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Add [T]::as_chunks(_mut) #76635

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Oct 27, 2020
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Add [T]::as_chunks(_mut) #76635

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19 changes: 4 additions & 15 deletions library/core/src/slice/iter.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2102,13 +2102,8 @@ pub struct ArrayChunks<'a, T: 'a, const N: usize> {
impl<'a, T, const N: usize> ArrayChunks<'a, T, N> {
#[inline]
pub(super) fn new(slice: &'a [T]) -> Self {
let len = slice.len() / N;
let (fst, snd) = slice.split_at(len * N);
// SAFETY: We cast a slice of `len * N` elements into
// a slice of `len` many `N` elements chunks.
let array_slice: &[[T; N]] = unsafe { from_raw_parts(fst.as_ptr().cast(), len) };

Self { iter: array_slice.iter(), rem: snd }
let (array_slice, rem) = slice.as_chunks();
Self { iter: array_slice.iter(), rem }
}

/// Returns the remainder of the original slice that is not going to be
Expand Down Expand Up @@ -2229,14 +2224,8 @@ pub struct ArrayChunksMut<'a, T: 'a, const N: usize> {
impl<'a, T, const N: usize> ArrayChunksMut<'a, T, N> {
#[inline]
pub(super) fn new(slice: &'a mut [T]) -> Self {
let len = slice.len() / N;
let (fst, snd) = slice.split_at_mut(len * N);
// SAFETY: We cast a slice of `len * N` elements into
// a slice of `len` many `N` elements chunks.
unsafe {
let array_slice: &mut [[T; N]] = from_raw_parts_mut(fst.as_mut_ptr().cast(), len);
Self { iter: array_slice.iter_mut(), rem: snd }
}
let (array_slice, rem) = slice.as_chunks_mut();
Self { iter: array_slice.iter_mut(), rem }
}

/// Returns the remainder of the original slice that is not going to be
Expand Down
67 changes: 67 additions & 0 deletions library/core/src/slice/mod.rs
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Expand Up @@ -883,6 +883,36 @@ impl<T> [T] {
ChunksExactMut::new(self, chunk_size)
}

/// Splits the slice into a slice of `N`-element arrays,
/// starting at the beginning of the slice,
/// and a remainder slice with length strictly less than `N`.
///
/// # Panics
///
/// Panics if `N` is 0. This check will most probably get changed to a compile time
/// error before this method gets stabilized.
///
/// # Examples
///
/// ```
/// #![feature(slice_as_chunks)]
/// let slice = ['l', 'o', 'r', 'e', 'm'];
/// let (chunks, remainder) = slice.as_chunks();
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Should the example be: let (...) = slice.as_chunks::<2>();? (Similar for as_chunks_mut.)

I imagine this example with implicit N only works because N is inferred from the subsequent assert_eq! structure. (But I'm pretty new to Rust, so maybe I'm missing something!)

Thanks!

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It does indeed infer N, and that's meant to be a good thing, but perhaps the example should have a comment pointing this out.

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We could either put slice.as_chunks() as comment or slice.as_chunks::<2>() as comment to show that it is being infered.

But in some cases type is not infered which may be confusing, like Some([]) cannot be infered as N for [T; N] so it may contradict to this comment here, so it would also be safer to say "most of the time it could be infered" for now. I recall seeing this in one of the pull request.

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Yes, it does infer N in these cases. I agree it would be nice to also have some examples that turbofish it.

Maybe some like these, to emphasize the length math:

let slice = ['-'; 100];
let (chunks, remainder) = slice.as_chunks::<20>();
assert_eq!((chunks.len(), remainder.len()), (5, 0));
let (chunks, remainder) = slice.as_chunks::<30>();
assert_eq!((chunks.len(), remainder.len()), (3, 10));

/// assert_eq!(chunks, &[['l', 'o'], ['r', 'e']]);
/// assert_eq!(remainder, &['m']);
/// ```
#[unstable(feature = "slice_as_chunks", issue = "74985")]
#[inline]
pub fn as_chunks<const N: usize>(&self) -> (&[[T; N]], &[T]) {
assert_ne!(N, 0);
let len = self.len() / N;
let (multiple_of_n, remainder) = self.split_at(len * N);
// SAFETY: We cast a slice of `len * N` elements into
// a slice of `len` many `N` elements chunks.
let array_slice: &[[T; N]] = unsafe { from_raw_parts(multiple_of_n.as_ptr().cast(), len) };
(array_slice, remainder)
}

/// Returns an iterator over `N` elements of the slice at a time, starting at the
/// beginning of the slice.
///
Expand Down Expand Up @@ -917,6 +947,43 @@ impl<T> [T] {
ArrayChunks::new(self)
}

/// Splits the slice into a slice of `N`-element arrays,
/// starting at the beginning of the slice,
/// and a remainder slice with length strictly less than `N`.
///
/// # Panics
///
/// Panics if `N` is 0. This check will most probably get changed to a compile time
/// error before this method gets stabilized.
///
/// # Examples
///
/// ```
/// #![feature(slice_as_chunks)]
/// let v = &mut [0, 0, 0, 0, 0];
/// let mut count = 1;
///
/// let (chunks, remainder) = v.as_chunks_mut();
/// remainder[0] = 9;
/// for chunk in chunks {
/// *chunk = [count; 2];
/// count += 1;
/// }
/// assert_eq!(v, &[1, 1, 2, 2, 9]);
/// ```
#[unstable(feature = "slice_as_chunks", issue = "74985")]
#[inline]
pub fn as_chunks_mut<const N: usize>(&mut self) -> (&mut [[T; N]], &mut [T]) {
assert_ne!(N, 0);
let len = self.len() / N;
let (multiple_of_n, remainder) = self.split_at_mut(len * N);
let array_slice: &mut [[T; N]] =
// SAFETY: We cast a slice of `len * N` elements into
// a slice of `len` many `N` elements chunks.
unsafe { from_raw_parts_mut(multiple_of_n.as_mut_ptr().cast(), len) };
(array_slice, remainder)
}

/// Returns an iterator over `N` elements of the slice at a time, starting at the
/// beginning of the slice.
///
Expand Down