From 4837fb45ff41ad8ff4c6dfc9bd3f8056959f3d4e Mon Sep 17 00:00:00 2001
From: Adrian Friedli <adi@koalatux.ch>
Date: Sat, 16 Feb 2019 22:34:28 +0100
Subject: [PATCH 1/2] implement nth_back for Box

---
 src/liballoc/boxed.rs | 3 +++
 src/liballoc/lib.rs   | 1 +
 2 files changed, 4 insertions(+)

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index 51549f92d4dbf..1e3ccf90eb87f 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -662,6 +662,9 @@ impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
     fn next_back(&mut self) -> Option<I::Item> {
         (**self).next_back()
     }
+    fn nth_back(&mut self, n: usize) -> Option<I::Item> {
+        (**self).nth_back(n)
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index 95b9dacf8565a..da1afc2ff8aa3 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -115,6 +115,7 @@
 #![feature(maybe_uninit)]
 #![feature(alloc_layout_extra)]
 #![feature(try_trait)]
+#![feature(iter_nth_back)]
 
 // Allow testing this library
 

From 739ba0417ae6477ffc9c5e8298b5075725d52420 Mon Sep 17 00:00:00 2001
From: Adrian Friedli <adi@koalatux.ch>
Date: Sun, 24 Feb 2019 00:22:41 +0100
Subject: [PATCH 2/2] implement nth_back for Windows

---
 src/libcore/slice/mod.rs   | 13 +++++++++++++
 src/libcore/tests/slice.rs | 13 +++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index acca9748372ca..b048a12e402a5 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -3835,6 +3835,19 @@ impl<'a, T> DoubleEndedIterator for Windows<'a, T> {
             ret
         }
     }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        let (end, overflow) = self.v.len().overflowing_sub(n);
+        if end < self.size || overflow {
+            self.v = &[];
+            None
+        } else {
+            let ret = &self.v[end-self.size..end];
+            self.v = &self.v[..end-1];
+            Some(ret)
+        }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
diff --git a/src/libcore/tests/slice.rs b/src/libcore/tests/slice.rs
index 04d646ea01d03..1e92b6f787870 100644
--- a/src/libcore/tests/slice.rs
+++ b/src/libcore/tests/slice.rs
@@ -578,6 +578,19 @@ fn test_windows_nth() {
     assert_eq!(c2.next(), None);
 }
 
+#[test]
+fn test_windows_nth_back() {
+    let v: &[i32] = &[0, 1, 2, 3, 4, 5];
+    let mut c = v.windows(2);
+    assert_eq!(c.nth_back(2).unwrap()[0], 2);
+    assert_eq!(c.next_back().unwrap()[1], 2);
+
+    let v2: &[i32] = &[0, 1, 2, 3, 4];
+    let mut c2 = v2.windows(4);
+    assert_eq!(c2.nth_back(1).unwrap()[1], 1);
+    assert_eq!(c2.next_back(), None);
+}
+
 #[test]
 fn test_windows_last() {
     let v: &[i32] = &[0, 1, 2, 3, 4, 5];