Regarding ac46eb4ad6662cf6d771b20d8963658b2186c48c:
Module states come and go with the modules that contain them; if a "get_global_ast_state" or "astmodulestate_global" needs to be accessed from outside the module, it shouldn't be module state :/

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So, the main issue here is that the AST types are not used only by the _ast module, but by the interpreter itself: the compile() builtin and Py_CompileStringObject.

I see two ways of fixing this properly:

1. All the classes _ast provides should be built-in, like, say, `function`. (Currently, that means they should be static types; later they could be per-interpreter.)
The _ast module should merely expose them from Python, like the `types` module exposes the function type.
This would mean that calling Py_CompileStringObject with PyCF_ONLY_AST will be independent of the _ast module.

2. The mod2obj/obj2mod functions, called by e.g. compile(..., PyCF_ONLY_AST), should:
* import the _ast module
* call a Python-accessible function, e.g. _ast._mod2obj
This would mean replacing the _ast module (in sys.modules or through an import hook, which you can do from Python code) will affect what AST types will be used throughout the interpreter.