Function combine_substructure

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