make walk/visit_mac opt-in only

macros can expand into arbitrary items, exprs, etc. This
means that using a default walker or folder on an AST before
macro expansion is complete will miss things (the things that
the macros expand into). As a partial fence against this, this
commit moves the default traversal of macros into a separate
procedure, and makes the default trait implementation signal
an error. This means that Folders and Visitors can traverse
macros if they want to, but they need to explicitly add an
impl that calls the walk_mac or fold_mac procedure

This should prevent problems down the road.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/librustc/plugin/load.rs b/src/librustc/plugin/load.rs
index 79d0690653f..499cffa42aa 100644
--- a/src/librustc/plugin/load.rs
+++ b/src/librustc/plugin/load.rs
@@ -72,6 +72,7 @@ pub fn load_plugins(sess: &Session, krate: &ast::Crate) -> Plugins {
     loader.plugins
 }
 
+// note that macros aren't expanded yet, and therefore macros can't add plugins.
 impl<'a> Visitor<()> for PluginLoader<'a> {
     fn visit_view_item(&mut self, vi: &ast::ViewItem, _: ()) {
         match vi.node {
@@ -109,6 +110,10 @@ impl<'a> Visitor<()> for PluginLoader<'a> {
             _ => (),
         }
     }
+    fn visit_mac(&mut self, _: &ast::Mac, _:()) {
+        // bummer... can't see plugins inside macros.
+        // do nothing.
+    }
 }
 
 impl<'a> PluginLoader<'a> {