Expand nested items within a backtrace.

Fixes a regression from #4913 which causes items to be exanded with spans lacking expn_info from the context's current backtrace.

src/libsyntax/ext/expand.rs

@@ -248,7 +248,7 @@ macro_rules! with_exts_frame (
 // When we enter a module, record it, for the sake of `module!`
 pub fn expand_item(it: @ast::Item, fld: &mut MacroExpander)
                    -> SmallVector<@ast::Item> {
-    let mut decorator_items = SmallVector::zero();
+    let mut decorator_items: SmallVector<@ast::Item> = SmallVector::zero();
     for attr in it.attrs.rev_iter() {
         let mname = attr.name();
 
@@ -262,20 +262,21 @@ pub fn expand_item(it: @ast::Item, fld: &mut MacroExpander)
                         span: None
                     }
                 });
+
                 // we'd ideally decorator_items.push_all(expand_item(item, fld)),
                 // but that double-mut-borrows fld
+                let mut items: SmallVector<@ast::Item> = SmallVector::zero();
                 dec_fn(fld.cx, attr.span, attr.node.value, it,
-                       |item| decorator_items.push(item));
+                       |item| items.push(item));
+                decorator_items.extend(&mut items.move_iter()
+                    .flat_map(|item| expand_item(item, fld).move_iter()));
+
                 fld.cx.bt_pop();
             }
             _ => {}
         }
     }
 
-    let decorator_items = decorator_items.move_iter()
-        .flat_map(|item| expand_item(item, fld).move_iter())
-        .collect();
-
     let mut new_items = match it.node {
         ast::ItemMac(..) => expand_item_mac(it, fld),
         ast::ItemMod(_) | ast::ItemForeignMod(_) => {

src/libsyntax/util/small_vector.rs

@@ -39,6 +39,14 @@ impl<T> FromIterator<T> for SmallVector<T> {
     }
 }
 
+impl<T> Extendable<T> for SmallVector<T> {
+    fn extend<I: Iterator<T>>(&mut self, iter: &mut I) {
+        for val in *iter {
+            self.push(val);
+        }
+    }
+}
+
 impl<T> SmallVector<T> {
     pub fn zero() -> SmallVector<T> {
         Zero