Specialise the Range::last

Author: nagisa

src/libcore/iter/range.rs

@@ -249,8 +249,32 @@ impl<A: Step> Iterator for ops::Range<A> where
         })
     }
 
-    // TODO: last specialisation if `A: Sub` and/or Range<A> is a DoubleEndedIterator.
-    // so self.end.sub_one() or self.next_back()
+    #[inline]
+    fn last(self) -> Option<Self::Item> {
+        <Self as SpecLast>::last(self)
+    }
+}
+
+trait SpecLast {
+    type Item;
+    fn last(self) -> Option<Self::Item>;
+}
+
+impl<It> SpecLast for It
+where It: Iterator {
+    type Item = <Self as Iterator>::Item;
+    #[inline]
+    default fn last(self) -> Option<Self::Item> {
+        <Self as Iterator>::last(self)
+    }
+}
+
+impl<It> SpecLast for It
+where It: DoubleEndedIterator {
+    #[inline]
+    fn last(mut self) -> Option<Self::Item> {
+        <Self as DoubleEndedIterator>::next_back(&mut self)
+    }
 }
 
 // These macros generate `ExactSizeIterator` impls for various range types.
@@ -319,7 +343,7 @@ impl<A: Step> Iterator for ops::RangeFrom<A> where
 
     #[cold]
     fn last(self) -> Option<Self::Item> {
-        panic!("Iterator::last on a `x..` cannot work")
+        panic!("Iterator::last on a `x..` will overflow")
     }
 }
 

src/libcore/tests/iter.rs

@@ -1157,12 +1157,6 @@ fn test_usize_range_no_last() {
     assert_eq!((0..).last(), None);
 }
 
-#[test]
-#[should_panic]
-fn test_usize_range_no_last() {
-    assert_eq!((0..).last(), None);
-}
-
 #[test]
 #[should_panic]
 fn test_isize_range_count_oflow() {
@@ -1172,6 +1166,7 @@ fn test_isize_range_count_oflow() {
 #[test]
 fn test_range_last() {
     assert_eq!((0..10).last(), Some(9));
+    assert_eq!((0u32..!0).last(), Some(!0 - 1));
     assert_eq!(RangeInclusive { start: 0, end: 10 }.last(), Some(10));
 }