Skip Rust extension build for free-threaded Python

Free-threaded Python (3.13t) doesn't support Rust extensions yet.
The build hook now detects free-threaded Python and skips the
Rust extension build to avoid GIL-related import errors.

Author: aclark4life

hatch_build.py

@@ -145,11 +145,18 @@ def initialize(self, version, build_data):
 
         # Build Rust extension (optional)
         # Only build if PYMONGO_BUILD_RUST is set or Rust is available
+        # Skip for free-threaded Python (not yet supported)
+        is_free_threaded = hasattr(sys, "_is_gil_enabled") and not sys._is_gil_enabled()
         build_rust = os.environ.get("PYMONGO_BUILD_RUST", "").lower() in ("1", "true", "yes")
-        if build_rust:
+        if build_rust and is_free_threaded:
+            warnings.warn(
+                "Rust extension is not yet supported on free-threaded Python. Skipping build.",
+                stacklevel=2,
+            )
+        elif build_rust:
             # If PYMONGO_BUILD_RUST is explicitly set, the build must succeed
             self._build_rust_extension(here, required=True)
-        elif shutil.which("cargo"):
+        elif shutil.which("cargo") and not is_free_threaded:
             # If Rust is available but not explicitly requested, build is optional
             self._build_rust_extension(here, required=False)