If interested, you can see their work-in-progress main.rs in the related code revision[0], which includes their Rust code calling down to the C function Py_Initialize() to spin up the now-embedded CPython interpreter that is living "inside" a Rust program:

    unsafe {
        Py_Initialize();
        PySys_SetArgv(args.len() as c_int,
                      argv.as_ptr());
        PyEval_InitThreads();
        let _thread_state = PyEval_SaveThread();
    }

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[0] https://phab.mercurial-scm.org/D1581#change-t24aVkGEJ5Xh