Builds are failing on free-threaded Python

[jeertmans]

Hi!

I searched quite a lot, but I could not find any issue mentioning the fact that one cannot build this library with free-threaded Python, which can arise automatically with recent PyO3 versions and the use of --find-interpreter to build the library, which finds Python3.13t on Linux platforms.

So with the recent releases of PyO3 (started with 0.23.2, I think), my CI builds started failing. I noticed the following issue:

error[E0432]: unresolved import `pyo3::sync::GILProtected`
  --> /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/numpy-0.23.0/src/datetime.rs:63:12
   |
63 | use pyo3::{sync::GILProtected, Bound, Py, Python};
   |            ^^^^^^^^^^^^^^^^^^ no `GILProtected` in `sync`
   |
note: found an item that was configured out
  --> /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pyo3-0.23.3/src/sync.rs:47:12
   |
47 | pub struct GILProtected<T> {
   |            ^^^^^^^^^^^^
note: the item is gated here
  --> /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pyo3-0.23.3/src/sync.rs:46:1
   |
46 | #[cfg(not(Py_GIL_DISABLED))]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0432]: unresolved import `pyo3::sync::GILProtected`
  --> /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/numpy-0.23.0/src/strings.rs:15:5
   |
15 |     sync::GILProtected,
   |     ^^^^^^^^^^^^^^^^^^ no `GILProtected` in `sync`
   |
   = note: unresolved item `crate::datetime::units::GILProtected` exists but is inaccessible
note: found an item that was configured out
  --> /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pyo3-0.23.3/src/sync.rs:47:12
   |
47 | pub struct GILProtected<T> {
   |            ^^^^^^^^^^^^
note: the item is gated here
  --> /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pyo3-0.23.3/src/sync.rs:46:1
   |
46 | #[cfg(not(Py_GIL_DISABLED))]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0432`.

The use of GILProtected seems quite restricted in your project, and I was wondering if this issue could be solved by using a Mutex instead, as done by @davidhewitt in the jiter module, see crates/jiter/src/py_string_cache.rs.

If free-threaded Python is incompatible with this project, maybe it could be better to emit a compile_error!(...) instead?

[Icxolu]

We are working on supporting the free-threaded build over in #471. We still need to make sure that we get rust-numpys dynamic borrow checking correct in the new environment, which is a bit more involved than just replacing the GILProtected.

[jeertmans]

Thanks for pointing to that PR @Icxolu!