fix underflow in DoubleEndedIterator::next_back

Author: durka

src/libcore/iter.rs

@@ -4665,19 +4665,23 @@ impl<A: Step + One> DoubleEndedIterator for ops::RangeInclusive<A> where
 
             NonEmpty { ref mut start, ref mut end } => {
                 let one = A::one();
-                let mut n = &*end - &one;
-                mem::swap(&mut n, end);
+                if start <= end {
+                    let mut n = &*end - &one;
+                    mem::swap(&mut n, end);
 
-                (if n == *start { Some(mem::replace(start, one)) } else { None },
-                 n)
+                    (if n == *start { Some(mem::replace(start, one)) } else { None },
+                     Some(n))
+                } else {
+                    (Some(mem::replace(end, one)), None)
+                }
             }
         };
 
         if let Some(start) = finishing {
             *self = Empty { at: start };
         }
 
-        Some(n)
+        n
     }
 }
 

src/test/run-pass/range_inclusive.rs

@@ -99,6 +99,11 @@ pub fn main() {
     }
     assert_eq!(long, RangeInclusive::Empty { at: 251 });
 
+    // check underflow
+    let mut narrow = 1...0;
+    assert_eq!(narrow.next_back(), None);
+    assert_eq!(narrow, RangeInclusive::Empty { at: 0 });
+
     // what happens if you have a nonsense range?
     let mut nonsense = 10...5;
     assert_eq!(nonsense.next(), None);