Make the ffi crate no_std

[Person-93]

I've added no_std to the python-ffi crate.

Most of the changes in this PR are a search-and-replace from std:: to core::. The only other change I've made is in the datetime file. It used the Once type from std. I've updated it to use AtomicBool instead.

This is the first PR from this converstaion: #5856 (comment)

[codspeed-hq]

Merging this PR will not alter performance

✅ 125 untouched benchmarks

---

_{Comparing Person-93:no_std_ffi (f33c321) with main (65d6a36)}

[Person-93]

The ffi crate currently uses std::thread::park to work around the interpreter calling pthread_exit for python versions older than 3.14.

pyo3/pyo3-ffi/src/pystate.rs

Lines 82 to 92 in f57bda7

	#[cfg(not(any(Py_3_14, target_arch = "wasm32")))]
	struct HangThread;
	#[cfg(not(any(Py_3_14, target_arch = "wasm32")))]
	impl Drop for HangThread {
	fn drop(&mut self) {
	loop {
	std::thread::park(); // Block forever.
	}
	}
	}

I can think of two ways to handle this.

• move the usage into the main crate
• use a feature-flag to conditionally use a separate crate that can sleep in no_std

The first way is technically a breaking change if anyone is using the PyGILState_Ensure method from the ffi crate as they would have to deal with pinned stack objects some other way, but I think it's simpler, so I will go with that one unless you object.

[davidhewitt]

Thanks, overall this looks good. Couple of suggestions / thoughts / questions about the nontrivial bits.

[ngoldbaum]

Neat, I'm glad this is mostly mechanical. I'll try to give this a once-over.

[ngoldbaum]

I agree with all of David's comments. I also think we need a CI job that compiles the crate with no_std. Might be worth adding that to the nox configuration so all the CI jobs do it if it's a pretty quick build.

[Person-93]

I agree with all of David's comments. I also think we need a CI job that compiles the crate with no_std. Might be worth adding that to the nox configuration so all the CI jobs do it if it's a pretty quick build.

The ffi crate will always compile with no_std if python >= 3.14 or wasm build. It's not feature-gated.