Skip to content

What if we just add <[MaybeUninit<T>; N]>::assume_init directly? #104475

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Closed
wants to merge 1 commit into from
Closed
Changes from all commits
Commits
File filter

Filter by extension

Filter by extension

Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
2 changes: 1 addition & 1 deletion library/alloc/src/lib.rs
Original file line number Diff line number Diff line change
@@ -92,6 +92,7 @@
#![feature(allocator_api)]
#![feature(array_chunks)]
#![feature(array_into_iter_constructors)]
#![feature(array_maybe_uninit_assume_init)]
#![feature(array_methods)]
#![feature(array_windows)]
#![feature(assert_matches)]
@@ -127,7 +128,6 @@
#![feature(layout_for_ptr)]
#![feature(maybe_uninit_slice)]
#![feature(maybe_uninit_uninit_array)]
#![feature(maybe_uninit_uninit_array_transpose)]
#![cfg_attr(test, feature(new_uninit))]
#![feature(nonnull_slice_from_raw_parts)]
#![feature(pattern)]
4 changes: 2 additions & 2 deletions library/alloc/src/vec/into_iter.rs
Original file line number Diff line number Diff line change
@@ -223,7 +223,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {

self.ptr = self.ptr.wrapping_byte_add(N);
// Safety: ditto
return Ok(unsafe { raw_ary.transpose().assume_init() });
return Ok(unsafe { raw_ary.assume_init() });
}

if len < N {
@@ -241,7 +241,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
return unsafe {
ptr::copy_nonoverlapping(self.ptr, raw_ary.as_mut_ptr() as *mut T, N);
self.ptr = self.ptr.add(N);
Ok(raw_ary.transpose().assume_init())
Ok(raw_ary.assume_init())
};
}

4 changes: 2 additions & 2 deletions library/core/src/array/iter.rs
Original file line number Diff line number Diff line change
@@ -104,7 +104,7 @@ impl<T, const N: usize> IntoIter<T, N> {
///
/// ```
/// #![feature(array_into_iter_constructors)]
/// #![feature(maybe_uninit_uninit_array_transpose)]
/// #![feature(array_maybe_uninit_assume_init)]
/// #![feature(maybe_uninit_uninit_array)]
/// use std::array::IntoIter;
/// use std::mem::MaybeUninit;
@@ -133,7 +133,7 @@ impl<T, const N: usize> IntoIter<T, N> {
/// }
///
/// // SAFETY: We've initialized all N items
/// unsafe { Ok(buffer.transpose().assume_init()) }
/// unsafe { Ok(buffer.assume_init()) }
/// }
///
/// let r: [_; 4] = next_chunk(&mut (10..16)).unwrap();
47 changes: 46 additions & 1 deletion library/core/src/array/mod.rs
Original file line number Diff line number Diff line change
@@ -799,6 +799,51 @@ impl<T, const N: usize> [T; N] {
}
}

impl<T, const N: usize> [MaybeUninit<T>; N] {
/// Extracts the values from an array of [`MaybeUninit<T>`] containers.
///
/// This is essentially `.map(MaybeUninit::assume_init)`, but in a way
/// that's *much* clearer to the optimizer how to do efficiently.
///
/// # Safety
///
/// It is up to the caller to guarantee that all elements of the array are
/// in an initialized state.
///
/// # Examples
///
/// ```
/// #![feature(inline_const)]
/// #![feature(array_maybe_uninit_assume_init)]
/// use std::mem::MaybeUninit;
///
/// let mut array = [const { MaybeUninit::<i32>::uninit() }; 3];
/// array[0].write(0);
/// array[1].write(1);
/// array[2].write(2);
///
/// // SAFETY: we initialised all three elements
/// let array = unsafe { array.assume_init() };
///
/// assert_eq!(array, [0, 1, 2]);
/// ```
#[unstable(feature = "array_maybe_uninit_assume_init", issue = "96097")]
#[rustc_const_unstable(feature = "const_array_maybe_uninit_assume_init", issue = "96097")]
#[inline(always)]
#[track_caller]
pub const unsafe fn assume_init(self) -> [T; N] {
// SAFETY:
// * The caller guarantees that all elements of the array are initialized
// * `MaybeUninit<T>` and T are guaranteed to have the same layout
// * `MaybeUninit` does not drop, so there are no double-frees
// And thus the conversion is safe
unsafe {
crate::intrinsics::assert_inhabited::<[T; N]>();
mem::transmute_copy(&mem::ManuallyDrop::new(self))
}
}
}

/// Pulls `N` items from `iter` and returns them as an array. If the iterator
/// yields fewer than `N` items, this function exhibits undefined behavior.
///
@@ -896,7 +941,7 @@ where

mem::forget(guard);
// SAFETY: All elements of the array were populated in the loop above.
let output = unsafe { array.transpose().assume_init() };
let output = unsafe { array.assume_init() };
Ok(Try::from_output(output))
}

2 changes: 1 addition & 1 deletion library/core/tests/lib.rs
Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
#![feature(alloc_layout_extra)]
#![feature(array_chunks)]
#![feature(array_maybe_uninit_assume_init)]
#![feature(array_methods)]
#![feature(array_windows)]
#![feature(bigint_helper_methods)]
@@ -53,7 +54,6 @@
#![feature(split_as_slice)]
#![feature(maybe_uninit_uninit_array)]
#![feature(maybe_uninit_write_slice)]
#![feature(maybe_uninit_uninit_array_transpose)]
#![feature(min_specialization)]
#![feature(numfmt)]
#![feature(step_trait)]
4 changes: 2 additions & 2 deletions library/core/tests/mem.rs
Original file line number Diff line number Diff line change
@@ -194,11 +194,11 @@ fn uninit_array_assume_init() {
array[3].write(1);
array[4].write(5);

let array = unsafe { array.transpose().assume_init() };
let array = unsafe { array.assume_init() };

assert_eq!(array, [3, 1, 4, 1, 5]);

let [] = unsafe { [MaybeUninit::<!>::uninit(); 0].transpose().assume_init() };
let [] = unsafe { [MaybeUninit::<!>::uninit(); 0].assume_init() };
}

#[test]