[fizixer]

> What exactly are you going to expose? How should the build system interact with this?

The case of AST is relatively clear. You create a library module, or a standalone program, that takes source code and generates an AST. The AST could be in JSON format, s-expression (that would be my choice), heck even XML. It's better if the schema adheres to some standard (I think LLVM AST can be studied for inspiration, or even taken as standard) but even if it's not, that's a problem orthogonal to that of monolithic vs modular. Once you have an source-to-AST converter, it can be used inside a compiler, text-editor, build system, or something else. These are all "clients" of the source-to-AST module.

I'm not too sure about semantic-analysis since I'm studying it myself at the moment. All I can say at the moment (without expert knowledge of formal semantics) that once AST is used in more and more tools, semantic analysis would follow, and hopefully conventions would emerge for treating it in a canonical fashion. Short of that, every "AST client" can roll their own ad-hoc semantic-analyzer built-into the tool itself. Note that it would still be way more modular than a monolithic design.