Allow inheritance between structs.

No subtyping, no interaction with traits. Partially addresses #9912.

src/libsyntax/print/pprust.rs

@@ -640,6 +640,9 @@ impl<'a> State<'a> {
                 ));
             }
             ast::ItemStruct(struct_def, ref generics) => {
+                if struct_def.is_virtual {
+                    try!(self.word_space("virtual"));
+                }
                 try!(self.head(visibility_qualified(item.vis, "struct")));
                 try!(self.print_struct(struct_def, generics, item.ident, item.span));
             }
@@ -754,6 +757,13 @@ impl<'a> State<'a> {
                         span: codemap::Span) -> IoResult<()> {
         try!(self.print_ident(ident));
         try!(self.print_generics(generics));
+        match struct_def.super_struct {
+            Some(t) => {
+                try!(self.word_space(":"));
+                try!(self.print_type(t));
+            },
+            None => {},
+        }
         if ast_util::struct_def_is_tuple_like(struct_def) {
             if !struct_def.fields.is_empty() {
                 try!(self.popen());