C bitfields guarantees

[Steven Schveighoffer]

On Friday, 5 July 2024 at 19:56:49 UTC, Tim wrote:
> On Friday, 5 July 2024 at 19:35:10 UTC, Steven Schveighoffer 
> wrote:
>>
>> What if you need > 32 bits or want to pack into a `ulong`? Is 
>> the behavior sane across compilers?
>
> The following struct has a different layout for different 
> platforms:

...

Thanks for this.

I also tested the following, and found it too shows discrepancies.

```c
struct S {
     unsigned short x;
     unsigned int a : 12;
     unsigned int b : 12;
     unsigned int c : 8;
};
```

Here there are only `uint` bitfields, yet the compiler chooses to 
layout the bits differently based on the *preceding* field.

Walter, I have to unfortunately withdraw my support for defining 
D bitfields to just be the same as C bitfields -- the minefields 
are too subtle. The statement that "If you use uint as the field 
type, you'll get the same layout across every C compiler" is not 
true. And I don't think we can really specify the true nature of 
what you must do for portable bitfields in a way that is 
straightforward. Saying something like "you can only use `uint` 
bitfields in structs that contain only `uint` types" is not a 
good feature.

I'm back to requesting that we have a mechanism to request C 
bitfields (such as marking a struct as `extern(C)`), or picking 
one C style and going with that.

-Steve