Neat project: add pointer capability to std.bitmanip.bitfields

[Vladimir Panteleev via Digitalmars-d]

On Thursday, 3 December 2015 at 01:31:05 UTC, rsw0x wrote:
> On Wednesday, 2 December 2015 at 19:54:26 UTC, Vladimir 
> Panteleev wrote:
>> On Wednesday, 2 December 2015 at 19:39:47 UTC, Andrei 
>> Alexandrescu wrote:
>>> Once done, this is a fantastic example of (a) the power of 
>>> generative programming, and (b) the advantages of using 
>>> library facilities instead of built-in features.
>>>
>>> https://issues.dlang.org/show_bug.cgi?id=15397
>>>
>>> Who would want to take it?
>>
>> Warning, this is very unsafe and incompatible with the GC. 
>> Bit-twiddling GC pointers can lead to memory corruption and 
>> very hard-to-track bugs. Such a feature must be opt-in in a 
>> very explicit way.
>
> Iirc I'm the one that originally brought this up. There's no 
> reason for lsb smuggling in pointers to be unsafe

True, assuming that:

1. The pointers are still aligned at machine word boundaries
2. The underlying storage type is one that the GC will scan for 
pointers (e.g. void or void*, not size_t/ubyte)
3. The setters enforce that the discarded pointer bits were zero
4. No more than 4 bits are reused (as the smallest GC object size 
is 16 bytes)

Point 4 actually relies on the GC's current implementation, which 
could be an issue (it ties what is allowed to compile in code 
using the standard library with an implementation detail).