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Auto merge of #78818 - scottmcm:as_rchunks, r=KodrAus
Add `as_rchunks` (and friends) to slices `@est31` mentioned (https://github.com/rust-lang/rust/issues/76354#issuecomment-717027175) that, for completeness, there needed to be an `as_chunks`-like method that chunks from the end (with the remainder at the beginning) like `rchunks` does. So here's a PR for `as_rchunks: &[T] -> (&[T], &[[T; N]])` and `as_rchunks_mut: &mut [T] -> (&mut [T], &mut [[T; N]])`. But as I was doing this and copy-pasting `from_raw_parts` calls, I thought that I should extract that into an unsafe method. It started out a private helper, but it seemed like `as_chunks_unchecked` could be reasonable as a "real" method, so I added docs and made it public. Let me know if you think it doesn't pull its weight.
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@ -888,6 +888,46 @@ impl<T> [T] {
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ChunksExactMut::new(self, chunk_size)
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}
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/// Splits the slice into a slice of `N`-element arrays,
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/// assuming that there's no remainder.
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///
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/// # Safety
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///
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/// This may only be called when
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/// - The slice splits exactly into `N`-element chunks (aka `self.len() % N == 0`).
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/// - `N != 0`.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(slice_as_chunks)]
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/// let slice: &[char] = &['l', 'o', 'r', 'e', 'm', '!'];
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/// let chunks: &[[char; 1]] =
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/// // SAFETY: 1-element chunks never have remainder
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/// unsafe { slice.as_chunks_unchecked() };
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/// assert_eq!(chunks, &[['l'], ['o'], ['r'], ['e'], ['m'], ['!']]);
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/// let chunks: &[[char; 3]] =
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/// // SAFETY: The slice length (6) is a multiple of 3
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/// unsafe { slice.as_chunks_unchecked() };
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/// assert_eq!(chunks, &[['l', 'o', 'r'], ['e', 'm', '!']]);
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///
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/// // These would be unsound:
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/// // let chunks: &[[_; 5]] = slice.as_chunks_unchecked() // The slice length is not a multiple of 5
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/// // let chunks: &[[_; 0]] = slice.as_chunks_unchecked() // Zero-length chunks are never allowed
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/// ```
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#[unstable(feature = "slice_as_chunks", issue = "74985")]
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#[inline]
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pub unsafe fn as_chunks_unchecked<const N: usize>(&self) -> &[[T; N]] {
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debug_assert_ne!(N, 0);
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debug_assert_eq!(self.len() % N, 0);
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let new_len =
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// SAFETY: Our precondition is exactly what's needed to call this
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unsafe { crate::intrinsics::exact_div(self.len(), N) };
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// SAFETY: We cast a slice of `new_len * N` elements into
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// a slice of `new_len` many `N` elements chunks.
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unsafe { from_raw_parts(self.as_ptr().cast(), new_len) }
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}
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/// Splits the slice into a slice of `N`-element arrays,
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/// starting at the beginning of the slice,
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/// and a remainder slice with length strictly less than `N`.
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@ -912,12 +952,42 @@ impl<T> [T] {
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assert_ne!(N, 0);
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let len = self.len() / N;
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let (multiple_of_n, remainder) = self.split_at(len * N);
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// SAFETY: We cast a slice of `len * N` elements into
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// a slice of `len` many `N` elements chunks.
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let array_slice: &[[T; N]] = unsafe { from_raw_parts(multiple_of_n.as_ptr().cast(), len) };
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// SAFETY: We already panicked for zero, and ensured by construction
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// that the length of the subslice is a multiple of N.
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let array_slice = unsafe { multiple_of_n.as_chunks_unchecked() };
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(array_slice, remainder)
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}
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/// Splits the slice into a slice of `N`-element arrays,
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/// starting at the end of the slice,
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/// and a remainder slice with length strictly less than `N`.
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///
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/// # Panics
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///
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/// Panics if `N` is 0. This check will most probably get changed to a compile time
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/// error before this method gets stabilized.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(slice_as_chunks)]
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/// let slice = ['l', 'o', 'r', 'e', 'm'];
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/// let (remainder, chunks) = slice.as_rchunks();
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/// assert_eq!(remainder, &['l']);
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/// assert_eq!(chunks, &[['o', 'r'], ['e', 'm']]);
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/// ```
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#[unstable(feature = "slice_as_chunks", issue = "74985")]
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#[inline]
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pub fn as_rchunks<const N: usize>(&self) -> (&[T], &[[T; N]]) {
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assert_ne!(N, 0);
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let len = self.len() / N;
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let (remainder, multiple_of_n) = self.split_at(self.len() - len * N);
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// SAFETY: We already panicked for zero, and ensured by construction
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// that the length of the subslice is a multiple of N.
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let array_slice = unsafe { multiple_of_n.as_chunks_unchecked() };
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(remainder, array_slice)
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}
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/// Returns an iterator over `N` elements of the slice at a time, starting at the
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/// beginning of the slice.
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///
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@ -952,6 +1022,48 @@ impl<T> [T] {
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ArrayChunks::new(self)
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}
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/// Splits the slice into a slice of `N`-element arrays,
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/// assuming that there's no remainder.
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///
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/// # Safety
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///
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/// This may only be called when
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/// - The slice splits exactly into `N`-element chunks (aka `self.len() % N == 0`).
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/// - `N != 0`.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(slice_as_chunks)]
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/// let slice: &mut [char] = &mut ['l', 'o', 'r', 'e', 'm', '!'];
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/// let chunks: &mut [[char; 1]] =
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/// // SAFETY: 1-element chunks never have remainder
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/// unsafe { slice.as_chunks_unchecked_mut() };
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/// chunks[0] = ['L'];
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/// assert_eq!(chunks, &[['L'], ['o'], ['r'], ['e'], ['m'], ['!']]);
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/// let chunks: &mut [[char; 3]] =
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/// // SAFETY: The slice length (6) is a multiple of 3
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/// unsafe { slice.as_chunks_unchecked_mut() };
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/// chunks[1] = ['a', 'x', '?'];
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/// assert_eq!(slice, &['L', 'o', 'r', 'a', 'x', '?']);
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///
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/// // These would be unsound:
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/// // let chunks: &[[_; 5]] = slice.as_chunks_unchecked_mut() // The slice length is not a multiple of 5
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/// // let chunks: &[[_; 0]] = slice.as_chunks_unchecked_mut() // Zero-length chunks are never allowed
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/// ```
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#[unstable(feature = "slice_as_chunks", issue = "74985")]
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#[inline]
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pub unsafe fn as_chunks_unchecked_mut<const N: usize>(&mut self) -> &mut [[T; N]] {
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debug_assert_ne!(N, 0);
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debug_assert_eq!(self.len() % N, 0);
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let new_len =
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// SAFETY: Our precondition is exactly what's needed to call this
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unsafe { crate::intrinsics::exact_div(self.len(), N) };
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// SAFETY: We cast a slice of `new_len * N` elements into
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// a slice of `new_len` many `N` elements chunks.
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unsafe { from_raw_parts_mut(self.as_mut_ptr().cast(), new_len) }
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}
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/// Splits the slice into a slice of `N`-element arrays,
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/// starting at the beginning of the slice,
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/// and a remainder slice with length strictly less than `N`.
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@ -982,13 +1094,48 @@ impl<T> [T] {
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assert_ne!(N, 0);
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let len = self.len() / N;
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let (multiple_of_n, remainder) = self.split_at_mut(len * N);
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let array_slice: &mut [[T; N]] =
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// SAFETY: We cast a slice of `len * N` elements into
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// a slice of `len` many `N` elements chunks.
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unsafe { from_raw_parts_mut(multiple_of_n.as_mut_ptr().cast(), len) };
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// SAFETY: We already panicked for zero, and ensured by construction
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// that the length of the subslice is a multiple of N.
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let array_slice = unsafe { multiple_of_n.as_chunks_unchecked_mut() };
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(array_slice, remainder)
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}
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/// Splits the slice into a slice of `N`-element arrays,
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/// starting at the end of the slice,
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/// and a remainder slice with length strictly less than `N`.
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///
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/// # Panics
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///
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/// Panics if `N` is 0. This check will most probably get changed to a compile time
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/// error before this method gets stabilized.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(slice_as_chunks)]
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/// let v = &mut [0, 0, 0, 0, 0];
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/// let mut count = 1;
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///
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/// let (remainder, chunks) = v.as_rchunks_mut();
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/// remainder[0] = 9;
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/// for chunk in chunks {
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/// *chunk = [count; 2];
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/// count += 1;
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/// }
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/// assert_eq!(v, &[9, 1, 1, 2, 2]);
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/// ```
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#[unstable(feature = "slice_as_chunks", issue = "74985")]
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#[inline]
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pub fn as_rchunks_mut<const N: usize>(&mut self) -> (&mut [T], &mut [[T; N]]) {
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assert_ne!(N, 0);
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let len = self.len() / N;
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let (remainder, multiple_of_n) = self.split_at_mut(self.len() - len * N);
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// SAFETY: We already panicked for zero, and ensured by construction
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// that the length of the subslice is a multiple of N.
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let array_slice = unsafe { multiple_of_n.as_chunks_unchecked_mut() };
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(remainder, array_slice)
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}
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/// Returns an iterator over `N` elements of the slice at a time, starting at the
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/// beginning of the slice.
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///
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33
src/test/codegen/slice-as_chunks.rs
Normal file
33
src/test/codegen/slice-as_chunks.rs
Normal file
@ -0,0 +1,33 @@
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// no-system-llvm
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// compile-flags: -O
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// only-64bit (because the LLVM type of i64 for usize shows up)
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// ignore-debug: the debug assertions get in the way
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#![crate_type = "lib"]
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#![feature(slice_as_chunks)]
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// CHECK-LABEL: @chunks4
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#[no_mangle]
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pub fn chunks4(x: &[u8]) -> &[[u8; 4]] {
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// CHECK-NEXT: start:
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// CHECK-NEXT: lshr i64 %x.1, 2
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// CHECK-NOT: shl
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// CHECK-NOT: mul
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// CHECK-NOT: udiv
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// CHECK-NOT: urem
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// CHECK: ret
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x.as_chunks().0
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}
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// CHECK-LABEL: @chunks4_with_remainder
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#[no_mangle]
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pub fn chunks4_with_remainder(x: &[u8]) -> (&[[u8; 4]], &[u8]) {
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// CHECK: and i64 %x.1, -4
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// CHECK: and i64 %x.1, 3
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// CHECK: lshr exact
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// CHECK-NOT: mul
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// CHECK-NOT: udiv
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// CHECK-NOT: urem
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// CHECK: ret
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x.as_chunks()
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}
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