Rollup merge of rust-lang#47126 - sdroege:exact-chunks, r=bluss

Add slice::ExactChunks and ::ExactChunksMut iterators

These guarantee that always the requested slice size will be returned
and any leftoever elements at the end will be ignored. It allows llvm to
get rid of bounds checks in the code using the iterator.

This is inspired by the same iterators provided by ndarray.

Fixes rust-lang#47115

I'll add unit tests for all this if the general idea and behaviour makes sense for everybody.
Also see rust-lang#47115 (comment) for an example what this improves.

src/liballoc/lib.rs

@@ -124,6 +124,7 @@
 #![feature(unsize)]
 #![feature(allocator_internals)]
 #![feature(on_unimplemented)]
+#![feature(exact_chunks)]
 
 #![cfg_attr(not(test), feature(fused, fn_traits, placement_new_protocol, swap_with_slice, i128))]
 #![cfg_attr(test, feature(test, box_heap))]

src/liballoc/slice.rs

@@ -123,6 +123,8 @@ pub use core::slice::{from_raw_parts, from_raw_parts_mut};
 pub use core::slice::{from_ref, from_ref_mut};
 #[unstable(feature = "slice_get_slice", issue = "35729")]
 pub use core::slice::SliceIndex;
+#[unstable(feature = "exact_chunks", issue = "47115")]
+pub use core::slice::{ExactChunks, ExactChunksMut};
 
 ////////////////////////////////////////////////////////////////////////////////
 // Basic slice extension methods
@@ -611,6 +613,9 @@ impl<T> [T] {
     /// not divide the length of the slice, then the last chunk will
     /// not have length `chunk_size`.
     ///
+    /// See [`exact_chunks`] for a variant of this iterator that returns chunks
+    /// of always exactly `chunk_size` elements.
+    ///
     /// # Panics
     ///
     /// Panics if `chunk_size` is 0.
@@ -631,11 +636,44 @@ impl<T> [T] {
         core_slice::SliceExt::chunks(self, chunk_size)
     }
 
+    /// Returns an iterator over `chunk_size` elements of the slice at a
+    /// time. The chunks are slices and do not overlap. If `chunk_size` does
+    /// not divide the length of the slice, then the last up to `chunk_size-1`
+    /// elements will be omitted.
+    ///
+    /// Due to each chunk having exactly `chunk_size` elements, the compiler
+    /// can often optimize the resulting code better than in the case of
+    /// [`chunks`].
+    ///
+    /// # Panics
+    ///
+    /// Panics if `chunk_size` is 0.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(exact_chunks)]
+    ///
+    /// let slice = ['l', 'o', 'r', 'e', 'm'];
+    /// let mut iter = slice.exact_chunks(2);
+    /// assert_eq!(iter.next().unwrap(), &['l', 'o']);
+    /// assert_eq!(iter.next().unwrap(), &['r', 'e']);
+    /// assert!(iter.next().is_none());
+    /// ```
+    #[unstable(feature = "exact_chunks", issue = "47115")]
+    #[inline]
+    pub fn exact_chunks(&self, chunk_size: usize) -> ExactChunks<T> {
+        core_slice::SliceExt::exact_chunks(self, chunk_size)
+    }
+
     /// Returns an iterator over `chunk_size` elements of the slice at a time.
     /// The chunks are mutable slices, and do not overlap. If `chunk_size` does
     /// not divide the length of the slice, then the last chunk will not
     /// have length `chunk_size`.
     ///
+    /// See [`exact_chunks_mut`] for a variant of this iterator that returns chunks
+    /// of always exactly `chunk_size` elements.
+    ///
     /// # Panics
     ///
     /// Panics if `chunk_size` is 0.
@@ -660,6 +698,42 @@ impl<T> [T] {
         core_slice::SliceExt::chunks_mut(self, chunk_size)
     }
 
+    /// Returns an iterator over `chunk_size` elements of the slice at a time.
+    /// The chunks are mutable slices, and do not overlap. If `chunk_size` does
+    /// not divide the length of the slice, then the last up to `chunk_size-1`
+    /// elements will be omitted.
+    ///
+    ///
+    /// Due to each chunk having exactly `chunk_size` elements, the compiler
+    /// can often optimize the resulting code better than in the case of
+    /// [`chunks_mut`].
+    ///
+    /// # Panics
+    ///
+    /// Panics if `chunk_size` is 0.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(exact_chunks)]
+    ///
+    /// let v = &mut [0, 0, 0, 0, 0];
+    /// let mut count = 1;
+    ///
+    /// for chunk in v.exact_chunks_mut(2) {
+    ///     for elem in chunk.iter_mut() {
+    ///         *elem += count;
+    ///     }
+    ///     count += 1;
+    /// }
+    /// assert_eq!(v, &[1, 1, 2, 2, 0]);
+    /// ```
+    #[unstable(feature = "exact_chunks", issue = "47115")]
+    #[inline]
+    pub fn exact_chunks_mut(&mut self, chunk_size: usize) -> ExactChunksMut<T> {
+        core_slice::SliceExt::exact_chunks_mut(self, chunk_size)
+    }
+
     /// Divides one slice into two at an index.
     ///
     /// The first will contain all indices from `[0, mid)` (excluding

src/liballoc/tests/lib.rs

@@ -30,6 +30,7 @@
 #![feature(string_retain)]
 #![feature(unboxed_closures)]
 #![feature(unicode)]
+#![feature(exact_chunks)]
 
 extern crate alloc_system;
 extern crate std_unicode;

src/liballoc/tests/slice.rs

@@ -945,6 +945,30 @@ fn test_chunksator_0() {
     let _it = v.chunks(0);
 }
 
+#[test]
+fn test_exact_chunksator() {
+    let v = &[1, 2, 3, 4, 5];
+
+    assert_eq!(v.exact_chunks(2).len(), 2);
+
+    let chunks: &[&[_]] = &[&[1, 2], &[3, 4]];
+    assert_eq!(v.exact_chunks(2).collect::<Vec<_>>(), chunks);
+    let chunks: &[&[_]] = &[&[1, 2, 3]];
+    assert_eq!(v.exact_chunks(3).collect::<Vec<_>>(), chunks);
+    let chunks: &[&[_]] = &[];
+    assert_eq!(v.exact_chunks(6).collect::<Vec<_>>(), chunks);
+
+    let chunks: &[&[_]] = &[&[3, 4], &[1, 2]];
+    assert_eq!(v.exact_chunks(2).rev().collect::<Vec<_>>(), chunks);
+}
+
+#[test]
+#[should_panic]
+fn test_exact_chunksator_0() {
+    let v = &[1, 2, 3, 4];
+    let _it = v.exact_chunks(0);
+}
+
 #[test]
 fn test_reverse_part() {
     let mut values = [1, 2, 3, 4, 5];
@@ -1159,7 +1183,7 @@ fn test_mut_chunks() {
         }
     }
     let result = [0, 0, 0, 1, 1, 1, 2];
-    assert!(v == result);
+    assert_eq!(v, result);
 }
 
 #[test]
@@ -1171,7 +1195,7 @@ fn test_mut_chunks_rev() {
         }
     }
     let result = [2, 2, 2, 1, 1, 1, 0];
-    assert!(v == result);
+    assert_eq!(v, result);
 }
 
 #[test]
@@ -1181,6 +1205,38 @@ fn test_mut_chunks_0() {
     let _it = v.chunks_mut(0);
 }
 
+#[test]
+fn test_mut_exact_chunks() {
+    let mut v = [0, 1, 2, 3, 4, 5, 6];
+    assert_eq!(v.exact_chunks_mut(2).len(), 3);
+    for (i, chunk) in v.exact_chunks_mut(3).enumerate() {
+        for x in chunk {
+            *x = i as u8;
+        }
+    }
+    let result = [0, 0, 0, 1, 1, 1, 6];
+    assert_eq!(v, result);
+}
+
+#[test]
+fn test_mut_exact_chunks_rev() {
+    let mut v = [0, 1, 2, 3, 4, 5, 6];
+    for (i, chunk) in v.exact_chunks_mut(3).rev().enumerate() {
+        for x in chunk {
+            *x = i as u8;
+        }
+    }
+    let result = [1, 1, 1, 0, 0, 0, 6];
+    assert_eq!(v, result);
+}
+
+#[test]
+#[should_panic]
+fn test_mut_exact_chunks_0() {
+    let mut v = [1, 2, 3, 4];
+    let _it = v.exact_chunks_mut(0);
+}
+
 #[test]
 fn test_mut_last() {
     let mut x = [1, 2, 3, 4, 5];