Compile time executable calling?

[Justin Whear]

On Sat, 13 Jul 2013 00:36:21 +0200, Peter Alexander wrote:

> On Friday, 12 July 2013 at 20:42:50 UTC, Tofu Ninja wrote:
>> So I had an idea recently, wouldn't it be cool to have the ability to
>> call an executable at compile time and capture its output.
> 
> How long until D compilers are able to read mail? :-)
> 
> There's many obvious applications of this proposed feature, but I say
> such things should be delegated to the build process. Just run those
> executables using a makefile, or whatever build system you use.
> 
> If you need compile time inputs from your D code then just run a
> separate build to extract them. Yes it's more work and less convenient,
> but I think we need to be careful with how much is added to the D
> compilers. We don't want to turn them into operating systems.

Other compilers allow symbols/macros to be defined on the command line, 
e.g. gcc's `-D` flag;  it'd be nice for DMD to define a similar mechanism 
for defining enums, e.g.

---D code---
  void main() { writeln(VERSION_NUMBER); }
------------

dmd main.d -DVERSION_NUMBER=1.2

Currently I have to use my makefiles to output a whole file which defines 
the various build constants.