Auto merge of #150 - ehuss:extend-resume, r=emilio

Fix `extend` from assuming a fused iterator.

Iterators may resume after returning None.  I think it is generally expected that `extend` should stop on the first None.

Fixes #147.  Specifically, in rustc, there are some situations where an iterator of Results are collected into a `Result<SmallVec, _>` (such as [here](https://github.com/rust-lang/rust/blob/c1c60d292e2dd2deff7084208274f9a02f750d43/src/librustc/ty/relate.rs#L139-L142) which ends up in [this call to `collect`](https://github.com/rust-lang/rust/blob/c1c60d292e2dd2deff7084208274f9a02f750d43/src/librustc/ty/context.rs#L3002)).  The [Result adapter](https://github.com/rust-lang/rust/blob/c1c60d292e2dd2deff7084208274f9a02f750d43/src/libcore/result.rs#L1245-L1258) returns None for the first `Err` in the sequence.  However, it is possible for an iterator to have additional elements after the first Err.  With this bug, SmallVec was falling through to the slow-path `for` loop, and resuming the iterator grabbing too many elements.  I believe this was having some bad interactions with the type interner where the additional elements after the `Err` were getting processed (via a `map()`) and interned when they shouldn't be (or otherwise having some side effects from the `map`).

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lib.rs

@@ -1356,7 +1356,7 @@ impl<A: Array> Extend<A::Item> for SmallVec<A> {
                     ptr::write(ptr.offset(len.get() as isize), out);
                     len.increment_len(1);
                 } else {
-                    break;
+                    return;
                 }
             }
         }
@@ -2329,4 +2329,16 @@ mod tests {
         v.push(4);
         assert_eq!(v[..], [4]);
     }
+
+    #[test]
+    fn resumable_extend() {
+        let s = "a b c";
+        // This iterator yields: (Some('a'), None, Some('b'), None, Some('c')), None
+        let it = s
+            .chars()
+            .scan(0, |_, ch| if ch.is_whitespace() { None } else { Some(ch) });
+        let mut v: SmallVec<[char; 4]> = SmallVec::new();
+        v.extend(it);
+        assert_eq!(v[..], ['a']);
+    }
 }