In that case, I'd rather just have explicit bitfield getter/setter functions à la

    slct(var, 6, 2) = expr;  // var = (var &~ (0b111111 << 2)) | ((expr & 0b111111) << 2);
    var = slct(expr, 6, 2);   // var = (expr >> 2) & 0b111111;

Freaking BCPL had those in 1976, and that language was more primitive than C (it didn't even have any types beyond "machine word, reinterpreted accordingly to the operation applied to it").