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Showing content from https://github.com/rust-lang/libs-team/issues/69 below:

{split_,}{first,last}_chunk · Issue #69 · rust-lang/libs-team · GitHub

This is an ACP for rust-lang/rust#95198, which I believe predates ACPs. Either that or I just didn't know about them at the time.

In some cases, it's valuable to convert between slices and arrays, and there's not really a good way to do this safely.

The current best API we have for this is the unstable as_chunks, which leads to the below being used fairly often:

slice.as_chunks::<N>().0[0]

Which just isn't as readable.

The proposed solution:

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])>;
}

This borrows directly from the first/last APIs:

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])>;
}

The PR with the proposed solution: rust-lang/rust#95198
Discussion in the array_chunks tracking issue: rust-lang/rust#74985

This issue is part of the libs-api team API change proposal process. Once this issue is filed the libs-api team will review open proposals in its weekly meeting. You should receive feedback within a week or two.

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