C bitfields guarantees

[Walter Bright]

On 7/5/2024 8:23 PM, Steven Schveighoffer wrote:
> My recommendation still is either:
> 
> 1. Denote D bitfields by a specified layout system (pick the most common C one 
> and do that). C bitfields can match the C compiler.
> 2. Simply forbid problematic alignments at compile time:
> 
> ```d
> struct S {
>     uint x;
>     uint64 a : 24;
>     uint64 b : 24;
>     uint64 c : 16;
> }
> 
> // error, alignment of bitfield `a` may not match C layout, please use padding 
> or aligned bitfields to specify intended layout.
> 
> // these are OK.
> struct SWithPadding {
>     uint x;
>     uint _; // padding
>     uint64 a : 24;
>     uint64 b : 24;
>     uint64 c : 16;
> }
> 
> struct SPacked {
>     uint64 x : 32;
>     uint64 a : 24;
>     uint64 b : 24;
>     uint64 c : 16;
> }
> ```
> 
> Maybe the error only occurs if you specify a compiler switch?

It's clear how to get the desired portable layout.

Consider that nobody ever requested a warning on:

```
struct S {
     uint x;
     ulong y;
}
```
which is the equivalent. Not for D, C, or C++. At least none directed at my 
compilers.

I would be good with a note about this technique in the specification.

P.S. We've already got soooo many compiler switches, adding another one needs a 
strong case. Every such switch is a bug :-/