The `span_fatal`s were mirroring `bytes!()`'s behavior here, as that's what I used to figure out how syntax extensions work. I assume you mean returning `cx.expr_unreachable()` as the return value of the entire function? So it would be something like

    _ => {
        cx.span_err(expr.span, "blah");
        return MRExpr(cx.expr_unreachable());
    }