Do not expand a derive invocation when derive is not allowed

1. Change the return type of `expand_invoc()` and its subroutines to
   `Option<Expansion>` from `Expansion`.
2. Return `None` when expanding a derive invocation if the item cannot
   have derive on it (in `expand_derive_invoc()`).

1 files changed, 12 insertions, 0 deletions

diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index d53335f5be7..be71d6e038c 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -96,6 +96,18 @@ impl Annotatable {
             _ => panic!("expected Item")
         }
     }
+
+    pub fn derive_allowed(&self) -> bool {
+        match *self {
+            Annotatable::Item(ref item) => match item.node {
+                ast::ItemKind::Struct(..) |
+                ast::ItemKind::Enum(..) |
+                ast::ItemKind::Union(..) => true,
+                _ => false,
+            },
+            _ => false,
+        }
+    }
 }
 
 // A more flexible ItemDecorator.