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Rollup merge of #100076 - tspiteri:const_slice_split_at, r=oli-obk

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make slice::{split_at,split_at_unchecked} const functions

Now that `slice::from_raw_parts` is const in stable 1.64, it makes sense to have `split_at` const as well, otherwise unsafe code is required to achieve a const equivalent.
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Yuki Okushi 2022-08-26 09:51:43 +09:00 • committed by GitHub
commit e193f4697f
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2 changed files with 14 additions and 4 deletions
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1
library/core/src/lib.rs
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@ -149,6 +149,7 @@
#![feature(maybe_uninit_uninit_array)] #![feature(maybe_uninit_uninit_array)]
#![feature(ptr_metadata)] #![feature(ptr_metadata)]
#![feature(slice_ptr_get)] #![feature(slice_ptr_get)]
#![feature(slice_split_at_unchecked)]
#![feature(str_internals)] #![feature(str_internals)]
#![feature(utf16_extra)] #![feature(utf16_extra)]
#![feature(utf16_extra_const)] #![feature(utf16_extra_const)]

17
library/core/src/slice/mod.rs
View file

@ -1541,13 +1541,14 @@ impl<T> [T] {
/// } /// }
/// ``` /// ```
#[stable(feature = "rust1", since = "1.0.0")] #[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_slice_split_at_not_mut", issue = "none")]
#[inline] #[inline]
#[track_caller] #[track_caller]
#[must_use] #[must_use]
pub fn split_at(&self, mid: usize) -> (&[T], &[T]) { pub const fn split_at(&self, mid: usize) -> (&[T], &[T]) {
assert!(mid <= self.len()); assert!(mid <= self.len());
// SAFETY: `[ptr; mid]` and `[mid; len]` are inside `self`, which // SAFETY: `[ptr; mid]` and `[mid; len]` are inside `self`, which
// fulfills the requirements of `from_raw_parts_mut`. // fulfills the requirements of `split_at_unchecked`.
unsafe { self.split_at_unchecked(mid) } unsafe { self.split_at_unchecked(mid) }
} }
@ -1626,11 +1627,19 @@ impl<T> [T] {
/// } /// }
/// ``` /// ```
#[unstable(feature = "slice_split_at_unchecked", reason = "new API", issue = "76014")] #[unstable(feature = "slice_split_at_unchecked", reason = "new API", issue = "76014")]
#[rustc_const_unstable(feature = "slice_split_at_unchecked", issue = "76014")]
#[inline] #[inline]
#[must_use] #[must_use]
pub unsafe fn split_at_unchecked(&self, mid: usize) -> (&[T], &[T]) { pub const unsafe fn split_at_unchecked(&self, mid: usize) -> (&[T], &[T]) {
// HACK: the const function `from_raw_parts` is used to make this
// function const; previously the implementation used
// `(self.get_unchecked(..mid), self.get_unchecked(mid..))`
let len = self.len();
let ptr = self.as_ptr();
// SAFETY: Caller has to check that `0 <= mid <= self.len()` // SAFETY: Caller has to check that `0 <= mid <= self.len()`
unsafe { (self.get_unchecked(..mid), self.get_unchecked(mid..)) } unsafe { (from_raw_parts(ptr, mid), from_raw_parts(ptr.add(mid), len - mid)) }
} }
/// Divides one mutable slice into two at an index, without doing bounds checking. /// Divides one mutable slice into two at an index, without doing bounds checking.