Forward nth(), nth_back() and last() in exhaust::Iter.

Author: kpreid

CHANGELOG.md

@@ -6,6 +6,8 @@
 
 * Iterators for several standard library enums now implement `ExactSizeIterator`.
 * `NonZero` iterators now have an improved but not perfect `size_hint()`.
+* `exhaust::Iter` now forwards `nth()`, `nth_back()`, and `last()` to the underlying iterator,
+  which may improve performance.
 
 ### Fixed
 

src/lib.rs

@@ -411,6 +411,17 @@ impl<T: Exhaust> Iterator for Iter<T> {
             f(state, T::from_factory(item_factory))
         })
     }
+
+    // Ideally we would forward `try_fold()`, but that is not possible until the `Try` trait
+    // is stabilized.
+
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        self.0.nth(n).map(T::from_factory)
+    }
+
+    fn last(self) -> Option<Self::Item> {
+        self.0.last().map(T::from_factory)
+    }
 }
 
 impl<T: Exhaust<Iter: DoubleEndedIterator>> DoubleEndedIterator for Iter<T> {
@@ -427,6 +438,10 @@ impl<T: Exhaust<Iter: DoubleEndedIterator>> DoubleEndedIterator for Iter<T> {
             f(state, T::from_factory(item_factory))
         })
     }
+
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        self.0.nth_back(n).map(T::from_factory)
+    }
 }
 
 impl<T: Exhaust> FusedIterator for Iter<T> {

tests/core_impls_and_misc/iter.rs

@@ -16,3 +16,31 @@ fn clone() {
     assert_eq!(it2.len(), 1);
     assert_eq!(it2.next(), Some(true));
 }
+
+#[test]
+fn nth() {
+    let mut it = exhaust::Iter::<u8>::default();
+    assert_eq!(it.nth(100), Some(100));
+    assert_eq!(it.next(), Some(101));
+    assert_eq!(it.nth(1000), None);
+}
+
+#[test]
+fn nth_back() {
+    let mut it = exhaust::Iter::<u8>::default();
+    assert_eq!(it.nth_back(10), Some(245));
+    assert_eq!(it.next_back(), Some(244));
+    assert_eq!(it.nth_back(1000), None);
+}
+
+#[test]
+fn last_simple() {
+    assert_eq!(exhaust::Iter::<u8>::default().last(), Some(255));
+}
+
+#[test]
+fn last_mutated() {
+    let mut it = exhaust::Iter::<u8>::default();
+    it.next_back();
+    assert_eq!(it.last(), Some(254));
+}