Setting configs incompatible with Py_GIL_DISABLED should error harder

[ngoldbaum]

I noticed working on cryptography, which tries to build against the Python 3.7 limited API, that it has this pair of configs set:

cargo:rustc-cfg=Py_3_7
cargo:rustc-cfg=Py_GIL_DISABLED

This leads to confusing compiler errors due to the conditional compilation in pyo3-ffi not being written to handle this case since it's impossible:

DEBUG    Compiling cryptography-x509 v0.1.0 (/Users/goldbaum/Documents/cryptography/src/rust/cryptography-x509)
DEBUG error[E0432]: unresolved import `crate::PyMutex`
DEBUG  --> /Users/goldbaum/Documents/pyo3/pyo3-ffi/src/object.rs:3:5
DEBUG   |
DEBUG 3 | use crate::PyMutex;
DEBUG   |     ^^^^^^^^^^^^^^ no `PyMutex` in the root
DEBUG
DEBUG error[E0425]: cannot find value `_Py_IMMORTAL_REFCNT` in this scope
DEBUG    --> /Users/goldbaum/Documents/pyo3/pyo3-ffi/src/object.rs:162:20
DEBUG     |
DEBUG 29  | const _Py_IMMORTAL_REFCNT_LOCAL: u32 = u32::MAX;
DEBUG     | ------------------------------------------------ similarly named constant `_Py_IMMORTAL_REFCNT_LOCAL` defined here
DEBUG ...
DEBUG 162 |             return _Py_IMMORTAL_REFCNT;
DEBUG     |                    ^^^^^^^^^^^^^^^^^^^ help: a constant with a similar name exists: `_Py_IMMORTAL_REFCNT_LOCAL`
DEBUG
DEBUG error[E0560]: struct `object::PyObject` has no field named `ob_refcnt`
DEBUG   --> /Users/goldbaum/Documents/pyo3/pyo3-ffi/src/object.rs:55:5
DEBUG    |
DEBUG 55 |     ob_refcnt: 1,
DEBUG    |     ^^^^^^^^^ `object::PyObject` does not have this field
DEBUG    |
DEBUG    = note: all struct fields are already assigned

We should panic earlier because this configuration will never work.

[davidhewitt]

Hmm, I wonder, does this imply that it's actually really hard for cryptography to build a free-threaded wheel because they enabled the abi3 features?

[ngoldbaum]

No, I think it just means they'll need to upload a cp313t wheel alongside the abi3 wheel. This is true for any project that normally uploads version-independent wheels but wants to support free-threaded 3.13.

The limited API is a subset of the full API and I don't think normally compiling against the 3.7 limited ABI prevents compiling for a newer non-limited ABI.