A (builtin) function, returning bit offset for bitfields within structs, could be useful to pass bit "addresses" as function parameters. In analogy to offsetof()/sizeof(), a similar function returning number of bits of a bitfield could be created, too, although it would be less useful than the offset.
I thought I saw a paper about adding this and even bit sized types.
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2763.pdf It seems like they did not add this ... But they do have _Bitwidthof now ...
Thanks for the comments and the link. Small embedded is generally frowned upon. The proposal is characteristically heavyweight and unwieldy. Maybe in C5x.