> +1 on exposing f,__builtins__.

This change is related to bpo-43228 "Regression in function builtins": cloudpickle is broken by this issue (new PyFunctionObject.func_builtins member).

> Of course, the thing I'd really want is a way to state that all references to builtins are meant to have the exact semantics of those builtins, so the compiler can translate e.g. len(x) into a new opcode that just calls PyObject_Size().  (I can dream, can't I?)

I tried to implement such optimization in my old https://faster-cpython.readthedocs.io/fat_python.html project. I implemented guards to de-optimize the code if a builtin is overriden.

> I suppose enough checking of dict version tags can get us there, or at least close enough.

The dict version comes from my FAT Python project: PEP 509. It is used to optimize method calls. See also PEP 510 and PEP 511 ;-)