Deprecate [T]::rotate in favor of [T]::rotate_{left,right}.

Background
==========

Slices currently have an unstable [`rotate`] method which rotates
elements in the slice to the _left_ N positions. [Here][tracking] is the
tracking issue for this unstable feature.

```rust
let mut a = ['a', 'b' ,'c', 'd', 'e', 'f'];
a.rotate(2);
assert_eq!(a, ['c', 'd', 'e', 'f', 'a', 'b']);
```

Proposal
========

Deprecate the [`rotate`] method and introduce `rotate_left` and
`rotate_right` methods.

```rust
let mut a = ['a', 'b' ,'c', 'd', 'e', 'f'];
a.rotate_left(2);
assert_eq!(a, ['c', 'd', 'e', 'f', 'a', 'b']);
```

```rust
let mut a = ['a', 'b' ,'c', 'd', 'e', 'f'];
a.rotate_right(2);
assert_eq!(a, ['e', 'f', 'a', 'b', 'c', 'd']);
```

Justification
=============

I used this method today for my first time and (probably because I’m a
naive westerner who reads LTR) was surprised when the docs mentioned that
elements get rotated in a left-ward direction. I was in a situation
where I needed to shift elements in a right-ward direction and had to
context switch from the main problem I was working on and think how much
to rotate left in order to accomplish the right-ward rotation I needed.

Ruby’s `Array.rotate` shifts left-ward, Python’s `deque.rotate` shifts
right-ward. Both of their implementations allow passing negative numbers
to shift in the opposite direction respectively.

Introducing `rotate_left` and `rotate_right` would:

- remove ambiguity about direction (alleviating need to read docs 😉)
- make it easier for people who need to rotate right

[`rotate`]: https://doc.rust-lang.org/std/primitive.slice.html#method.rotate
[tracking]: https://github.com/rust-lang/rust/issues/41891

src/liballoc/tests/slice.rs

@@ -494,37 +494,72 @@ fn test_sort_stability() {
 }
 
 #[test]
-fn test_rotate() {
+fn test_rotate_left() {
     let expected: Vec<_> = (0..13).collect();
     let mut v = Vec::new();
 
     // no-ops
     v.clone_from(&expected);
-    v.rotate(0);
+    v.rotate_left(0);
     assert_eq!(v, expected);
-    v.rotate(expected.len());
+    v.rotate_left(expected.len());
     assert_eq!(v, expected);
     let mut zst_array = [(), (), ()];
-    zst_array.rotate(2);
+    zst_array.rotate_left(2);
 
     // happy path
     v = (5..13).chain(0..5).collect();
-    v.rotate(8);
+    v.rotate_left(8);
     assert_eq!(v, expected);
 
     let expected: Vec<_> = (0..1000).collect();
 
     // small rotations in large slice, uses ptr::copy
     v = (2..1000).chain(0..2).collect();
-    v.rotate(998);
+    v.rotate_left(998);
     assert_eq!(v, expected);
     v = (998..1000).chain(0..998).collect();
-    v.rotate(2);
+    v.rotate_left(2);
     assert_eq!(v, expected);
 
     // non-small prime rotation, has a few rounds of swapping
     v = (389..1000).chain(0..389).collect();
-    v.rotate(1000-389);
+    v.rotate_left(1000-389);
+    assert_eq!(v, expected);
+}
+
+#[test]
+fn test_rotate_right() {
+    let expected: Vec<_> = (0..13).collect();
+    let mut v = Vec::new();
+
+    // no-ops
+    v.clone_from(&expected);
+    v.rotate_right(0);
+    assert_eq!(v, expected);
+    v.rotate_right(expected.len());
+    assert_eq!(v, expected);
+    let mut zst_array = [(), (), ()];
+    zst_array.rotate_right(2);
+
+    // happy path
+    v = (5..13).chain(0..5).collect();
+    v.rotate_right(5);
+    assert_eq!(v, expected);
+
+    let expected: Vec<_> = (0..1000).collect();
+
+    // small rotations in large slice, uses ptr::copy
+    v = (2..1000).chain(0..2).collect();
+    v.rotate_right(2);
+    assert_eq!(v, expected);
+    v = (998..1000).chain(0..998).collect();
+    v.rotate_right(998);
+    assert_eq!(v, expected);
+
+    // non-small prime rotation, has a few rounds of swapping
+    v = (389..1000).chain(0..389).collect();
+    v.rotate_right(389);
     assert_eq!(v, expected);
 }