Make insert_many panic-safe

Fixes servo#96.

lib.rs

@@ -772,23 +772,30 @@ impl<A: Array> SmallVec<A> {
             let old_len = self.len();
             assert!(index <= old_len);
             let ptr = self.as_mut_ptr().offset(index as isize);
+
+            // Move the trailing elements.
             ptr::copy(ptr, ptr.offset(lower_size_bound as isize), old_len - index);
-            for (off, element) in iter.enumerate() {
-                if off < lower_size_bound {
-                    ptr::write(ptr.offset(off as isize), element);
-                    let len = self.len() + 1;
-                    self.set_len(len);
-                } else {
-                    // Iterator provided more elements than the hint.
-                    assert!(index + off >= index);  // Protect against overflow.
-                    self.insert(index + off, element);
+
+            // In case the iterator panics, don't double-drop the items we just copied above.
+            self.set_len(index);
+
+            let mut num_added = 0;
+            for element in iter {
+                let cur = ptr.offset(num_added as isize);
+                if num_added >= lower_size_bound {
+                    // Iterator provided more elements than the hint.  Move trailing items again.
+                    self.reserve(1);
+                    ptr::copy(cur, cur.offset(1), old_len - index);
                 }
+                ptr::write(cur, element);
+                num_added += 1;
             }
-            let num_added = self.len() - old_len;
             if num_added < lower_size_bound {
                 // Iterator provided fewer elements than the hint
                 ptr::copy(ptr.offset(lower_size_bound as isize), ptr.offset(num_added as isize), old_len - index);
             }
+
+            self.set_len(old_len + num_added);
         }
     }