Use array::IntoIter for the ArrayChunks remainder

Author: rossmacarthur

library/core/src/array/iter.rs

@@ -84,6 +84,16 @@ impl<T, const N: usize> IntoIter<T, N> {
         IntoIterator::into_iter(array)
     }
 
+    /// Creates a new iterator from a partially initalized array.
+    ///
+    /// # Safety
+    ///
+    /// The caller must guarantee that all and only the `alive` elements of
+    /// `data` are initialized.
+    pub(crate) unsafe fn with_partial(data: [MaybeUninit<T>; N], alive: Range<usize>) -> Self {
+        Self { data, alive }
+    }
+
     /// Creates an iterator over the elements in a partially-initialized buffer.
     ///
     /// If you have a fully-initialized array, then use [`IntoIterator`].

library/core/src/iter/adapters/array_chunks.rs

@@ -1,68 +1,10 @@
+use crate::array;
 use crate::iter::{Fuse, FusedIterator, Iterator, TrustedLen};
 use crate::mem;
 use crate::mem::MaybeUninit;
 use crate::ops::{ControlFlow, Try};
 use crate::ptr;
 
-#[derive(Debug)]
-struct Remainder<T, const N: usize> {
-    array: [MaybeUninit<T>; N],
-    init: usize,
-}
-
-impl<T, const N: usize> Remainder<T, N> {
-    fn new() -> Self {
-        Self { array: MaybeUninit::uninit_array(), init: 0 }
-    }
-
-    unsafe fn with_init(array: [MaybeUninit<T>; N], init: usize) -> Self {
-        Self { array, init }
-    }
-
-    fn as_slice(&self) -> &[T] {
-        debug_assert!(self.init <= N);
-        // SAFETY: This raw slice will only contain the initialized objects
-        // within the buffer.
-        unsafe {
-            let slice = self.array.get_unchecked(..self.init);
-            MaybeUninit::slice_assume_init_ref(slice)
-        }
-    }
-
-    fn as_mut_slice(&mut self) -> &mut [T] {
-        debug_assert!(self.init <= N);
-        // SAFETY: This raw slice will only contain the initialized objects
-        // within the buffer.
-        unsafe {
-            let slice = self.array.get_unchecked_mut(..self.init);
-            MaybeUninit::slice_assume_init_mut(slice)
-        }
-    }
-}
-
-impl<T, const N: usize> Clone for Remainder<T, N>
-where
-    T: Clone,
-{
-    fn clone(&self) -> Self {
-        let mut new = Self::new();
-        // SAFETY: The new array is the same size and `init` is always less than
-        // or equal to `N`.
-        let this = unsafe { new.array.get_unchecked_mut(..self.init) };
-        MaybeUninit::write_slice_cloned(this, self.as_slice());
-        new.init = self.init;
-        new
-    }
-}
-
-impl<T, const N: usize> Drop for Remainder<T, N> {
-    fn drop(&mut self) {
-        // SAFETY: This raw slice will only contain the initialized objects
-        // within the buffer.
-        unsafe { ptr::drop_in_place(self.as_mut_slice()) }
-    }
-}
-
 /// An iterator over `N` elements of the iterator at a time.
 ///
 /// The chunks do not overlap. If `N` does not divide the length of the
@@ -75,7 +17,7 @@ impl<T, const N: usize> Drop for Remainder<T, N> {
 #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "none")]
 pub struct ArrayChunks<I: Iterator, const N: usize> {
     iter: Fuse<I>,
-    remainder: Remainder<I::Item, N>,
+    remainder: Option<array::IntoIter<I::Item, N>>,
 }
 
 impl<I, const N: usize> ArrayChunks<I, N>
@@ -84,25 +26,16 @@ where
 {
     pub(in crate::iter) fn new(iter: I) -> Self {
         assert!(N != 0, "chunk size must be non-zero");
-        Self { iter: iter.fuse(), remainder: Remainder::new() }
-    }
-
-    /// Returns a reference to the remaining elements of the original iterator
-    /// that are not going to be returned by this iterator. The returned slice
-    /// has at most `N-1` elements.
-    #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "none")]
-    #[inline]
-    pub fn remainder(&self) -> &[I::Item] {
-        self.remainder.as_slice()
+        Self { iter: iter.fuse(), remainder: None }
     }
 
-    /// Returns a mutable reference to the remaining elements of the original
-    /// iterator that are not going to be returned by this iterator. The
-    /// returned slice has at most `N-1` elements.
+    /// Returns an iterator over the remaining elements of the original iterator
+    /// that are not going to be returned by this iterator. The returned
+    /// iterator will yield at most `N-1` elements.
     #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "none")]
     #[inline]
-    pub fn remainder_mut(&mut self) -> &mut [I::Item] {
-        self.remainder.as_mut_slice()
+    pub fn into_remainder(self) -> Option<array::IntoIter<I::Item, N>> {
+        self.remainder
     }
 }
 
@@ -129,8 +62,10 @@ where
                     if guard.init > 0 {
                         let init = guard.init;
                         mem::forget(guard);
-                        // SAFETY: `array` was initialized with `init` elements.
-                        self.remainder = unsafe { Remainder::with_init(array, init) };
+                        self.remainder = {
+                            // SAFETY: `array` was initialized with `init` elements.
+                            Some(unsafe { array::IntoIter::with_partial(array, 0..init) })
+                        };
                     }
                     return None;
                 }
@@ -189,7 +124,7 @@ where
                     let init = guard.init;
                     mem::forget(guard);
                     // SAFETY: `array` was initialized with `init` elements.
-                    self.remainder = unsafe { Remainder::with_init(array, init) };
+                    self.remainder = Some(unsafe { array::IntoIter::with_partial(array, 0..init) });
                 }
                 R::from_output(o)
             }
@@ -370,7 +305,7 @@ where
         // SAFETY: `array` was initialized with exactly `init` elements.
         self.remainder = unsafe {
             array.get_unchecked_mut(..init).reverse();
-            Remainder::with_init(array, init)
+            Some(array::IntoIter::with_partial(array, 0..init))
         };
         Some(())
     }

library/core/src/iter/traits/iterator.rs

@@ -3092,7 +3092,9 @@ pub trait Iterator {
     /// Returns an iterator over `N` elements of the iterator at a time.
     ///
     /// The chunks do not overlap. If `N` does not divide the length of the
-    /// iterator, then the last up to `N-1` elements will be omitted.
+    /// iterator, then the last up to `N-1` elements will be omitted and can be
+    /// retrieved from the [`.into_remainder()`][ArrayChunks::into_remainder]
+    /// function of the iterator.
     ///
     /// # Panics
     ///
@@ -3109,7 +3111,7 @@ pub trait Iterator {
     /// assert_eq!(iter.next(), Some(['l', 'o']));
     /// assert_eq!(iter.next(), Some(['r', 'e']));
     /// assert_eq!(iter.next(), None);
-    /// assert_eq!(iter.remainder(), &['m']);
+    /// assert_eq!(iter.into_remainder().unwrap().as_slice(), &['m']);
     /// ```
     ///
     /// ```

library/core/tests/iter/adapters/array_chunks.rs

@@ -15,39 +15,24 @@ fn test_iterator_array_chunks_infer() {
 fn test_iterator_array_chunks_clone_and_drop() {
     let count = Cell::new(0);
     let mut it = (0..5).map(|_| CountDrop::new(&count)).array_chunks::<3>();
-
     assert_eq!(it.by_ref().count(), 1);
     assert_eq!(count.get(), 3);
-    assert_eq!(it.remainder().len(), 2);
-
     let mut it2 = it.clone();
     assert_eq!(count.get(), 3);
-    assert_eq!(it2.remainder().len(), 2);
-
-    drop(it);
+    assert_eq!(it.into_remainder().unwrap().len(), 2);
     assert_eq!(count.get(), 5);
-    assert_eq!(it2.remainder().len(), 2);
     assert!(it2.next().is_none());
-
-    drop(it2);
+    assert_eq!(it2.into_remainder().unwrap().len(), 2);
     assert_eq!(count.get(), 7);
 }
 
 #[test]
 fn test_iterator_array_chunks_remainder() {
     let mut it = (0..11).array_chunks::<4>();
-    assert_eq!(it.remainder(), &[]);
-    assert_eq!(it.remainder_mut(), &[]);
     assert_eq!(it.next(), Some([0, 1, 2, 3]));
-    assert_eq!(it.remainder(), &[]);
-    assert_eq!(it.remainder_mut(), &[]);
     assert_eq!(it.next(), Some([4, 5, 6, 7]));
-    assert_eq!(it.remainder(), &[]);
-    assert_eq!(it.remainder_mut(), &[]);
-    assert_eq!(it.next(), None);
     assert_eq!(it.next(), None);
-    assert_eq!(it.remainder(), &[8, 9, 10]);
-    assert_eq!(it.remainder_mut(), &[8, 9, 10]);
+    assert_eq!(it.into_remainder().unwrap().as_slice(), &[8, 9, 10]);
 }
 
 #[test]
@@ -105,8 +90,7 @@ fn test_iterator_array_chunks_next_and_next_back() {
     assert_eq!(it.next(), None);
     assert_eq!(it.next_back(), None);
     assert_eq!(it.next(), None);
-    assert_eq!(it.remainder(), &[9, 10]);
-    assert_eq!(it.remainder_mut(), &[9, 10]);
+    assert_eq!(it.into_remainder().unwrap().as_slice(), &[9, 10]);
 }
 
 #[test]
@@ -119,7 +103,7 @@ fn test_iterator_array_chunks_rev_remainder() {
         assert_eq!(it.next(), None);
         assert_eq!(it.next(), None);
     }
-    assert_eq!(it.remainder(), &[8, 9, 10]);
+    assert_eq!(it.into_remainder().unwrap().as_slice(), &[8, 9, 10]);
 }
 
 #[test]
@@ -128,7 +112,6 @@ fn test_iterator_array_chunks_try_fold() {
     let mut it = (0..10).map(|_| CountDrop::new(&count)).array_chunks::<3>();
     let result: Result<_, ()> = it.by_ref().try_fold(0, |acc, _item| Ok(acc + 1));
     assert_eq!(result, Ok(3));
-    assert_eq!(it.remainder().len(), 1);
     assert_eq!(count.get(), 9);
     drop(it);
     assert_eq!(count.get(), 10);
@@ -137,7 +120,6 @@ fn test_iterator_array_chunks_try_fold() {
     let mut it = (0..10).map(|_| CountDrop::new(&count)).array_chunks::<3>();
     let result = it.by_ref().try_fold(0, |acc, _item| if acc < 2 { Ok(acc + 1) } else { Err(acc) });
     assert_eq!(result, Err(2));
-    assert_eq!(it.remainder().len(), 0);
     assert_eq!(count.get(), 9);
     drop(it);
     assert_eq!(count.get(), 9);
@@ -166,7 +148,6 @@ fn test_iterator_array_chunks_try_rfold() {
     let mut it = (0..10).map(|_| CountDrop::new(&count)).array_chunks::<3>();
     let result: Result<_, ()> = it.try_rfold(0, |acc, _item| Ok(acc + 1));
     assert_eq!(result, Ok(3));
-    assert_eq!(it.remainder().len(), 1);
     assert_eq!(count.get(), 9);
     drop(it);
     assert_eq!(count.get(), 10);