Auto merge of #54580 - sdroege:rchunks, r=SimonSapin

Add slice::rchunks(), rchunks_mut(), rchunks_exact() and rchunks_exact_mut()

These work exactly like the normal chunks iterators but start creating
chunks from the end of the slice.

----

The new iterators were motivated by a [comment](#47115 (comment)) by @DutchGhost.

~~~This currently includes the commits from #54537 to not have to rename things twice or have merge conflicts. I'll force-push a new version of the branch ones those are in master.~~~

Also the stabilization tracking issue is just some number right now. I'll create the corresponding issue once this is reviewed and otherwise mergeable.

cc @DutchGhost

src/liballoc/lib.rs

@@ -120,6 +120,7 @@
 #![feature(const_vec_new)]
 #![feature(slice_partition_dedup)]
 #![feature(maybe_uninit)]
+#![feature(rchunks)]
 
 // Allow testing this library
 

src/liballoc/slice.rs

@@ -125,6 +125,8 @@ pub use core::slice::{from_ref, from_mut};
 pub use core::slice::SliceIndex;
 #[unstable(feature = "chunks_exact", issue = "47115")]
 pub use core::slice::{ChunksExact, ChunksExactMut};
+#[unstable(feature = "rchunks", issue = "55177")]
+pub use core::slice::{RChunks, RChunksMut, RChunksExact, RChunksExactMut};
 
 ////////////////////////////////////////////////////////////////////////////////
 // Basic slice extension methods

src/liballoc/tests/lib.rs

@@ -20,6 +20,7 @@
 #![feature(try_reserve)]
 #![feature(unboxed_closures)]
 #![feature(chunks_exact)]
+#![feature(rchunks)]
 #![feature(repeat_generic_slice)]
 
 extern crate alloc_system;

src/liballoc/tests/slice.rs

@@ -998,6 +998,54 @@ fn test_chunks_exactator_0() {
     let _it = v.chunks_exact(0);
 }
 
+#[test]
+fn test_rchunksator() {
+    let v = &[1, 2, 3, 4, 5];
+
+    assert_eq!(v.rchunks(2).len(), 3);
+
+    let chunks: &[&[_]] = &[&[4, 5], &[2, 3], &[1]];
+    assert_eq!(v.rchunks(2).collect::<Vec<_>>(), chunks);
+    let chunks: &[&[_]] = &[&[3, 4, 5], &[1, 2]];
+    assert_eq!(v.rchunks(3).collect::<Vec<_>>(), chunks);
+    let chunks: &[&[_]] = &[&[1, 2, 3, 4, 5]];
+    assert_eq!(v.rchunks(6).collect::<Vec<_>>(), chunks);
+
+    let chunks: &[&[_]] = &[&[1], &[2, 3], &[4, 5]];
+    assert_eq!(v.rchunks(2).rev().collect::<Vec<_>>(), chunks);
+}
+
+#[test]
+#[should_panic]
+fn test_rchunksator_0() {
+    let v = &[1, 2, 3, 4];
+    let _it = v.rchunks(0);
+}
+
+#[test]
+fn test_rchunks_exactator() {
+    let v = &[1, 2, 3, 4, 5];
+
+    assert_eq!(v.rchunks_exact(2).len(), 2);
+
+    let chunks: &[&[_]] = &[&[4, 5], &[2, 3]];
+    assert_eq!(v.rchunks_exact(2).collect::<Vec<_>>(), chunks);
+    let chunks: &[&[_]] = &[&[3, 4, 5]];
+    assert_eq!(v.rchunks_exact(3).collect::<Vec<_>>(), chunks);
+    let chunks: &[&[_]] = &[];
+    assert_eq!(v.rchunks_exact(6).collect::<Vec<_>>(), chunks);
+
+    let chunks: &[&[_]] = &[&[2, 3], &[4, 5]];
+    assert_eq!(v.rchunks_exact(2).rev().collect::<Vec<_>>(), chunks);
+}
+
+#[test]
+#[should_panic]
+fn test_rchunks_exactator_0() {
+    let v = &[1, 2, 3, 4];
+    let _it = v.rchunks_exact(0);
+}
+
 #[test]
 fn test_reverse_part() {
     let mut values = [1, 2, 3, 4, 5];
@@ -1205,7 +1253,7 @@ fn test_get_mut() {
 #[test]
 fn test_mut_chunks() {
     let mut v = [0, 1, 2, 3, 4, 5, 6];
-    assert_eq!(v.chunks_mut(2).len(), 4);
+    assert_eq!(v.chunks_mut(3).len(), 3);
     for (i, chunk) in v.chunks_mut(3).enumerate() {
         for x in chunk {
             *x = i as u8;
@@ -1237,7 +1285,7 @@ fn test_mut_chunks_0() {
 #[test]
 fn test_mut_chunks_exact() {
     let mut v = [0, 1, 2, 3, 4, 5, 6];
-    assert_eq!(v.chunks_exact_mut(2).len(), 3);
+    assert_eq!(v.chunks_exact_mut(3).len(), 2);
     for (i, chunk) in v.chunks_exact_mut(3).enumerate() {
         for x in chunk {
             *x = i as u8;
@@ -1266,6 +1314,70 @@ fn test_mut_chunks_exact_0() {
     let _it = v.chunks_exact_mut(0);
 }
 
+#[test]
+fn test_mut_rchunks() {
+    let mut v = [0, 1, 2, 3, 4, 5, 6];
+    assert_eq!(v.rchunks_mut(3).len(), 3);
+    for (i, chunk) in v.rchunks_mut(3).enumerate() {
+        for x in chunk {
+            *x = i as u8;
+        }
+    }
+    let result = [2, 1, 1, 1, 0, 0, 0];
+    assert_eq!(v, result);
+}
+
+#[test]
+fn test_mut_rchunks_rev() {
+    let mut v = [0, 1, 2, 3, 4, 5, 6];
+    for (i, chunk) in v.rchunks_mut(3).rev().enumerate() {
+        for x in chunk {
+            *x = i as u8;
+        }
+    }
+    let result = [0, 1, 1, 1, 2, 2, 2];
+    assert_eq!(v, result);
+}
+
+#[test]
+#[should_panic]
+fn test_mut_rchunks_0() {
+    let mut v = [1, 2, 3, 4];
+    let _it = v.rchunks_mut(0);
+}
+
+#[test]
+fn test_mut_rchunks_exact() {
+    let mut v = [0, 1, 2, 3, 4, 5, 6];
+    assert_eq!(v.rchunks_exact_mut(3).len(), 2);
+    for (i, chunk) in v.rchunks_exact_mut(3).enumerate() {
+        for x in chunk {
+            *x = i as u8;
+        }
+    }
+    let result = [0, 1, 1, 1, 0, 0, 0];
+    assert_eq!(v, result);
+}
+
+#[test]
+fn test_mut_rchunks_exact_rev() {
+    let mut v = [0, 1, 2, 3, 4, 5, 6];
+    for (i, chunk) in v.rchunks_exact_mut(3).rev().enumerate() {
+        for x in chunk {
+            *x = i as u8;
+        }
+    }
+    let result = [0, 0, 0, 0, 1, 1, 1];
+    assert_eq!(v, result);
+}
+
+#[test]
+#[should_panic]
+fn test_mut_rchunks_exact_0() {
+    let mut v = [1, 2, 3, 4];
+    let _it = v.rchunks_exact_mut(0);
+}
+
 #[test]
 fn test_mut_last() {
     let mut x = [1, 2, 3, 4, 5];