Auto merge of #75021 - cuviper:array_chunks_mut, r=scottmcm

Add `slice::array_chunks_mut`

This follows `array_chunks` from #74373 with a mutable version, `array_chunks_mut`. The implementation is identical apart from mutability. The new tests are adaptations of the `chunks_exact_mut` tests, plus an inference test like the one for `array_chunks`.

I reused the unstable feature `array_chunks` and tracking issue #74985, but I can separate that if desired.

r? `@withoutboats`
cc `@lcnr`

Author: bors

library/alloc/src/slice.rs

@@ -93,6 +93,8 @@ use crate::vec::Vec;
 
 #[unstable(feature = "array_chunks", issue = "74985")]
 pub use core::slice::ArrayChunks;
+#[unstable(feature = "array_chunks", issue = "74985")]
+pub use core::slice::ArrayChunksMut;
 #[stable(feature = "slice_get_slice", since = "1.28.0")]
 pub use core::slice::SliceIndex;
 #[stable(feature = "from_ref", since = "1.28.0")]

library/core/src/slice/mod.rs

@@ -996,9 +996,9 @@ impl<T> [T] {
     /// Returns an iterator over `N` elements of the slice at a time, starting at the
     /// beginning of the slice.
     ///
-    /// The chunks are slices and do not overlap. If `N` does not divide the length of the
-    /// slice, then the last up to `N-1` elements will be omitted and can be retrieved
-    /// from the `remainder` function of the iterator.
+    /// The chunks are array references and do not overlap. If `N` does not divide the
+    /// length of the slice, then the last up to `N-1` elements will be omitted and can be
+    /// retrieved from the `remainder` function of the iterator.
     ///
     /// This method is the const generic equivalent of [`chunks_exact`].
     ///
@@ -1032,6 +1032,49 @@ impl<T> [T] {
         ArrayChunks { iter: array_slice.iter(), rem: snd }
     }
 
+    /// Returns an iterator over `N` elements of the slice at a time, starting at the
+    /// beginning of the slice.
+    ///
+    /// The chunks are mutable array references and do not overlap. If `N` does not divide
+    /// the length of the slice, then the last up to `N-1` elements will be omitted and
+    /// can be retrieved from the `into_remainder` function of the iterator.
+    ///
+    /// This method is the const generic equivalent of [`chunks_exact_mut`].
+    ///
+    /// # 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(array_chunks)]
+    /// let v = &mut [0, 0, 0, 0, 0];
+    /// let mut count = 1;
+    ///
+    /// for chunk in v.array_chunks_mut() {
+    ///     *chunk = [count; 2];
+    ///     count += 1;
+    /// }
+    /// assert_eq!(v, &[1, 1, 2, 2, 0]);
+    /// ```
+    ///
+    /// [`chunks_exact_mut`]: #method.chunks_exact_mut
+    #[unstable(feature = "array_chunks", issue = "74985")]
+    #[inline]
+    pub fn array_chunks_mut<const N: usize>(&mut self) -> ArrayChunksMut<'_, T, N> {
+        assert_ne!(N, 0);
+        let len = self.len() / N;
+        let (fst, snd) = self.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);
+            ArrayChunksMut { iter: array_slice.iter_mut(), rem: snd }
+        }
+    }
+
     /// Returns an iterator over `chunk_size` elements of the slice at a time, starting at the end
     /// of the slice.
     ///
@@ -5826,7 +5869,7 @@ unsafe impl<'a, T> TrustedRandomAccess for ChunksExactMut<'a, T> {
 /// time), starting at the beginning of the slice.
 ///
 /// When the slice len is not evenly divided by the chunk size, the last
-/// up to `chunk_size-1` elements will be omitted but can be retrieved from
+/// up to `N-1` elements will be omitted but can be retrieved from
 /// the [`remainder`] function from the iterator.
 ///
 /// This struct is created by the [`array_chunks`] method on [slices].
@@ -5843,7 +5886,7 @@ pub struct ArrayChunks<'a, T: 'a, const N: usize> {
 
 impl<'a, T, const N: usize> ArrayChunks<'a, T, N> {
     /// Returns the remainder of the original slice that is not going to be
-    /// returned by the iterator. The returned slice has at most `chunk_size-1`
+    /// returned by the iterator. The returned slice has at most `N-1`
     /// elements.
     #[unstable(feature = "array_chunks", issue = "74985")]
     pub fn remainder(&self) -> &'a [T] {
@@ -5929,6 +5972,105 @@ unsafe impl<'a, T, const N: usize> TrustedRandomAccess for ArrayChunks<'a, T, N>
     }
 }
 
+/// An iterator over a slice in (non-overlapping) mutable chunks (`N` elements
+/// at a time), starting at the beginning of the slice.
+///
+/// When the slice len is not evenly divided by the chunk size, the last
+/// up to `N-1` elements will be omitted but can be retrieved from
+/// the [`into_remainder`] function from the iterator.
+///
+/// This struct is created by the [`array_chunks_mut`] method on [slices].
+///
+/// [`array_chunks_mut`]: ../../std/primitive.slice.html#method.array_chunks_mut
+/// [`into_remainder`]: ../../std/slice/struct.ArrayChunksMut.html#method.into_remainder
+/// [slices]: ../../std/primitive.slice.html
+#[derive(Debug)]
+#[unstable(feature = "array_chunks", issue = "74985")]
+pub struct ArrayChunksMut<'a, T: 'a, const N: usize> {
+    iter: IterMut<'a, [T; N]>,
+    rem: &'a mut [T],
+}
+
+impl<'a, T, const N: usize> ArrayChunksMut<'a, T, N> {
+    /// Returns the remainder of the original slice that is not going to be
+    /// returned by the iterator. The returned slice has at most `N-1`
+    /// elements.
+    #[unstable(feature = "array_chunks", issue = "74985")]
+    pub fn into_remainder(self) -> &'a mut [T] {
+        self.rem
+    }
+}
+
+#[unstable(feature = "array_chunks", issue = "74985")]
+impl<'a, T, const N: usize> Iterator for ArrayChunksMut<'a, T, N> {
+    type Item = &'a mut [T; N];
+
+    #[inline]
+    fn next(&mut self) -> Option<&'a mut [T; N]> {
+        self.iter.next()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+
+    #[inline]
+    fn count(self) -> usize {
+        self.iter.count()
+    }
+
+    #[inline]
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        self.iter.nth(n)
+    }
+
+    #[inline]
+    fn last(self) -> Option<Self::Item> {
+        self.iter.last()
+    }
+
+    unsafe fn get_unchecked(&mut self, i: usize) -> &'a mut [T; N] {
+        // SAFETY: The safety guarantees of `get_unchecked` are transferred to
+        // the caller.
+        unsafe { self.iter.get_unchecked(i) }
+    }
+}
+
+#[unstable(feature = "array_chunks", issue = "74985")]
+impl<'a, T, const N: usize> DoubleEndedIterator for ArrayChunksMut<'a, T, N> {
+    #[inline]
+    fn next_back(&mut self) -> Option<&'a mut [T; N]> {
+        self.iter.next_back()
+    }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        self.iter.nth_back(n)
+    }
+}
+
+#[unstable(feature = "array_chunks", issue = "74985")]
+impl<T, const N: usize> ExactSizeIterator for ArrayChunksMut<'_, T, N> {
+    fn is_empty(&self) -> bool {
+        self.iter.is_empty()
+    }
+}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<T, const N: usize> TrustedLen for ArrayChunksMut<'_, T, N> {}
+
+#[unstable(feature = "array_chunks", issue = "74985")]
+impl<T, const N: usize> FusedIterator for ArrayChunksMut<'_, T, N> {}
+
+#[doc(hidden)]
+#[unstable(feature = "array_chunks", issue = "74985")]
+unsafe impl<'a, T, const N: usize> TrustedRandomAccess for ArrayChunksMut<'a, T, N> {
+    fn may_have_side_effect() -> bool {
+        false
+    }
+}
+
 /// An iterator over a slice in (non-overlapping) chunks (`chunk_size` elements at a
 /// time), starting at the end of the slice.
 ///

library/core/tests/slice.rs

@@ -564,6 +564,99 @@ fn test_array_chunks_zip() {
     assert_eq!(res, vec![14, 22]);
 }
 
+#[test]
+fn test_array_chunks_mut_infer() {
+    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5, 6];
+    for a in v.array_chunks_mut() {
+        let sum = a.iter().sum::<i32>();
+        *a = [sum; 3];
+    }
+    assert_eq!(v, &[3, 3, 3, 12, 12, 12, 6]);
+
+    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4, 5, 6];
+    v2.array_chunks_mut().for_each(|[a, b]| core::mem::swap(a, b));
+    assert_eq!(v2, &[1, 0, 3, 2, 5, 4, 6]);
+}
+
+#[test]
+fn test_array_chunks_mut_count() {
+    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
+    let c = v.array_chunks_mut::<3>();
+    assert_eq!(c.count(), 2);
+
+    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
+    let c2 = v2.array_chunks_mut::<2>();
+    assert_eq!(c2.count(), 2);
+
+    let v3: &mut [i32] = &mut [];
+    let c3 = v3.array_chunks_mut::<2>();
+    assert_eq!(c3.count(), 0);
+}
+
+#[test]
+fn test_array_chunks_mut_nth() {
+    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
+    let mut c = v.array_chunks_mut::<2>();
+    assert_eq!(c.nth(1).unwrap(), &[2, 3]);
+    assert_eq!(c.next().unwrap(), &[4, 5]);
+
+    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4, 5, 6];
+    let mut c2 = v2.array_chunks_mut::<3>();
+    assert_eq!(c2.nth(1).unwrap(), &[3, 4, 5]);
+    assert_eq!(c2.next(), None);
+}
+
+#[test]
+fn test_array_chunks_mut_nth_back() {
+    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
+    let mut c = v.array_chunks_mut::<2>();
+    assert_eq!(c.nth_back(1).unwrap(), &[2, 3]);
+    assert_eq!(c.next().unwrap(), &[0, 1]);
+    assert_eq!(c.next(), None);
+
+    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
+    let mut c2 = v2.array_chunks_mut::<3>();
+    assert_eq!(c2.nth_back(0).unwrap(), &[0, 1, 2]);
+    assert_eq!(c2.next(), None);
+    assert_eq!(c2.next_back(), None);
+
+    let v3: &mut [i32] = &mut [0, 1, 2, 3, 4];
+    let mut c3 = v3.array_chunks_mut::<10>();
+    assert_eq!(c3.nth_back(0), None);
+}
+
+#[test]
+fn test_array_chunks_mut_last() {
+    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
+    let c = v.array_chunks_mut::<2>();
+    assert_eq!(c.last().unwrap(), &[4, 5]);
+
+    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
+    let c2 = v2.array_chunks_mut::<2>();
+    assert_eq!(c2.last().unwrap(), &[2, 3]);
+}
+
+#[test]
+fn test_array_chunks_mut_remainder() {
+    let v: &mut [i32] = &mut [0, 1, 2, 3, 4];
+    let c = v.array_chunks_mut::<2>();
+    assert_eq!(c.into_remainder(), &[4]);
+}
+
+#[test]
+fn test_array_chunks_mut_zip() {
+    let v1: &mut [i32] = &mut [0, 1, 2, 3, 4];
+    let v2: &[i32] = &[6, 7, 8, 9, 10];
+
+    for (a, b) in v1.array_chunks_mut::<2>().zip(v2.array_chunks::<2>()) {
+        let sum = b.iter().sum::<i32>();
+        for v in a {
+            *v += sum;
+        }
+    }
+    assert_eq!(v1, [13, 14, 19, 20, 4]);
+}
+
 #[test]
 fn test_rchunks_count() {
     let v: &[i32] = &[0, 1, 2, 3, 4, 5];