include: Safely discard the const qualifier using the discard_const macro

[Andrew Talbot]

James Hawkins wrote:

> Hi,
> 
> This patch is based on a patch written by Stefan Huehner in 2005.
> With the -Wcast-qual warning turned on, these five wine-internal
> unicode functions produce over 1400 warnings:
> 
> strchrW, strrchrW, strpbrkW, memchrW, memrchrW
> 
> This patch gets rid of those warnings.  Any ideas about this guys?
> 
[...]

I think this sort of double casting is the way to go. I would just mention a
couple of possible caveats. When I discussed this with Alexandre, some time
back, he expressed a concern that he didn't want to see this kind of thing
proliferate throughout the code base, so I would suggest that this type of
fix should be confined to a small number of very specific functions
(typically string functions that take in a const string and return a
non-const version: there are others like this elsewhere) and be applied
inline, rather than with a macro, so as not to put temptation in anyone's
way. The second point I would make is that - and I haven't tested this on
gcc - if, say, ULONG_PTR is bigger than the size of a WCHAR * on any system
we support, the compiler might warn of a possible truncation loss when the
outer (WCHAR *) cast converts back to a pointer, which would be rather
unfortunate. :)

I presume ULONG_PTR is good for both Win32 and Win64.

-- Andy.