Upgrade implementation of StepBy<RangeInclusive<_>>.

Author: kennytm

src/libcore/iter/mod.rs

@@ -787,17 +787,19 @@ where
     #[inline]
     fn spec_next(&mut self) -> Option<Self::Item> {
         self.first_take = false;
-        if !(self.iter.start <= self.iter.end) {
+        if self.iter.is_empty() {
+            self.iter.is_iterating = Some(false);
             return None;
         }
         // add 1 to self.step to get original step size back
         // it was decremented for the general case on construction
         if let Some(n) = self.iter.start.add_usize(self.step+1) {
+            self.iter.is_iterating = Some(n <= self.iter.end);
             let next = mem::replace(&mut self.iter.start, n);
             Some(next)
         } else {
-            let last = self.iter.start.replace_one();
-            self.iter.end.replace_zero();
+            let last = self.iter.start.clone();
+            self.iter.is_iterating = Some(false);
             Some(last)
         }
     }