BUG: use meson compile wrapper on Windows

The meson compile wrapper is required on Windows to setup the MSVC
compiler environment. Using the --ninja-args option to meson compile
allows to provide the exact same semantics for the compile arguments.
The format in which ninja command line arguments need to be passed to
--ninja-args makes the interface a bit awkward to use and adds some
complexity to this otherwise simple patch.

Author: lithomas1

docs/how-to-guides/meson-args.rst

@@ -44,6 +44,20 @@ building a Python wheel. User options specified via ``pyproject.toml``
 or via Python build front-end config settings override the
 ``meson-python`` defaults.
 
+When building on Windows, ``meson-python`` invokes the ``ninja``
+command via the ``meson compile`` wrapper. When the GCC or the LLVM
+compilers are not found on the ``$PATH``, this activates the Visual
+Studio environment and allows ``ninja`` to use the MSVC compilers. To
+activate the Visual Studio environment unconditionally, pass the
+``--vsenv`` option to ``meson setup``, see this :ref:`example
+<vsenv-example>`. When using the ``meson compile`` wrapper, the user
+supplied options for the compilation command are passed via the
+``--ninja-args`` option. This ensures that the behaviour is
+independent of how the build is initiated. Refer to the `Meson
+documentation`__ for more details.
+
+__ https://mesonbuild.com/Commands.html#backend-specific-arguments
+
 
 Examples
 ========
@@ -152,3 +166,44 @@ To set this option temporarily at build-time:
         .. code-block:: console
 
 	   $ python -m pip wheel . --config-settings=setup-args="-Doptimization=3" .
+
+
+.. _vsenv-example:
+
+Force the use of the MSVC compilers on Windows
+----------------------------------------------
+
+The MSVC compilers are not installed in the ``$PATH``. The Visual
+Studio environment needs to be activated for ``ninja`` to be able to
+use these compilers. This is taken care of by ``meson compile`` but
+only when the GCC compilers or the LLVM compilers are not found on the
+``$PATH``. Passing the ``--vsenv`` option to ``meson setup`` forces
+the activation of the Visual Studio environment and generates an error
+when the activation fails.
+
+This option has no effect on other platforms thus, if your project
+requires to be compiled with MSVC, you can consider to set this option
+permanently in the project's ``pyproject.toml``:
+
+.. code-block:: toml
+
+   [tool.meson-python.args]
+   setup = ['--vsenv']
+
+To set this option temporarily at build-time:
+
+.. tab-set::
+
+    .. tab-item:: pypa/build
+        :sync: key_pypa_build
+
+        .. code-block:: console
+
+	   $ python -m build -Csetup-args="--vsenv" .
+
+    .. tab-item:: pip
+        :sync: key_pip
+
+        .. code-block:: console
+
+	   $ python -m pip wheel . --config-settings=setup-args="--vsenv" .

mesonpy/__init__.py

@@ -489,7 +489,10 @@ def _wheel_write_metadata(self, whl: mesonpy._wheelfile.WheelFile) -> None:
             )
 
     def build(self, directory: Path) -> pathlib.Path:
-        self._project.build()  # ensure project is built
+        # ensure project is built
+        self._project.build()
+        # install the project
+        self._project.install()
 
         wheel_file = pathlib.Path(directory, f'{self.name}.whl')
 
@@ -521,7 +524,8 @@ def build(self, directory: Path) -> pathlib.Path:
         return wheel_file
 
     def build_editable(self, directory: Path, verbose: bool = False) -> pathlib.Path:
-        self._project.build()  # ensure project is built
+        # ensure project is built
+        self._project.build()
 
         wheel_file = pathlib.Path(directory, f'{self.name}.whl')
 
@@ -534,14 +538,13 @@ def build_editable(self, directory: Path, verbose: bool = False) -> pathlib.Path
 
             # install loader module
             loader_module_name = f'_{self.normalized_name.replace(".", "_")}_editable_loader'
-            build_cmd = [self._project._ninja, *self._project._meson_args['compile']]
             whl.writestr(
                 f'{loader_module_name}.py',
                 read_binary('mesonpy', '_editable.py') + textwrap.dedent(f'''
                    install(
                        {self.top_level_modules!r},
                        {os.fspath(self._build_dir)!r},
-                       {build_cmd},
+                       {self._project._build_command!r},
                        {verbose!r},
                    )''').encode('utf-8'))
 
@@ -787,18 +790,30 @@ def _wheel_builder(self) -> _WheelBuilder:
             self._install_plan,
         )
 
-    def build_commands(self) -> Sequence[Sequence[str]]:
+    @property
+    def _build_command(self) -> List[str]:
         assert self._ninja is not None  # help mypy out
-        return (
-            (self._ninja, *self._meson_args['compile'],),
-            ('meson', 'install', '--no-rebuild', '--destdir', os.fspath(self._install_dir), *self._meson_args['install']),
-        )
+        if platform.system() == 'Windows':
+            # On Windows use 'meson compile' to setup the MSVC compiler
+            # environment. Using the --ninja-args option allows to
+            # provide the exact same semantics for the compile arguments
+            # provided by the users.
+            cmd = ['meson', 'compile']
+            args = list(self._meson_args['compile'])
+            if args:
+                cmd.append(f'--ninja-args={args!r}')
+            return cmd
+        return [self._ninja, *self._meson_args['compile']]
 
     @functools.lru_cache(maxsize=None)
     def build(self) -> None:
-        """Trigger the Meson build."""
-        for cmd in self.build_commands():
-            self._run(cmd)
+        """Build the Meson project."""
+        self._run(self._build_command)
+
+    def install(self) -> None:
+        """Install the Meson project."""
+        destdir = os.fspath(self._install_dir)
+        self._run(['meson', 'install', '--no-rebuild', '--destdir', destdir, *self._meson_args['install']])
 
     @classmethod
     @contextlib.contextmanager

tests/test_project.py

@@ -2,6 +2,8 @@
 #
 # SPDX-License-Identifier: MIT
 
+import ast
+import os.path
 import platform
 import shutil
 import sys
@@ -60,12 +62,26 @@ def test_unsupported_python_version(package_unsupported_python_version):
 
 def test_user_args(package_user_args, tmp_path, monkeypatch):
     project_run = mesonpy.Project._run
-    call_args_list = []
+    cmds = []
+    args = []
 
     def wrapper(self, cmd):
         # intercept and filter out test arguments and forward the call
-        call_args_list.append(tuple(cmd))
-        return project_run(self, [x for x in cmd if not x.startswith(('config-', 'cli-'))])
+        if cmd[:2] == ['meson', 'compile']:
+            # when using meson compile instead on ninja directly, the
+            # arguments needs to be unmarshalled from the form used to
+            # pass them to the --ninja-args option
+            assert cmd[-1].startswith('--ninja-args=')
+            cmds.append(cmd[:2])
+            args.append(ast.literal_eval(cmd[-1].split('=')[1]))
+        elif cmd[:1] == ['meson']:
+            cmds.append(cmd[:2])
+            args.append(cmd[2:])
+        else:
+            # direct ninja invocation
+            cmds.append([os.path.basename(cmd[0])])
+            args.append(cmd[1:])
+        return project_run(self, [x for x in cmd if not x.startswith(('config-', 'cli-', '--ninja-args'))])
 
     monkeypatch.setattr(mesonpy.Project, '_run', wrapper)
 
@@ -79,19 +95,32 @@ def wrapper(self, cmd):
     mesonpy.build_sdist(tmp_path, config_settings)
     mesonpy.build_wheel(tmp_path, config_settings)
 
-    expected = [
+    # check that the right commands are executed, namely that 'meson
+    # compile' is used on Windows trather than a 'ninja' direct
+    # invocation.
+    assert cmds == [
         # sdist: calls to 'meson setup' and 'meson dist'
-        ('config-setup', 'cli-setup'),
-        ('config-dist', 'cli-dist'),
+        ['meson', 'setup'],
+        ['meson', 'dist'],
         # wheel: calls to 'meson setup', 'meson compile', and 'meson install'
-        ('config-setup', 'cli-setup'),
-        ('config-compile', 'cli-compile'),
-        ('config-install', 'cli-install'),
+        ['meson', 'setup'],
+        ['meson', 'compile'] if platform.system() == 'Windows' else ['ninja'],
+        ['meson', 'install']
     ]
 
-    for expected_args, call_args in zip(expected, call_args_list):
+    # check that the user options are passed to the invoked commands
+    expected = [
+        # sdist: calls to 'meson setup' and 'meson dist'
+        ['config-setup', 'cli-setup'],
+        ['config-dist', 'cli-dist'],
+        # wheel: calls to 'meson setup', 'meson compile', and 'meson install'
+        ['config-setup', 'cli-setup'],
+        ['config-compile', 'cli-compile'],
+        ['config-install', 'cli-install'],
+    ]
+    for expected_args, cmd_args in zip(expected, args):
         for arg in expected_args:
-            assert arg in call_args
+            assert arg in cmd_args
 
 
 @pytest.mark.parametrize('package', ('top-level', 'meson-args'))