forcing weak purity

[Steven Schveighoffer]

I have come across a dilemma.

Alex Rønne Petersen has a pull request changing some things in the GC to  
pure.  I think gc_collect() should be weak-pure, because it could  
technically run on any memory allocation (which is already allowed in pure  
functions), and it runs in a context that doesn't really affect execution  
of the pure function.

So I think it should be able to be run inside a strong pure function.  But  
because it has no parameters and no return, marking it as pure makes it  
strong pure, and an optimizing compiler can effectively remove the call  
completely!

So how do we force something to be weak-pure?  What I want is:

1. it can be called from a pure function
2. it will not be optimized out in any way.

This solution looks crappy to me:

void gc_collect(void *unused = null);

any other ideas?

-Steve