Rollup merge of rust-lang#59328 - koalatux:iter-nth-back, r=scottmcm

Implement specialized nth_back() for Box and Windows.

Hi there, this is my first pull request to rust :-)

I started implementing some specializations for DoubleEndedIterator::nth_back() and these are the first two. The problem has been discussed in rust-lang#54054 and nth_back() is tracked in rust-lang#56995.

I'm stuck with the next implementation so I though I do a PR for the ones I'm confident with to get some feedback.

src/liballoc/boxed.rs

@@ -677,6 +677,9 @@ 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> {

src/liballoc/lib.rs

@@ -115,6 +115,7 @@
 #![feature(maybe_uninit, maybe_uninit_slice, maybe_uninit_array)]
 #![feature(alloc_layout_extra)]
 #![feature(try_trait)]
+#![feature(iter_nth_back)]
 
 // Allow testing this library
 

src/libcore/slice/mod.rs

@@ -3867,6 +3867,19 @@ 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")]

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];