1 files changed, 56 insertions, 0 deletions
diff --git a/library/alloc/src/collections/vec_deque/ring_slices.rs b/library/alloc/src/collections/vec_deque/ring_slices.rs
new file mode 100644
index 000000000..dd0fa7d60
--- /dev/null
+++ b/library/alloc/src/collections/vec_deque/ring_slices.rs
@@ -0,0 +1,56 @@
+use core::ptr::{self};
+
+/// Returns the two slices that cover the `VecDeque`'s valid range
+pub trait RingSlices: Sized {
+    fn slice(self, from: usize, to: usize) -> Self;
+    fn split_at(self, i: usize) -> (Self, Self);
+
+    fn ring_slices(buf: Self, head: usize, tail: usize) -> (Self, Self) {
+        let contiguous = tail <= head;
+        if contiguous {
+            let (empty, buf) = buf.split_at(0);
+            (buf.slice(tail, head), empty)
+        } else {
+            let (mid, right) = buf.split_at(tail);
+            let (left, _) = mid.split_at(head);
+            (right, left)
+        }
+    }
+}
+
+impl<T> RingSlices for &[T] {
+    fn slice(self, from: usize, to: usize) -> Self {
+        &self[from..to]
+    }
+    fn split_at(self, i: usize) -> (Self, Self) {
+        (*self).split_at(i)
+    }
+}
+
+impl<T> RingSlices for &mut [T] {
+    fn slice(self, from: usize, to: usize) -> Self {
+        &mut self[from..to]
+    }
+    fn split_at(self, i: usize) -> (Self, Self) {
+        (*self).split_at_mut(i)
+    }
+}
+
+impl<T> RingSlices for *mut [T] {
+    fn slice(self, from: usize, to: usize) -> Self {
+        assert!(from <= to && to < self.len());
+        // Not using `get_unchecked_mut` to keep this a safe operation.
+        let len = to - from;
+        ptr::slice_from_raw_parts_mut(self.as_mut_ptr().wrapping_add(from), len)
+    }
+
+    fn split_at(self, mid: usize) -> (Self, Self) {
+        let len = self.len();
+        let ptr = self.as_mut_ptr();
+        assert!(mid <= len);
+        (
+            ptr::slice_from_raw_parts_mut(ptr, mid),
+            ptr::slice_from_raw_parts_mut(ptr.wrapping_add(mid), len - mid),
+        )
+    }
+}