Reject --build-options for PEP 517 builds

news/6305.feature

@@ -0,0 +1,2 @@
+Using ``--build-options`` in a PEP 517 build now fails with an error,
+rather than silently ignoring the option.

src/pip/_internal/wheel.py

@@ -910,6 +910,11 @@ def _build_one_pep517(self, req, tempd, python_tag=None):
         Returns path to wheel if successfully built. Otherwise, returns None.
         """
         assert req.metadata_directory is not None
+        if self.build_options:
+            # PEP 517 does not support --build-options
+            logger.error('Cannot build wheel for %s using PEP 517 when '
+                         '--build-options is present' % (req.name,))
+            return None
         try:
             req.spin_message = 'Building wheel for %s (PEP 517)' % (req.name,)
             logger.debug('Destination directory: %s', tempd)

tests/functional/test_pep517.py

@@ -198,3 +198,17 @@ def test_explicit_setuptools_backend(script, tmpdir, data, common_wheels):
         project_dir,
     )
     result.assert_installed(name, editable=False)
+
+
+def test_pep517_and_build_options(script, tmpdir, data, common_wheels):
+    """Backend generated requirements are installed in the build env"""
+    project_dir, name = make_pyproject_with_setup(tmpdir)
+    result = script.pip(
+        'wheel', '--wheel-dir', tmpdir,
+        '--build-option', 'foo',
+        '-f', common_wheels,
+        project_dir,
+        expect_error=True
+    )
+    assert 'Cannot build wheel' in result.stderr
+    assert 'when --build-options is present' in result.stderr