Function rustc_builtin_macros::deriving::generic::combine_substructure

pub fn combine_substructure(
    f: Box<dyn FnMut(&mut ExtCtxt<'_>, Span, &Substructure<'_>) -> P<Expr> + '_>
) -> RefCell<Box<dyn FnMut(&mut ExtCtxt<'_>, Span, &Substructure<'_>) -> P<Expr> + '_>>