impl a {
    fn trans_trait_alias(&mut self, ident: String, generics: &ast::Generics, bounds: &ast::GenericBounds)
    -> TraitAlias {
        TraitAlias {
            name: ident,
            generics: self.trans_generics(generics),
            bounds: self.trans_type_param_bounds(bounds),
        }
    }
}