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Stabilize `slice_first_last_chunk` This PR does a few different things based around stabilizing `slice_first_last_chunk`. They are split up so this PR can be by-commit reviewed, I can move parts to a separate PR if desired. This feature provides a very elegant API to extract arrays from either end of a slice, such as for parsing integers from binary data. ## Stabilize `slice_first_last_chunk` ACP: https://github.com/rust-lang/libs-team/issues/69 Implementation: https://github.com/rust-lang/rust/issues/90091 Tracking issue: https://github.com/rust-lang/rust/issues/111774 This stabilizes the functionality from https://github.com/rust-lang/rust/issues/111774: ```rust impl [T] { pub const fn first_chunk<const N: usize>(&self) -> Option<&[T; N]>; pub fn first_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>; pub const fn last_chunk<const N: usize>(&self) -> Option<&[T; N]>; pub fn last_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>; pub const fn split_first_chunk<const N: usize>(&self) -> Option<(&[T; N], &[T])>; pub fn split_first_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T; N], &mut [T])>; pub const fn split_last_chunk<const N: usize>(&self) -> Option<(&[T], &[T; N])>; pub fn split_last_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T], &mut [T; N])>; } ``` Const stabilization is included for all non-mut methods, which are blocked on `const_mut_refs`. This change includes marking the trivial function `slice_split_at_unchecked` const-stable for internal use (but not fully stable). ## Remove `split_array` slice methods Tracking issue: https://github.com/rust-lang/rust/issues/90091 Implementation: https://github.com/rust-lang/rust/pull/83233#pullrequestreview-780315524 This PR also removes the following unstable methods from the `split_array` feature, https://github.com/rust-lang/rust/issues/90091: ```rust impl<T> [T] { pub fn split_array_ref<const N: usize>(&self) -> (&[T; N], &[T]); pub fn split_array_mut<const N: usize>(&mut self) -> (&mut [T; N], &mut [T]); pub fn rsplit_array_ref<const N: usize>(&self) -> (&[T], &[T; N]); pub fn rsplit_array_mut<const N: usize>(&mut self) -> (&mut [T], &mut [T; N]); } ``` This is done because discussion at #90091 and its implementation PR indicate a strong preference for nonpanicking APIs that return `Option`. The only difference between functions under the `split_array` and `slice_first_last_chunk` features is `Option` vs. panic, so remove the duplicates as part of this stabilization. This does not affect the array methods from `split_array`. We will want to revisit these once `generic_const_exprs` is further along. ## Reverse order of return tuple for `split_last_chunk{,_mut}` An unresolved question for #111774 is whether to return `(preceding_slice, last_chunk)` (`(&[T], &[T; N])`) or the reverse (`(&[T; N], &[T])`), from `split_last_chunk` and `split_last_chunk_mut`. It is currently implemented as `(last_chunk, preceding_slice)` which matches `split_last -> (&T, &[T])`. The first commit changes these to `(&[T], &[T; N])` for these reasons: - More consistent with other splitting methods that return multiple values: `str::rsplit_once`, `slice::split_at{,_mut}`, `slice::align_to` all return tuples with the items in order - More intuitive (arguably opinion, but it is consistent with other language elements like pattern matching `let [a, b, rest @ ..] ...` - If we ever added a varidic way to obtain multiple chunks, it would likely return something in order: `.split_many_last::<(2, 4)>() -> (&[T], &[T; 2], &[T; 4])` - It is the ordering used in the `rsplit_array` methods I think the inconsistency with `split_last` could be acceptable in this case, since for `split_last` the scalar `&T` doesn't have any internal order to maintain with the other items. ## Unresolved questions Do we want to reserve the same names on `[u8; N]` to avoid inference confusion? https://github.com/rust-lang/rust/pull/117561#issuecomment-1793388647 --- `slice_first_last_chunk` has only been around since early 2023, but `split_array` has been around since 2021. `@rustbot` label -T-libs +T-libs-api -T-libs +needs-fcp cc `@rust-lang/wg-const-eval,` `@scottmcm` who raised this topic, `@clarfonthey` implementer of `slice_first_last_chunk` `@jethrogb` implementer of `split_array` Zulip discussion: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Stabilizing.20array-from-slice.20*something*.3F Fixes: #111774 |
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