better handling of iterator edge cases

Author: dwrensha

capnp/src/traits.rs

@@ -137,12 +137,13 @@ impl <U, T : IndexMove<u32, U>> ::std::iter::Iterator for ListIter<T, U> {
     }
 
     fn nth(&mut self, p: usize) -> Option<U>{
-        if p < self.size as usize {
+        if self.index + (p as u32) < self.size  {
             self.index += p as u32;
             let result = self.list.index_move(self.index);
             self.index += 1;
             Some(result)
         } else {
+            self.index = self.size;
             None
         }
     }

capnpc/test/test.rs

@@ -1461,5 +1461,16 @@ mod tests {
             assert_eq!(c, s.get_u_int32_field());
             c += 1;
         }
+
+        {
+            let mut overflow_iter = structs.iter();
+            assert!(overflow_iter.nth(4).is_some());
+
+            // The first four elements have been consumed, so going another 4 should overflow.
+            assert!(overflow_iter.nth(4).is_none());
+
+            // The previous call pushed us to the end, even though it returned None.
+            assert!(overflow_iter.next().is_none());
+        }
     }
 }