Add VecDeque::range* methods

This patch adds `VecDeque::range` and `VecDeque::range_mut` to provide
iterators over a sub-range of a `VecDeque`. This behavior can be
emulated with `skip` and `take`, but directly providing a `Range` is
more ergonomic. This also partially makes up for `VecDeque`'s lack of
`SliceIndex` support.

src/liballoc/collections/vec_deque.rs

@@ -1084,6 +1084,104 @@ impl<T> VecDeque<T> {
         self.tail == self.head
     }
 
+    fn range_start_end<R>(&self, range: R) -> (usize, usize)
+    where
+        R: RangeBounds<usize>,
+    {
+        let len = self.len();
+        let start = match range.start_bound() {
+            Included(&n) => n,
+            Excluded(&n) => n + 1,
+            Unbounded => 0,
+        };
+        let end = match range.end_bound() {
+            Included(&n) => n + 1,
+            Excluded(&n) => n,
+            Unbounded => len,
+        };
+        assert!(start <= end, "lower bound was too large");
+        assert!(end <= len, "upper bound was too large");
+        (start, end)
+    }
+
+    /// Creates an iterator that covers the specified range in the `VecDeque`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the starting point is greater than the end point or if
+    /// the end point is greater than the length of the vector.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let v: VecDeque<_> = vec![1, 2, 3].into_iter().collect();
+    /// let range = v.iter_rage(2..).copied().collect::<VecDeque<_>>();
+    /// assert_eq!(range, [3]);
+    ///
+    /// // A full range covers all contents
+    /// let all = v.range(..);
+    /// assert_eq!(all.len(), 3);
+    /// ```
+    #[inline]
+    #[unstable(feature = "deque_range", issue = "none")]
+    pub fn range<R>(&self, range: R) -> Iter<'_, T>
+    where
+        R: RangeBounds<usize>,
+    {
+        let (start, end) = self.range_start_end(range);
+        let tail = self.wrap_add(self.tail, start);
+        let head = self.wrap_add(self.tail, end);
+        Iter {
+            tail,
+            head,
+            // The shared reference we have in &self is maintained in the '_ of Iter.
+            ring: unsafe { self.buffer_as_slice() },
+        }
+    }
+
+    /// Creates an iterator that covers the specified mutable range in the `VecDeque`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the starting point is greater than the end point or if
+    /// the end point is greater than the length of the vector.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut v: VecDeque<_> = vec![1, 2, 3].into_iter().collect();
+    /// for v in v.range_mut(2..) {
+    ///   *v *= 2;
+    /// }
+    /// assert_eq!(v, vec![1, 2, 6]);
+    ///
+    /// // A full range covers all contents
+    /// for v in v.range_mut(..) {
+    ///   *v *= 2;
+    /// }
+    /// assert_eq!(v, vec![2, 4, 12]);
+    /// ```
+    #[inline]
+    #[unstable(feature = "deque_range", issue = "none")]
+    pub fn range_mut<R>(&mut self, range: R) -> IterMut<'_, T>
+    where
+        R: RangeBounds<usize>,
+    {
+        let (start, end) = self.range_start_end(range);
+        let tail = self.wrap_add(self.tail, start);
+        let head = self.wrap_add(self.tail, end);
+        IterMut {
+            tail,
+            head,
+            // The shared reference we have in &mut self is maintained in the '_ of IterMut.
+            ring: unsafe { self.buffer_as_mut_slice() },
+        }
+    }
+
     /// Creates a draining iterator that removes the specified range in the
     /// `VecDeque` and yields the removed items.
     ///
@@ -1129,19 +1227,7 @@ impl<T> VecDeque<T> {
         // When finished, the remaining data will be copied back to cover the hole,
         // and the head/tail values will be restored correctly.
         //
-        let len = self.len();
-        let start = match range.start_bound() {
-            Included(&n) => n,
-            Excluded(&n) => n + 1,
-            Unbounded => 0,
-        };
-        let end = match range.end_bound() {
-            Included(&n) => n + 1,
-            Excluded(&n) => n,
-            Unbounded => len,
-        };
-        assert!(start <= end, "drain lower bound was too large");
-        assert!(end <= len, "drain upper bound was too large");
+        let (start, end) = self.range_start_end(range);
 
         // The deque's elements are parted into three segments:
         // * self.tail  -> drain_tail

src/liballoc/collections/vec_deque/tests.rs

@@ -246,6 +246,65 @@ fn test_remove() {
     }
 }
 
+#[test]
+fn test_range() {
+    let mut tester: VecDeque<usize> = VecDeque::with_capacity(7);
+
+    let cap = tester.capacity();
+    for len in 0..=cap {
+        for tail in 0..=cap {
+            for start in 0..=len {
+                for end in drain_start..=len {
+                    tester.tail = tail;
+                    tester.head = tail;
+                    for i in 0..len {
+                        tester.push_back(i);
+                    }
+
+                    // Check that we iterate over the correct values
+                    let range: VecDeque<_> = tester.range(start..end).copied().collect();
+                    let expected: VecDeque<_> = (start..end).collect();
+                    assert_eq!(range, expected);
+                }
+            }
+        }
+    }
+}
+
+#[test]
+fn test_range_mut() {
+    let mut tester: VecDeque<usize> = VecDeque::with_capacity(7);
+
+    let cap = tester.capacity();
+    for len in 0..=cap {
+        for tail in 0..=cap {
+            for start in 0..=len {
+                for end in drain_start..=len {
+                    tester.tail = tail;
+                    tester.head = tail;
+                    for i in 0..len {
+                        tester.push_back(i);
+                    }
+
+                    let head_was = tester.head;
+                    let tail_was = tester.tail;
+
+                    // Check that we iterate over the correct values
+                    let range: VecDeque<_> = tester.range_mut(start..end).copied().collect();
+                    let expected: VecDeque<_> = (start..end).collect();
+                    assert_eq!(range, expected);
+
+                    // We shouldn't have changed the capacity or made the
+                    // head or tail out of bounds
+                    assert_eq!(tester.capacity(), cap);
+                    assert_eq!(tester.tail, tail_was);
+                    assert_eq!(tester.head, head_was);
+                }
+            }
+        }
+    }
+}
+
 #[test]
 fn test_drain() {
     let mut tester: VecDeque<usize> = VecDeque::with_capacity(7);