Hmm, on second thought, I'm not so sure about this -- it might be confusing. Consider:

    bitflags!(
        struct Flags: u32 {
            FlagA = 1,
            FlagB = 2
        }
    )

The problem is that the stuff inside `{ ... }` doesn't look very `struct`-like. (If anything, it looks more like our current `enum`s.) So having a distinct pseudo keyword like `flags` may be the best approach.