Adding a small feature like #131198 (comment) becomes a complicated puzzle of moving functions and structs between header files to avoid cycles.
I was ultimately defeated in this case.

This is a bit silly. We should break up these dependencies, by breaking headers into structs and code.
Code depends on data, and headers that depend on some structs, but not the code, can import only the struct definitions. Breaking headers into smaller, more self contained chunks would also help.

As an example of the problem, to use _PyThreadState_GET() one must import pycore_pystate.h but that imports pycore_runtime.h which imports nineteen other "core" header files!

Linked PRs

• GH-131238: Core header refactor #131250
• gh-131238: Add explicit include to pycore headers #131257
• GH-131238: More refactoring of core header files #131351
• gh-131238: Convert pycore_pystate.h static inline to functions #131352
• gh-131238: Remove pycore_runtime.h from pycore_pystate.h #131356
• gh-131238: Remove many includes in pycore_interp.h #131472
• gh-131238: Remove more includes from pycore_interp.h #131480
• gh-131238: Use pycore_interp_structs.h header #131481
• gh-131238: Move pycore_obmalloc.h include to pycore_runtime_structs.h #131482
• gh-131238: Remove pycore_lock.h includes #131483
• gh-131238: Cleanup pycore_runtime.h includes #131486
• gh-131238: Remove includes from pycore_interp.h #131495
• gh-131238: Add pycore_interpframe.h to PYTHON_HEADERS #131545
• gh-131238: Remove pycore_object_deferred.h from pycore_object.h #131549
• gh-131238: Add pycore_interpframe_structs.h header #131553
• gh-131238: Move _Py_VISIT_STACKREF() to pycore_stackref.h #131560
• gh-131238: Add missing pycore_function.h includes for JIT compiler #131571