Fix custom deriving tests

src/libsyntax_ext/lib.rs

@@ -46,12 +46,14 @@ mod asm;
 mod cfg;
 mod concat;
 mod concat_idents;
-mod deriving;
 mod env;
 mod format;
 mod log_syntax;
 mod trace_macros;
 
+// for custom_derive
+pub mod deriving;
+
 pub fn register_builtins(env: &mut SyntaxEnv) {
     // utility function to simplify creating NormalTT syntax extensions
     fn builtin_normal_expander(f: MacroExpanderFn) -> SyntaxExtension {