C bitfields guarantees

[Walter Bright]

C and D programmers already know how to align things with pad fields. It's a 
basic skill.

If extern(C) and extern(D) were added to bitfields, then the programmer would 
have to learn two new syntactic constructs and what they mean. It's a 
distinction that is easily forgettable, too. Quick, what's the difference in 
calling convention between extern(C) and extern(D) functions?

With pad fields, there's nothing new to learn, and it's quite obvious even for a 
naive programmer what is happening with them.