librustc: Forbid destructors from being attached to any structs that might contain non-Owned fields. r=nmatsakis

src/libsyntax/parse/parser.rs

@@ -268,6 +268,7 @@ pub struct Parser {
 
 }
 
+#[unsafe_destructor]
 impl Drop for Parser {
     /* do not copy the parser; its state is tied to outside state */
     fn finalize(&self) {}