Fix build

Fixes #2096

src/items.rs

@@ -2740,6 +2740,8 @@ impl Rewrite for ast::ForeignItem {
                     format!("{}{}{};", prefix, sep, ty_str)
                 })
             }
+            // FIXME(#2097) support extern types.
+            ast::ForeignItemKind::Ty => unimplemented!(),
         }?;
 
         let missing_span = if self.attrs.is_empty() {