Extend the call site default argument expansion mechanism?

[Yuxuan Shui]

So in D I can use default argument like this:

int f(int line=__LINE__) {}

And because default argument is expanded at call site, f() will 
be called with the line number of the call site.

This is a really clever feature, and I think a similar feature 
can be useful in other ways.

Say I need to construct a bunch of data structure that takes an 
Allocator argument, I need to do this:

...
auto alloc = new SomeAllocator();
auto data1 = new DataStructure(..., alloc);
auto data2 = new DataStructure(..., alloc);
auto data3 = new DataStructure(..., alloc);
...

This looks redundant. But if we have the ability to define more 
special keywords like __LINE__, we can do something like this:

...
// constructor of DataStructure
this(Allocator alloc=__ALLOC__) {...}
...
auto alloc = new SomeAllocator();
define __ALLOC__ = alloc;
// And we don't need to pass alloc everytime
...

Is this a good idea?