I am sorry, but really I don't understand specifically which difference between C bitfields and Rust bitflags you regard as crucial. Is it about the feature being baked into the compiler vs. available as an external macro package? But the only reason it needs to be baked into the compiler in C is because there's no other choice - the C preprocessor syntax is too limited and awful.
I believe the point is that C bitfields allow sub-word addressing; the variable refers to specific bits within a word. The Rust flags is the same as using bitwise operations on word-sized values.
To be honest, I don't know if a functional difference, but I have not done any embedded programming where there might be a difference.
I've seen plenty of hardware registers that use one bit for some X, two bits for some Y, and 5 bits for some Z. (Yes, in embedded programming.) One way of dealing with that is with C bitfields.
Now, I never did it that way myself. I just and-ed it with a bitmask to get the part that I wanted. But it's a bit messier to do it that way, because if you want one of the fields as a number, and it's not from the least-significant bits, then you have to shift it as well as mask it, whereas with bitfields you can just read it as a number and get the right value.
