Split interfaces to the glue layer and backend into different files

[yebblies]

And add make rules to build a backendless dmd.

In order to generate header files for dmd, I need a working dmd binary with the c++ header generation enabled. Until the frontend headers are built it's impossible to build the dmd glue layer or backend.

So the steps are:

• Build a frontend-only version of dmd with C++ header support
• Generate the headers
• Build the full version of dmd

This process will let us move to generated headers without changing build dependencies.

[yebblies]

See also #5082

[ibuclaw]

I would prefer the idea of ddmd.backend to be a package containing what you've decided to call gluelayer, then have ddmd.backend.dmd as the place to keep your backend-proper.

[yebblies]

What would that gain? I don't see the point.

I'm thinking about splitting out the 'driver' code (mars, lib_, scan_, inifile, link etc) from the rest of the frontend, and as you know I'd like to ban glue/backend references from the rest of the frontend.

[ibuclaw]

Well, apart from gluelayer looking aesthetically unpleasing to my eyes... :-)

It could be at least be used as a connector for alternate backends, eg, ddmd.backend.gdc being where I put references to my functions, types, symbols, and so on...

As for the ban on references, I'm certainly not against the idea of using a common name/alias for backend types so long as they have a consistent naming scheme.
eg:

version (DM_Backend)
{
    alias Btype = TYPE;
    alias Bsymbol = Symbol
}
version (GCC_Backend)
{
    alias Btype = tree;
    alias Bsymbol = tree;
}

And so on...

[yebblies]

Well, apart from gluelayer looking aesthetically unpleasing to my eyes... :-)

Think of it as temporary!

The problem with the current system of using type/Symbol/etc is that's it's not enough. Some backends need members in more ast classes, some need less... If we really want to have the frontend as a re-usable library, then we really should be removing the backend references IMO.

[9rnsr]

I can accept this temporary change, after the Darwin_64_32 breaking is fixed.

[yebblies]

@9rnsr Updated.

[9rnsr]

Auto-merge toggled on

[yebblies]

Thanks!