expose full preprocessed .stan program through compiler

[bob-carpenter]

Summary:

The stan::lang::compile function should have a std::string& argument that gets instantiated to the fully expanded .stan file after preprocessing.

Current Version:

v2.18.0

[bgoodri]

@paul-buerkner I think I'll just wait for this and take the thing out of rstan

[paul-buerkner]

Sounds good to me.

[VMatthijs]

I'd be happy to implement this in stanc3. Does it have to be literally the source file (including comments and everything)? Or can it be a pretty printed version? The latter would be easy for me to add.

[VMatthijs]

This is implemented now in stanc3, using the --auto-format flag to the compiler. It exposes a pre-processed (done all the includes) and pretty printed (unfortunately, comments stripped out for now) version of your program.