Were these intended as part of an external interface for Gecko? In which case I would say using bitfields (as opposed to explicit masks) was a mistake. But ye

Were these intended as part of an external interface for Gecko? In which case I would say using bitfields (as opposed to explicit masks) was a mistake. But yes, I can imagine that for Mozilla there is a need to mix Rust and C++ right inside an application at boundaries that are not usually considered external, in which case all kinds of non-portable C constructs are acceptable.