout]: self#memory-layout
/// [considerations for unsafe code]: self#considerations-for-unsafe-code
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
// SAFETY: Upheld by caller.
Box(unsafe { Unique::new_unchecked(raw) }, alloc)
}
/// Constructs a box from a `NonNull` pointer in the given allocator.
///
/// After calling this function, the `NonNull` pointer is owned by
/// the resulting `Box`. Specifically, the `Box` destructor will call
/// the destructor of `T` and free the allocated memory. For this
/// to be safe, the memory must have been allocated in accordance
/// with the [memory layout] used by `Box` .
///
/// # Safety
///
/// This function is unsafe because improper use may lead to
/// memory problems. For example, a double-free may occur if the
/// function is called twice on the same raw pointer.
///
/// The non-null pointer must point to a block of memory allocated by `alloc`.
///
/// The safety conditions are described in the [memory layout] section.
/// Note that the [considerations for unsafe code] apply to all `Box<T, A>` values.
///
/// # Examples
///
/// Recreate a `Box` which was previously converted to a `NonNull` pointer
/// using [`Box::into_non_null_with_allocator`]:
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let x = Box::new_in(5, System);
/// let (non_null, alloc) = Box::into_non_null_with_allocator(x);
/// let x = unsafe { Box::from_non_null_in(non_null, alloc) };
/// ```
/// Manually create a `Box` from scratch by using the system allocator:
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::{Allocator, Layout, System};
///
/// unsafe {
/// let non_null = System.allocate(Layout::new::<i32>())?.cast::<i32>();
/// // In general .write is required to avoid attempting to destruct
/// // the (uninitialized) previous contents of `non_null`.
/// non_null.write(5);
/// let x = Box::from_non_null_in(non_null, System);
/// }
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
///
/// [memory layout]: self#memory-layout
/// [considerations for unsafe code]: self#considerations-for-unsafe-code
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub unsafe fn from_non_null_in(raw: NonNull<T>, alloc: A) -> Self {
// SAFETY: guaranteed by the caller.
unsafe { Box::from_raw_in(raw.as_ptr(), alloc) }
}
/// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
///
/// The pointer will be properly aligned and non-null.
///
/// After calling this function, the caller is responsible for the
/// memory previously managed by the `Box`. In particular, the
/// caller should properly destroy `T` and release the memory, taking
/// into account the [memory layout] used by `Box`. The easiest way to
/// do this is to convert the raw pointer back into a `Box` with the
/// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
/// the cleanup.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
/// is so that there is no conflict with a method on the inner type.
///
/// # Examples
/// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
/// for automatic cleanup:
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let x = Box::new_in(String::from("Hello"), System);
/// let (ptr, alloc) = Box::into_raw_with_allocator(x);
/// let x = unsafe { Box::from_raw_in(ptr, alloc) };
/// ```
/// Manual cleanup by explicitly running the destructor and deallocating
/// the memory:
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::{Allocator, Layout, System};
/// use std::ptr::{self, NonNull};
///
/// let x = Box::new_in(String::from("Hello"), System);
/// let (ptr, alloc) = Box::into_raw_with_allocator(x);
/// unsafe {
/// ptr::drop_in_place(ptr);
/// let non_null = NonNull::new_unchecked(ptr);
/// alloc.deallocate(non_null.cast(), Layout::new::<String>());
/// }
/// ```
///
/// [memory layout]: self#memory-layout
#[must_use = "losing the pointer will leak memory"]
#[unstable(feature = "allocator_api", issue = "32838")]
#[rustc_const_unstable(feature = "const_heap", issue = "79597")]
#[inline]
pub const fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
let mut b = mem::ManuallyDrop::new(b);
// We carefully get the raw pointer out in a way that Miri's aliasing model understands what
// is happening: using the primitive "deref" of `Box`. In case `A` is *not* `Global`, we
// want *no* aliasing requirements here!
// In case `A` *is* `Global`, this does not quite have the right behavior; `into_raw`
// works around that.
let ptr = &raw mut **b;
// SAFETY: See above.
let alloc = unsafe { ptr::read(&b.1) };
(ptr, alloc)
}
/// Consumes the `Box`, returning a wrapped `NonNull` pointer and the allocator.
///
/// The pointer will be properly aligned.
///
/// After calling this function, the caller is responsible for the
/// memory previously managed by the `Box`. In particular, the
/// caller should properly destroy `T` and release the memory, taking
/// into account the [memory layout] used by `Box`. The easiest way to
/// do this is to convert the `NonNull` pointer back into a `Box` with the
/// [`Box::from_non_null_in`] function, allowing the `Box` destructor to
/// perform the cleanup.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Box::into_non_null_with_allocator(b)` instead of
/// `b.into_non_null_with_allocator()`. This is so that there is no
/// conflict with a method on the inner type.
///
/// # Examples
/// Converting the `NonNull` pointer back into a `Box` with
/// [`Box::from_non_null_in`] for automatic cleanup:
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::System;
///
/// let x = Box::new_in(String::from("Hello"), System);
/// let (non_null, alloc) = Box::into_non_null_with_allocator(x);
/// let x = unsafe { Box::from_non_null_in(non_null, alloc) };
/// ```
/// Manual cleanup by explicitly running the destructor and deallocating
/// the memory:
/// ```
/// #![feature(allocator_api)]
///
/// use std::alloc::{Allocator, Layout, System};
///
/// let x = Box::new_in(String::from("Hello"), System);
/// let (non_null, alloc) = Box::into_non_null_with_allocator(x);
/// unsafe {
/// non_null.drop_in_place();
/// alloc.deallocate(non_null.cast::<u8>(), Layout::new::<String>());
/// }
/// ```
///
/// [memory layout]: self#memory-layout
#[must_use = "losing the pointer will leak memory"]
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn into_non_null_with_allocator(b: Self) -> (NonNull<T>, A) {
let (ptr, alloc) = Box::into_raw_with_allocator(b);
// SAFETY: `Box` is guaranteed to be non-null.
unsafe { (NonNull::new_unchecked(ptr), alloc) }
}
#[unstable(
feature = "ptr_internals",
issue = "none",
reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
)]
#[inline]
#[doc(hidden)]
pub fn into_unique(b: Self) -> (Unique<T>, A) {
let (ptr, alloc) = Box::into_raw_with_allocator(b);
// SAFETY: Pointer is valid and unique.
unsafe { (Unique::from(&mut *ptr), alloc) }
}
/// Returns a raw mutable pointer to the `Box`'s contents.
///
/// The caller must ensure that the `Box` outlives the pointer this
/// function returns, or else it will end up dangling.
///
/// This method guarantees that for the purpose of the aliasing model, this method
/// does not materialize a reference to the underlying memory, and thus the returned pointer
/// will remain valid when mixed with other calls to [`as_ptr`], [`as_mut_ptr`], and [`as_non_null`].
/// Note that calling other methods that materialize references to the memory
/// may still invalidate this pointer.
/// See the example below for how this guarantee can be used.
///
/// # Examples
///
/// Due to the aliasing guarantee, the following code is legal:
///
/// ```rust
/// unsafe {
/// let mut b = Box::new(0);
/// let ptr1 = Box::as_mut_ptr(&mut b);
/// ptr1.write(1);
/// let ptr2 = Box::as_mut_ptr(&mut b);
/// ptr2.write(2);
/// // Notably, the write to `ptr2` did *not* invalidate `ptr1`:
/// ptr1.write(3);
/// }
/// ```
///
/// [`as_mut_ptr`]: Self::as_mut_ptr
/// [`as_ptr`]: Self::as_ptr
/// [`as_non_null`]: Self::as_non_null
#[must_use]
#[stable(feature = "box_as_ptr", since = "1.98.0")]
#[rustc_never_returns_null_ptr]
#[rustc_as_ptr]
#[inline]
pub fn as_mut_ptr(b: &mut Self) -> *mut T {
// This is a primitive deref, not going through `DerefMut`, and therefore not materializing
// any references.
&raw mut **b
}
/// Returns a raw pointer to the `Box`'s contents.
///
/// The caller must ensure that the `Box` outlives the pointer this
/// function returns, or else it will end up dangling.
///
/// The caller must also ensure that the memory the pointer (non-transitively) points to
/// is never written to (except inside an `UnsafeCell`) using this pointer or any pointer
/// derived from it. If you need to mutate the contents of the `Box`, use [`as_mut_ptr`].
///
/// This method guarantees that for the purpose of the aliasing model, this method
/// does not materialize a reference to the underlying memory, and thus the returned pointer
/// will remain valid when mixed with other calls to [`as_ptr`], [`as_mut_ptr`], and [`as_non_null`].
/// Note that calling other methods that materialize mutable references to the memory,
/// as well as writing to this memory, may still invalidate this pointer.
/// See the example below for how this guarantee can be used.
///
/// # Examples
///
/// Due to the aliasing guarantee, the following code is legal:
///
/// ```rust
/// unsafe {
/// let mut v = Box::new(0);
/// let ptr1 = Box::as_ptr(&v);
/// let ptr2 = Box::as_mut_ptr(&mut v);
/// let _val = ptr2.read();
/// // No write to this memory has happened yet, so `ptr1` is still valid.
/// let _val = ptr1.read();
/// // However, once we do a write...
/// ptr2.write(1);
/// // ... `ptr1` is no longer valid.
/// // This would be UB: let _val = ptr1.read();
/// }
/// ```
///
/// [`as_mut_ptr`]: Self::as_mut_ptr
/// [`as_ptr`]: Self::as_ptr
/// [`as_non_null`]: Self::as_non_null
#[must_use]
#[stable(feature = "box_as_ptr", since = "1.98.0")]
#[rustc_never_returns_null_ptr]
#[rustc_as_ptr]
#[inline]
pub fn as_ptr(b: &Self) -> *const T {
// This is a primitive deref, not going through `DerefMut`, and therefore not materializing
// any references.
&raw const **b
}
/// Returns a `NonNull` pointer to the `Box`'s contents.
///
/// The caller must ensure that the `Box` outlives the pointer this
/// function returns, or else it will end up dangling.
///
/// This method guarantees that for the purpose of the aliasing model, this method
/// does not materialize a reference to the underlying memory, and thus the returned pointer
/// will remain valid when mixed with other calls to [`as_ptr`], [`as_mut_ptr`], and [`as_non_null`].
/// Note that calling other methods that materialize references to the memory
/// may still invalidate this pointer.
/// See the example below for how this guarantee can be used.
///
/// # Examples
///
/// Due to the aliasing guarantee, the following code is legal:
///
/// ```rust
/// #![feature(box_as_non_null)]
///
/// unsafe {
/// let mut b = Box::new(0);
/// let ptr1 = Box::as_non_null(&mut b);
/// ptr1.write(1);
/// let ptr2 = Box::as_non_null(&mut b);
/// ptr2.write(2);
/// // Notably, the write to `ptr2` did *not* invalidate `ptr1`:
/// ptr1.write(3);
/// }
/// ```
///
/// [`as_mut_ptr`]: Self::as_mut_ptr
/// [`as_ptr`]: Self::as_ptr
/// [`as_non_null`]: Self::as_non_null
#[must_use]
#[unstable(feature = "box_as_non_null", issue = "157345")]
#[rustc_as_ptr]
#[inline]
pub fn as_non_null(b: &mut Self) -> NonNull<T> {
// SAFETY: `Box` is guaranteed to be non-null.
unsafe { NonNull::new_unchecked(Self::as_mut_ptr(b)) }
}
/// Returns a reference to the underlying allocator.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
/// is so that there is no conflict with a method on the inner type.
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub fn allocator(b: &Self) -> &A {
&b.1
}
/// Consumes and leaks the `Box`, returning a mutable reference,
/// `&'a mut T`.
///
/// Note that the type `T` must outlive the chosen lifetime `'a`. If the type
/// has only static references, or none at all, then this may be chosen to be
/// `'static`.
///
/// This function is mainly useful for data that lives for the remainder of the program's life,
/// i.e., memory that is meant to leak. If the memory should eventually be freed, prefer to use
/// [`Box::into_raw`] or [`Box::into_non_null`] instead. Reconstructing ("unleaking") a `Box` from
/// the mutable reference returned here (e.g. via [`Box::from_raw`]) is only possible if the
/// allocator is `Global`, and even then it is a grey area (meaning it is possible under specific
/// circumstances but many seemingly harmless ways of doing it are undefined behavior) and should
/// be avoided.
///
/// Note: this is an associated function, which means that you have
/// to call it as `Box::leak(b)` instead of `b.leak()`. This
/// is so that there is no conflict with a method on the inner type.
///
/// # Examples
///
/// Simple usage:
///
/// ```
/// let x = Box::new(41);
/// let static_ref: &'static mut usize = Box::leak(x);
/// *static_ref += 1;
/// assert_eq!(*static_ref, 42);
/// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
/// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
/// # drop(unsafe { Box::from_raw(static_ref) });
/// ```
///
/// Unsized data:
///
/// ```
/// let x = vec![1, 2, 3].into_boxed_slice();
/// let static_ref = Box::leak(x);
/// static_ref[0] = 4;
/// assert_eq!(*static_ref, [4, 2, 3]);
/// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
/// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
/// # drop(unsafe { Box::from_raw(static_ref) });
/// ```
#[stable(feature = "box_leak", since = "1.26.0")]
#[inline]
pub fn leak<'a>(b: Self) -> &'a mut T
where
A: 'a,
{
let (ptr, alloc) = Box::into_raw_with_allocator(b);
mem::forget(alloc);
// SAFETY: Pointer is valid and unique.
unsafe { &mut *ptr }
}
/// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
/// `*boxed` will be pinned in memory and unable to be moved.
///
/// This conversion does not allocate on the heap and happens in place.
///
/// This is also available via [`From`].
///
/// Constructing and pinning a `Box` with <code>Box::into_pin([Box::new]\(x))</code>
/// can also be written more concisely using <code>[Box::pin]\(x)</code>.
/// This `into_pin` method is useful if you already have a `Box<T>`, or you are
/// constructing a (pinned) `Box` in a different way than with [`Box::new`].
///
/// # Notes
///
/// It's not recommended that crates add an impl like `From<Box<T>> for Pin<T>`,
/// as it'll introduce an ambiguity when calling `Pin::from`.
/// A demonstration of such a poor impl is shown below.
///
/// ```compile_fail
/// # use std::pin::Pin;
/// struct Foo; // A type defined in this crate.
/// impl From<Box<()>> for Pin<Foo> {
/// fn from(_: Box<()>) -> Pin<Foo> {
/// Pin::new(Foo)
/// }
/// }
///
/// let foo = Box::new(());
/// let bar = Pin::from(foo);
/// ```
#[stable(feature = "box_into_pin", since = "1.63.0")]
pub fn into_pin(boxed: Self) -> Pin<Self>
where
A: StaticAllocator,
{
// SAFETY: It's not possible to move or replace the insides of a
// `Pin<Box<T>>` when `T: !Unpin`, so it's safe to pin it directly
// so long as the allocator promises to not break the pinning invariants.
unsafe { Pin::new_unchecked(boxed) }
}
}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
#[inline]
fn drop(&mut self) {
// the T in the Box is dropped by the compiler before the destructor is run
let ptr = self.0;
// SAFETY: The construction site of the unsized box had ensured for us that the
// allocation was made with a valid layout (the size does not overflow an isize,
// possibly because the size of the type is 0).
let layout = unsafe { Layout::for_value_raw(ptr.as_ptr()) };
if layout.size() != 0 {
// SAFETY: Any nonzero allocation would have been created with the allocator
// of this box and `layout` would fit that allocation. We also are the only ones
// responsible for doing this deallocation and know that the pointer must be valid.
unsafe {
self.1.deallocate(From::from(ptr.cast()), layout);
}
}
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Default> Default for Box<T> {
/// Creates a `Box<T>`, with the `Default` value for `T`.
#[inline]
fn default() -> Self {
let mut x: Box<mem::MaybeUninit<T>> = Box::new_uninit();
// SAFETY: `x` is valid for writing and has the same layout as `T`.
// If `T::default()` panics, dropping `x` will just deallocate the Box as `MaybeUninit<T>`
// does not have a destructor.
//
// We use `ptr::write` as `MaybeUninit::write` creates
// extra stack copies of `T` in debug mode.
//
// See https://github.com/rust-lang/rust/issues/136043 for more context.
unsafe { ptr::write(&raw mut *x as *mut T, T::default()) };
// SAFETY: `x` was just initialized above.
unsafe { x.assume_init() }
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Default for Box<[T]> {
/// Creates an empty `[T]` inside a `Box`.
#[inline]
fn default() -> Self {
let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
Box(ptr, Global)
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "default_box_extra", since = "1.17.0")]
impl Default for Box<str> {
#[inline]
fn default() -> Self {
// SAFETY: This is the same as `Unique::cast<U>` but with an unsized `U = str`.
let ptr: Unique<str> = unsafe {
let bytes: Unique<[u8]> = Unique::<[u8; 0]>::dangling();
Unique::new_unchecked(bytes.as_ptr() as *mut str)
};
Box(ptr, Global)
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "pin_default_impls", since = "1.91.0")]
impl<T> Default for Pin<Box<T>>
where
T: ?Sized,
Box<T>: Default,
{
#[inline]
fn default() -> Self {
Box::into_pin(Box::<T>::default())
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
// NB: This is not `AllocatorClone` since we don't care about allocator
// equivalence when cloning boxes.
impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
/// Returns a new box with a `clone()` of this box's contents.
///
/// # Examples
///
/// ```
/// let x = Box::new(5);
/// let y = x.clone();
///
/// // The value is the same
/// assert_eq!(x, y);
///
/// // But they are unique objects
/// assert_ne!(&*x as *const i32, &*y as *const i32);
/// ```
#[inline]
fn clone(&self) -> Self {
// Pre-allocate memory to allow writing the cloned value directly.
let mut boxed = Self::new_uninit_in(self.1.clone());
// SAFETY: Destination pointer is valid and will then become initialised.
unsafe {
(**self).clone_to_uninit(boxed.as_mut_ptr().cast());
boxed.assume_init()
}
}
/// Copies `source`'s contents into `self` without creating a new allocation.
///
/// # Examples
///
/// ```
/// let x = Box::new(5);
/// let mut y = Box::new(10);
/// let yp: *const i32 = &*y;
///
/// y.clone_from(&x);
///
/// // The value is the same
/// assert_eq!(x, y);
///
/// // And no allocation occurred
/// assert_eq!(yp, &*y);
/// ```
#[inline]
fn clone_from(&mut self, source: &Self) {
(**self).clone_from(&(**source));
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_slice_clone", since = "1.3.0")]
impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
fn clone(&self) -> Self {
let alloc = Box::allocator(self).clone();
self.to_vec_in(alloc).into_boxed_slice()
}
/// Copies `source`'s contents into `self` without creating a new allocation,
/// so long as the two are of the same length.
///
/// # Examples
///
/// ```
/// let x = Box::new([5, 6, 7]);
/// let mut y = Box::new([8, 9, 10]);
/// let yp: *const [i32] = &*y;
///
/// y.clone_from(&x);
///
/// // The value is the same
/// assert_eq!(x, y);
///
/// // And no allocation occurred
/// assert_eq!(yp, &*y);
/// ```
fn clone_from(&mut self, source: &Self) {
if self.len() == source.len() {
self.clone_from_slice(source);
} else {
*self = source.clone();
}
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "box_slice_clone", since = "1.3.0")]
impl<A: Allocator + Clone> Clone for Box<str, A> {
fn clone(&self) -> Self {
let buf = Box::clone_from_ref_in(self.as_bytes(), self.1.clone());
// SAFETY: We know the [u8] is a valid str.
unsafe { from_boxed_utf8_unchecked_in(buf) }
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
#[inline]
fn eq(&self, other: &Self) -> bool {
PartialEq::eq(&**self, &**other)
}
#[inline]
fn ne(&self, other: &Self) -> bool {
PartialEq::ne(&**self, &**other)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
#[inline]
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
PartialOrd::partial_cmp(&**self, &**other)
}
#[inline]
fn lt(&self, other: &Self) -> bool {
PartialOrd::lt(&**self, &**other)
}
#[inline]
fn le(&self, other: &Self) -> bool {
PartialOrd::le(&**self, &**other)
}
#[inline]
fn ge(&self, other: &Self) -> bool {
PartialOrd::ge(&**self, &**other)
}
#[inline]
fn gt(&self, other: &Self) -> bool {
PartialOrd::gt(&**self, &**other)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
#[inline]
fn cmp(&self, other: &Self) -> Ordering {
Ord::cmp(&**self, &**other)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
fn hash<H: Hasher>(&self, state: &mut H) {
(**self).hash(state);
}
}
#[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
fn finish(&self) -> u64 {
(**self).finish()
}
fn write(&mut self, bytes: &[u8]) {
(**self).write(bytes)
}
fn write_u8(&mut self, i: u8) {
(**self).write_u8(i)
}
fn write_u16(&mut self, i: u16) {
(**self).write_u16(i)
}
fn write_u32(&mut self, i: u32) {
(**self).write_u32(i)
}
fn write_u64(&mut self, i: u64) {
(**self).write_u64(i)
}
fn write_u128(&mut self, i: u128) {
(**self).write_u128(i)
}
fn write_usize(&mut self, i: usize) {
(**self).write_usize(i)
}
fn write_i8(&mut self, i: i8) {
(**self).write_i8(i)
}
fn write_i16(&mut self, i: i16) {
(**self).write_i16(i)
}
fn write_i32(&mut self, i: i32) {
(**self).write_i32(i)
}
fn write_i64(&mut self, i: i64) {
(**self).write_i64(i)
}
fn write_i128(&mut self, i: i128) {
(**self).write_i128(i)
}
fn write_isize(&mut self, i: isize) {
(**self).write_isize(i)
}
fn write_length_prefix(&mut self, len: usize) {
(**self).write_length_prefix(len)
}
fn write_str(&mut self, s: &str) {
(**self).write_str(s)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(&**self, f)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
// It's not possible to extract the inner Uniq directly from the Box,
// instead we cast it to a *const which aliases the Unique
let ptr: *const T = &**self;
fmt::Pointer::fmt(&ptr, f)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
type Target = T;
fn deref(&self) -> &T {
self
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
fn deref_mut(&mut self) -> &mut T {
self
}
}
#[unstable(feature = "deref_pure_trait", issue = "87121")]
unsafe impl<T: ?Sized, A: Allocator> DerefPure for Box<T, A> {}
#[unstable(feature = "legacy_receiver_trait", issue = "none")]
impl<T: ?Sized, A: Allocator> LegacyReceiver for Box<T, A> {}
#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
impl<Args: Tuple, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
type Output = <F as FnOnce<Args>>::Output;
extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
<F as FnOnce<Args>>::call_once(*self, args)
}
}
#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
impl<Args: Tuple, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
<F as FnMut<Args>>::call_mut(self, args)
}
}
#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
impl<Args: Tuple, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
extern "rust-call" fn call(&self, args: Args) -> Self::Output {
<F as Fn<Args>>::call(self, args)
}
}
#[stable(feature = "async_closure", since = "1.85.0")]
impl<Args: Tuple, F: AsyncFnOnce<Args> + ?Sized, A: Allocator> AsyncFnOnce<Args> for Box<F, A> {
type Output = F::Output;
type CallOnceFuture = F::CallOnceFuture;
extern "rust-call" fn async_call_once(self, args: Args) -> Self::CallOnceFuture {
F::async_call_once(*self, args)
}
}
#[stable(feature = "async_closure", since = "1.85.0")]
impl<Args: Tuple, F: AsyncFnMut<Args> + ?Sized, A: Allocator> AsyncFnMut<Args> for Box<F, A> {
type CallRefFuture<'a>
= F::CallRefFuture<'a>
where
Self: 'a;
extern "rust-call" fn async_call_mut(&mut self, args: Args) -> Self::CallRefFuture<'_> {
F::async_call_mut(self, args)
}
}
#[stable(feature = "async_closure", since = "1.85.0")]
impl<Args: Tuple, F: AsyncFn<Args> + ?Sized, A: Allocator> AsyncFn<Args> for Box<F, A> {
extern "rust-call" fn async_call(&self, args: Args) -> Self::CallRefFuture<'_> {
F::async_call(self, args)
}
}
#[unstable(feature = "coerce_unsized", issue = "18598")]
impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
// A pointer can only be pin safe if it does not implement certain safe traits
// maliciously. Since `Box` is fundamental, downstream crates may be able to
// implement those traits for `Box<LocalType>`, so we must carefully check that
// this is not a problem for each trait.
//
// The `Box` type always implements `Deref` and `DerefMut`, so despite being
// fundamental, downstream crates cannot implement these traits for
// `Box<LocalType>`.
//
// Conversely, downstream crates are able to implement `Clone`, `Debug`, and
// `Display` for `Box<LocalType>` as long as `LocalType` does not implement
// said trait. However, the `Box<T>` type does not treat the existence of an
// `&Box<T>` as evidence that the `T` is not pinned, so this is not
// problematic.
//
// Finally, even if downstream crates provide their own implementation of
// `Clone` for `Box<LocalType>`, it is not problematic for the cloned box to be
// wrapped in `Pin`, since the same conversion could have been carried out
// safely as `Box::pin((*p).clone())`.
#[unstable(feature = "pin_coerce_unsized_trait", issue = "150112")]
unsafe impl<T: ?Sized, A: StaticAllocator> PinSafePointer for Box<T, A> {}
// It is quite crucial that we only allow the `Global` allocator here.
// Handling arbitrary custom allocators (which can affect the `Box` layout heavily!)
// would need a lot of codegen and interpreter adjustments.
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
#[stable(feature = "box_borrow", since = "1.1.0")]
impl<T: ?Sized, A: Allocator> Borrow<T> for Box<T, A> {
fn borrow(&self) -> &T {
self
}
}
#[stable(feature = "box_borrow", since = "1.1.0")]
impl<T: ?Sized, A: Allocator> BorrowMut<T> for Box<T, A> {
fn borrow_mut(&mut self) -> &mut T {
self
}
}
#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
fn as_ref(&self) -> &T {
self
}
}
#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
fn as_mut(&mut self) -> &mut T {
self
}
}
/* Nota bene
*
* We could have chosen not to add this impl, and instead have written a
* function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
* because Box<T> implements Unpin even when T does not, as a result of
* this impl.
*
* We chose this API instead of the alternative for a few reasons:
* - Logically, it is helpful to understand pinning in regard to the
* memory region being pointed to. For this reason none of the
* standard library pointer types support projecting through a pin
* (Box<T> is the only pointer type in std for which this would be
* safe.)
* - It is in practice very useful to have Box<T> be unconditionally
* Unpin because of trait objects, for which the structural auto
* trait functionality does not apply (e.g., Box<dyn Foo> would
* otherwise not be Unpin).
*
* Another type with the same semantics as Box but only a conditional
* implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
* could have a method to project a Pin<T> from it.
*/
#[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> {}
#[unstable(feature = "coroutine_trait", issue = "43122")]
impl<G: ?Sized + Coroutine<R> + Unpin, R, A: Allocator> Coroutine<R> for Box<G, A> {
type Yield = G::Yield;
type Return = G::Return;
fn resume(mut self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return> {
G::resume(Pin::new(&mut *self), arg)
}
}
#[unstable(feature = "coroutine_trait", issue = "43122")]
impl<G: ?Sized + Coroutine<R>, R, A: Allocator> Coroutine<R> for Pin<Box<G, A>>
where
A: 'static,
{
type Yield = G::Yield;
type Return = G::Return;
fn resume(mut self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return> {
G::resume((*self).as_mut(), arg)
}
}
#[stable(feature = "futures_api", since = "1.36.0")]
impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A> {
type Output = F::Output;
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
F::poll(Pin::new(&mut *self), cx)
}
}
#[stable(feature = "box_error", since = "1.8.0")]
impl<E: Error> Error for Box<E> {
#[allow(deprecated)]
fn cause(&self) -> Option<&dyn Error> {
Error::cause(&**self)
}
fn source(&self) -> Option<&(dyn Error + 'static)> {
Error::source(&**self)
}
fn provide<'b>(&'b self, request: &mut error::Request<'b>) {
Error::provide(&**self, request);
}
}
#[unstable(feature = "allocator_api", issue = "32838")]
unsafe impl<T: ?Sized + Allocator, A: Allocator> Allocator for Box<T, A> {
#[inline]
fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
(**self).allocate(layout)
}
#[inline]
fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
(**self).allocate_zeroed(layout)
}
#[inline]
unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
// SAFETY: the safety contract must be upheld by the caller
unsafe { (**self).deallocate(ptr, layout) }
}
#[inline]
unsafe fn grow(
&self,
ptr: NonNull<u8>,
old_layout: Layout,
new_layout: Layout,
) -> Result<NonNull<[u8]>, AllocError> {
// SAFETY: the safety contract must be upheld by the caller
unsafe { (**self).grow(ptr, old_layout, new_layout) }
}
#[inline]
unsafe fn grow_zeroed(
&self,
ptr: NonNull<u8>,
old_layout: Layout,
new_layout: Layout,
) -> Result<NonNull<[u8]>, AllocError> {
// SAFETY: the safety contract must be upheld by the caller
unsafe { (**self).grow_zeroed(ptr, old_layout, new_layout) }
}
#[inline]
unsafe fn shrink(
&self,
ptr: NonNull<u8>,
old_layout: Layout,
new_layout: Layout,
) -> Result<NonNull<[u8]>, AllocError> {
// SAFETY: the safety contract must be upheld by the caller
unsafe { (**self).shrink(ptr, old_layout, new_layout) }
}
}
#[unstable(feature = "random", issue = "130703")]
impl<R: core::random::Rng + ?Sized, A: Allocator> core::random::Rng for Box<R, A> {
#[inline]
fn fill_bytes(&mut self, bytes: &mut [u8]) {
(**self).fill_bytes(bytes)
}
}