Short-circuit DefIdForest::intersection()

If the forest is already empty, there is no point in intersecting
further.

Also handle the first element separately, so we don't compute an
unnecessary intersection between the full forest and the first
element, which is always equal to the first element.

src/librustc/ty/inhabitedness/def_id_forest.rs

@@ -74,10 +74,21 @@ impl<'a, 'gcx, 'tcx> DefIdForest {
                            iter: I) -> DefIdForest
             where I: IntoIterator<Item=DefIdForest>
     {
-        let mut ret = DefIdForest::full(tcx);
+        let mut iter = iter.into_iter();
+        let mut ret = if let Some(first) = iter.next() {
+            first
+        } else {
+            return DefIdForest::full(tcx);
+        };
+
         let mut next_ret = SmallVec::new();
         let mut old_ret: SmallVec<[DefId; 1]> = SmallVec::new();
         for next_forest in iter {
+            // No need to continue if the intersection is already empty.
+            if ret.is_empty() {
+                break;
+            }
+
             for id in ret.root_ids.drain() {
                 if next_forest.contains(tcx, id) {
                     next_ret.push(id);