Rollup merge of rust-lang#60023 - koalatux:nth-back, r=scottmcm

implement specialized nth_back() for Bytes, Fuse and Enumerate

Hi,

After my first PR has been successfully merged, here is my second pull request :-)

Also this PR contains some specializations for the problem discussed in rust-lang#54054.

src/libcore/iter/adapters/mod.rs

@@ -981,6 +981,16 @@ impl<I> DoubleEndedIterator for Enumerate<I> where
         })
     }
 
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<(usize, <I as Iterator>::Item)> {
+        self.iter.nth_back(n).map(|a| {
+            let len = self.iter.len();
+            // Can safely add, `ExactSizeIterator` promises that the number of
+            // elements fits into a `usize`.
+            (self.count + len, a)
+        })
+    }
+
     #[inline]
     fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
         Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
@@ -1790,6 +1800,17 @@ impl<I> DoubleEndedIterator for Fuse<I> where I: DoubleEndedIterator {
         }
     }
 
+    #[inline]
+    default fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
+        if self.done {
+            None
+        } else {
+            let nth = self.iter.nth_back(n);
+            self.done = nth.is_none();
+            nth
+        }
+    }
+
     #[inline]
     default fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
         Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
@@ -1878,6 +1899,11 @@ impl<I> DoubleEndedIterator for Fuse<I>
         self.iter.next_back()
     }
 
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
+        self.iter.nth_back(n)
+    }
+
     #[inline]
     fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
         Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>

src/libcore/str/mod.rs

@@ -795,6 +795,11 @@ impl DoubleEndedIterator for Bytes<'_> {
         self.0.next_back()
     }
 
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        self.0.nth_back(n)
+    }
+
     #[inline]
     fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item> where
         P: FnMut(&Self::Item) -> bool

src/libcore/tests/iter.rs

@@ -389,6 +389,24 @@ fn test_iterator_enumerate_nth() {
     assert_eq!(i, 3);
 }
 
+#[test]
+fn test_iterator_enumerate_nth_back() {
+    let xs = [0, 1, 2, 3, 4, 5];
+    let mut it = xs.iter().enumerate();
+    while let Some((i, &x)) = it.nth_back(0) {
+        assert_eq!(i, x);
+    }
+
+    let mut it = xs.iter().enumerate();
+    while let Some((i, &x)) = it.nth_back(1) {
+        assert_eq!(i, x);
+    }
+
+    let (i, &x) = xs.iter().enumerate().nth_back(3).unwrap();
+    assert_eq!(i, x);
+    assert_eq!(i, 2);
+}
+
 #[test]
 fn test_iterator_enumerate_count() {
     let xs = [0, 1, 2, 3, 4, 5];