Forward warning options to subprocess

src/pyproject_hooks/_impl.py

@@ -3,6 +3,7 @@
 import sys
 import tempfile
 from contextlib import contextmanager
+from itertools import chain
 from os.path import abspath
 from os.path import join as pjoin
 from subprocess import STDOUT, check_call, check_output
@@ -380,6 +381,15 @@ def build_sdist(
             },
         )
 
+    def _get_warnopts(self) -> Iterator[str]:
+        """
+        Reconstruct Python's warn options that are active for the current process,
+        so that it can be forwarded to a subprocess.
+        """
+        # `sys.warnoptions` is documented/mentioned in
+        # https://docs.python.org/3.13/library/warnings.html#describing-warning-filters
+        return chain.from_iterable(("-W", opt) for opt in sys.warnoptions)
+
     def _call_hook(self, hook_name: str, kwargs: Mapping[str, Any]) -> Any:
         extra_environ = {"_PYPROJECT_HOOKS_BUILD_BACKEND": self.build_backend}
 
@@ -394,8 +404,9 @@ def _call_hook(self, hook_name: str, kwargs: Mapping[str, Any]) -> Any:
             # Run the hook in a subprocess
             with _in_proc_script_path() as script:
                 python = self.python_executable
+                opts = self._get_warnopts()
                 self._subprocess_runner(
-                    [python, abspath(str(script)), hook_name, td],
+                    [python, *opts, abspath(str(script)), hook_name, td],
                     cwd=self.source_dir,
                     extra_environ=extra_environ,
                 )

tests/test_call_hooks.py

@@ -4,6 +4,7 @@
 import zipfile
 from os.path import abspath, dirname
 from os.path import join as pjoin
+from subprocess import CalledProcessError
 from unittest.mock import Mock
 
 import pytest
@@ -233,3 +234,25 @@ def test_warnings():
     with modified_env({"PYTHONPATH": BUILDSYS_PKGS}):
         with pytest.warns(BuildBackendWarning, match="this is my example warning"):
             hooks.get_requires_for_build_wheel({})
+
+
+def test_forward_warnopts(monkeypatch, recwarn):
+    hooks = get_hooks("pkg-with-warnings")
+
+    monkeypatch.setattr(
+        BuildBackendHookCaller, "_get_warnopts", lambda _: ["-W", "ignore"]
+    )
+    with modified_env({"PYTHONPATH": BUILDSYS_PKGS}):
+        hooks.get_requires_for_build_wheel({})
+        assert len(recwarn) == 0  # All subprocess warnings were ignored.
+
+    monkeypatch.setattr(
+        BuildBackendHookCaller, "_get_warnopts", lambda _: ["-W", "error"]
+    )
+    with modified_env({"PYTHONPATH": BUILDSYS_PKGS}):
+        with pytest.raises(CalledProcessError):
+            # The `match` argument does not work against the exception in the
+            # subprocess, but  we have a sanity check in place: the same call
+            # worked above when the warnings were ignored, so we known this is
+            # caused by the new warnings filter.
+            hooks.get_requires_for_build_wheel({})