Is it possible to set function attributes conditionally?

[wjoe]

Hi,

Consider Allocators, e.g.:

```d
struct Mallocator
{
    enum usesGC = false;

    /// implement alloc, free, etc. @nogc
}

struct GCAllocator
{
   enum usesGC = true;

    /// implement alloc, free, etc. via the GC
}
```

Now I want to have the function attributes set depending on the 
allocator implementation

```d
template AutoGC(ALLOCATOR) if (isAllocator!ALLOCATOR)
{
    static if (!ALLOCATOR.usesGC)
       AutoGC = pragma(attrib, @nogc);
    else
       AutoGC = pragma(attrib, none);
}

@AutoGC!ALLOCATOR void fun(ALLOCATOR)() if(isAllocator!ALLOCATOR)
{
    void* p = ALLOCATOR.alloc(1024);
    // do something with p
    ALLOCATOR.free(p);
}
```

So fun!Mallocator would be @nogc and fun!GCAllocator wouldn't be 
@nogc.

This is a contrived example. In reality I would use this with 
custom array, hash map and other container implementations so I 
could use them in @nogc territory by just switching out the 
allocator.

Is it possible to do something like this ?