TST: enable fatal meson warnings for all tests

Unless explicitly disabled in ``pyproject.toml``.

This uses a pytest ``autouse`` fixture to monkeypatch the function
that validates the ``pyproject.toml`` meson-python configuration to
add ``--fatal-meson-warnings`` to the ``meson setup`` arguments,
unless ``--no-fatal-meson-warnings`` is specified by the package.

Author: dnicolodi

tests/conftest.py

@@ -192,3 +192,33 @@ def cleanenv():
     # $MACOSX_DEPLOYMENT_TARGET affects the computation of the platform tag on macOS.
     yield mpatch.delenv('MACOSX_DEPLOYMENT_TARGET', raising=False)
     mpatch.undo()
+
+
+@pytest.fixture(autouse=True, scope='session')
+def meson_fatal_warnings():
+    # Cannot use the 'monkeypatch' fixture because of scope mismatch.
+    mpatch = pytest.MonkeyPatch()
+    mesonpy_project_init = mesonpy.Project.__init__
+
+    def __init__(self, source_dir, build_dir, meson_args=None, editable_verbose=False):
+        if pathlib.Path(source_dir).absolute().name not in {
+
+                # The CMake subproject emits ``WARNING: CMake Toolchain:
+                # Failed to determine CMake compilers state`` on some
+                # systems. This is probably related to missing Rust or ObjC
+                # toolchains.
+                'cmake-subproject',
+
+                # The ``link-against-local-lib`` package uses linker arguments
+                # to add RPATH entries. This functionality is deprecated in
+                # Meson but it is used in the wild thus we should make sure it
+                # keeps working.
+                'link-against-local-lib',
+
+        }:
+            if meson_args is None:
+                meson_args = {}
+            meson_args.setdefault('setup', []).append('--fatal-meson-warnings')
+        mesonpy_project_init(self, source_dir, build_dir, meson_args, editable_verbose)
+
+    mpatch.setattr(mesonpy.Project, '__init__', __init__)

tests/packages/link-against-local-lib/pyproject.toml

@@ -8,6 +8,3 @@ requires = ['meson-python']
 
 [tool.meson-python]
 allow-windows-internal-shared-libs = true
-
-[tool.meson-python.args]
-setup = ['--no-fatal-meson-warnings']

tests/test_project.py

@@ -399,7 +399,7 @@ def test_ios_project(package_simple, monkeypatch, multiarch, tmp_path):
     project = mesonpy.Project(source_dir=package_simple, build_dir=tmp_path)
 
     # Meson configuration points at the cross file
-    assert project._meson_args['setup'] == ['--cross-file', os.fspath(tmp_path / 'meson-python-cross-file.ini')]
+    assert project._meson_args['setup'][-2:] == ['--cross-file', os.fspath(tmp_path / 'meson-python-cross-file.ini')]
 
     # Meson config files exist, and have some relevant keys
     assert (tmp_path / 'meson-python-native-file.ini').exists()