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Rollup merge of #136976 - jedbrown:jed/doc-boxed-deferred-init, r=tgr…
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alloc boxed: docs: use MaybeUninit::write instead of as_mut_ptr

In the deferred initialization pattern, the docs were needlessly going through `as_mut_ptr().write()` to initialize, which is unnecessary use of a pointer, needs to be inside an `unsafe` block, and may weaken alias analysis.
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@matthiaskrgr
matthiaskrgr authored Feb 14, 2025
2 parents b5fce2a + 2f27236 commit 145e35a
Showing 1 changed file with 44 additions and 84 deletions.
128 changes: 44 additions & 84 deletions library/alloc/src/boxed.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -280,13 +280,9 @@ impl<T> Box<T> {
///
/// ```
/// let mut five = Box::<u32>::new_uninit();
///
/// let five = unsafe {
/// // Deferred initialization:
/// five.as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
/// // Deferred initialization:
/// five.write(5);
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5)
/// ```
Expand Down Expand Up @@ -367,13 +363,9 @@ impl<T> Box<T> {
/// #![feature(allocator_api)]
///
/// let mut five = Box::<u32>::try_new_uninit()?;
///
/// let five = unsafe {
/// // Deferred initialization:
/// five.as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
/// // Deferred initialization:
/// five.write(5);
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5);
/// # Ok::<(), std::alloc::AllocError>(())
Expand Down Expand Up @@ -435,10 +427,8 @@ impl<T, A: Allocator> Box<T, A> {
A: Allocator,
{
let mut boxed = Self::new_uninit_in(alloc);
unsafe {
boxed.as_mut_ptr().write(x);
boxed.assume_init()
}
boxed.write(x);
unsafe { boxed.assume_init() }
}

/// Allocates memory in the given allocator then places `x` into it,
Expand All @@ -463,10 +453,8 @@ impl<T, A: Allocator> Box<T, A> {
A: Allocator,
{
let mut boxed = Self::try_new_uninit_in(alloc)?;
unsafe {
boxed.as_mut_ptr().write(x);
Ok(boxed.assume_init())
}
boxed.write(x);
unsafe { Ok(boxed.assume_init()) }
}

/// Constructs a new box with uninitialized contents in the provided allocator.
Expand All @@ -479,13 +467,9 @@ impl<T, A: Allocator> Box<T, A> {
/// use std::alloc::System;
///
/// let mut five = Box::<u32, _>::new_uninit_in(System);
///
/// let five = unsafe {
/// // Deferred initialization:
/// five.as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
/// // Deferred initialization:
/// five.write(5);
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5)
/// ```
Expand Down Expand Up @@ -517,13 +501,9 @@ impl<T, A: Allocator> Box<T, A> {
/// use std::alloc::System;
///
/// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
///
/// let five = unsafe {
/// // Deferred initialization:
/// five.as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
/// // Deferred initialization:
/// five.write(5);
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5);
/// # Ok::<(), std::alloc::AllocError>(())
Expand Down Expand Up @@ -669,15 +649,11 @@ impl<T> Box<[T]> {
///
/// ```
/// let mut values = Box::<[u32]>::new_uninit_slice(3);
///
/// let values = unsafe {
/// // Deferred initialization:
/// values[0].as_mut_ptr().write(1);
/// values[1].as_mut_ptr().write(2);
/// values[2].as_mut_ptr().write(3);
///
/// values.assume_init()
/// };
/// // 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])
/// ```
Expand Down Expand Up @@ -722,13 +698,11 @@ impl<T> Box<[T]> {
/// #![feature(allocator_api)]
///
/// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
/// let values = unsafe {
/// // Deferred initialization:
/// values[0].as_mut_ptr().write(1);
/// values[1].as_mut_ptr().write(2);
/// values[2].as_mut_ptr().write(3);
/// values.assume_init()
/// };
/// // 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>(())
Expand Down Expand Up @@ -814,15 +788,11 @@ impl<T, A: Allocator> Box<[T], A> {
/// use std::alloc::System;
///
/// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
///
/// let values = unsafe {
/// // Deferred initialization:
/// values[0].as_mut_ptr().write(1);
/// values[1].as_mut_ptr().write(2);
/// values[2].as_mut_ptr().write(3);
///
/// values.assume_init()
/// };
/// // 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])
/// ```
Expand Down Expand Up @@ -873,13 +843,11 @@ impl<T, A: Allocator> Box<[T], A> {
/// use std::alloc::System;
///
/// let mut values = Box::<[u32], _>::try_new_uninit_slice_in(3, System)?;
/// let values = unsafe {
/// // Deferred initialization:
/// values[0].as_mut_ptr().write(1);
/// values[1].as_mut_ptr().write(2);
/// values[2].as_mut_ptr().write(3);
/// values.assume_init()
/// };
/// // 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>(())
Expand Down Expand Up @@ -959,13 +927,9 @@ impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
///
/// ```
/// let mut five = Box::<u32>::new_uninit();
///
/// let five: Box<u32> = unsafe {
/// // Deferred initialization:
/// five.as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
/// // Deferred initialization:
/// five.write(5);
/// let five: Box<u32> = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5)
/// ```
Expand Down Expand Up @@ -1030,15 +994,11 @@ impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
///
/// ```
/// let mut values = Box::<[u32]>::new_uninit_slice(3);
///
/// let values = unsafe {
/// // Deferred initialization:
/// values[0].as_mut_ptr().write(1);
/// values[1].as_mut_ptr().write(2);
/// values[2].as_mut_ptr().write(3);
///
/// values.assume_init()
/// };
/// // 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])
/// ```
Expand Down

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