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Tracking Issue for `slice_first_last_chunk` feature (`slice::{split_,}{first,last}_chunk{,_mut}`) · Issue #111774 · rust-lang/rust · GitHub

Feature gate: #![feature(slice_first_last_chunk)]

This is a tracking issue for the slice::{split_,}{first,last}_chunk{,_mut} API.

2024-09-06: Most of this is stable now, except for the const-ness of the mut methods.

impl [T] {
    pub const fn first_chunk<const N: usize>(&self) -> Option<&[T; N]>;
    pub const 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 const 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 const 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; N], &[T])>;
    pub const fn split_last_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T; N], &mut [T])>;
}

Note that this mirrors the existing, already-stabilised API:

impl [T] {
    pub const fn first(&self) -> Option<&T>;
    pub const fn first_mut(&mut self) -> Option<&mut T>;
    pub const fn last(&self) -> Option<&T>;
    pub const fn last_mut(&mut self) -> Option<&mut T>;
    pub const fn split_first(&self) -> Option<(&T, &[T])>;
    pub const fn split_first_mut(&mut self) -> Option<(&mut T, &mut [T])>;
    pub const fn split_last(&self) -> Option<(&T, &[T])>;
    pub const fn split_last_mut(&mut self) -> Option<(&mut T, &mut [T])>;
}

• API change proposal (ACP)¹: ACP: slice::{split_,}{first,last}_chunk libs-team#69
• Implementation: Add slice::{split_,}{first,last}_chunk{,_mut} #95198
• Final comment period (FCP)²
• Wait until Tracking Issue for feature(const_slice_split_at_mut) #101804 is stable
• Stabilization PR

• Is this the best order for the tuple in split_last_chunk? Consistency with split_last would have it be -> Option<(&[T;N], &[T])> as was ACPed, but it might be confusing to have the two slice-like things in the opposite order from every other API I can think of that returns two subslices of the original. In Add slice::{split_,}{first,last}_chunk{,_mut} #95198 (comment), @scottmcm expected -> Option<(&[T;N], &[T])> for split_first_chunk, but for it to be -> Option<(&[T], &[T;N])> in split_last_chunk so the tuple is "in order", in some sense.
• ...

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/api-change-proposals.html ↩

2. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

maxwase, stepancheg and GrayJack

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