C bitfields guarantees

[Richard Andrew Cattermole (Rikki)]

Today many people have spent some time to try and understand 
Walter's belief that C is "good enough" for bit fields in terms 
of guarantees.

I believe I have understood a core component to this.

 From the C23 standard:

> An implementation may allocate any addressable storage unit 
> large enough to hold a bit-field. If
enough space remains, a bit-field that immediately follows 
another bit-field in a structure shall be
packed into adjacent bits of the same unit. If insufficient space 
remains, whether a bit-field that
does not fit is put into the next unit or overlaps adjacent units 
is implementation-defined. The
order of allocation of bit-fields within a unit (high-order to 
low-order or low-order to high-order) is
implementation-defined. The alignment of the addressable storage 
unit is unspecified.

What matters is the _initial_ type in the bit-field, the rest of 
the types _do not_ matter.

As long as you _do not_ start and finish in two separate memory 
addresses for that initial type it will be predictable.

I have filed a 
[ticket](https://github.com/dlang-community/D-Scanner/issues/955) 
for dscanner to introduce a warning to tell you that the compiler 
is going to do a bad thing, that will cause you problems and the 
compiler will not assist you.

Ideally, we wouldn't allow it for ``extern(D)`` code at all.

As of right now, assuming we get Dscanner to give the warning I 
can withdraw my concerns, although I do think that ``extern(D)`` 
shouldn't be offering you such a heavy foot-gun.