Add --assert-config option

Motivation from Fedora:

When we updated tox from version 3 to 4, it no longer fails when here is no
suitable tox configuration found. This was a deliberate upstream choice.

Unfortunately, it means that packages that use %pyproject_buildrequires with -t
or -e now silently succeed if there is no tox configuration found.

I identified 95 packages that are affected by this.

Details: https://lists.fedoraproject.org/archives/list/python-devel@lists.fedoraproject.org/thread/ZSHSHZKVA4XJQBJD7FMMCALKN4UP5SAJ/

README.rst

@@ -36,6 +36,12 @@ The ``tox-current-env`` plugin adds these options:
     Use ``-`` for ``FILE`` to print to standard output.
     This option only exists with tox 4 and requires at least tox 4.22.
 
+``tox --assert-config``
+    In tox 4, this option ensures that tox fails (raises an exception) if no configuration is found.
+    By default, tox 4 does not terminate when no configuration exists.
+    In tox 3, this option has no effect, but it can still be specified without causing errors.
+    This option can be used alongside other options.
+
 It is possible to use the three printing options together, as long as the ``FILE`` is different.
 
 Invoking ``tox`` without any of the above options should behave as regular ``tox`` invocation without this plugin.

src/tox_current_env/hooks3.py

@@ -48,6 +48,12 @@ def tox_addoption(parser):
         help="Don't run tests, only print the  names of the required extras to the given file "
             + "(use `-` for stdout)",
     )
+    parser.add_argument(
+        "--assert-config",
+        action="store_true",
+        default=False,
+        help="Fail if no tox configuration is found (default on tox 3, does nothing)",
+    )
 
 
 def _plugin_active(option):

src/tox_current_env/hooks4.py

@@ -66,12 +66,26 @@ def tox_add_option(parser):
         help="Don't run tests, only print the names of the required dependency-groups to the given file "
         + "(use `-` for stdout)",
     )
+    parser.add_argument(
+        "--assert-config",
+        action="store_true",
+        default=False,
+        help="Fail if no tox configuration is found",
+    )
 
 
 @impl
 def tox_add_core_config(core_conf, state):
     opt = state.conf.options
 
+    if opt.assert_config and not state.conf.src_path.exists():
+        raise LookupError(
+            "tox configuration not found. "
+            "To use tox, please ensure tox configuration is located in current directory "
+            "(in tox.ini, setup.cfg, pyproject.toml, or tox.toml). "
+            "See https://tox.wiki/en/latest/config.html for details."
+        )
+
     if opt.current_env or opt.print_deps_to or opt.print_extras_to or opt.print_dependency_groups_to:
         # We do not want to install the main package.
         # no_package is the same as skipsdist.

tests/test_integration_tox3.py

@@ -580,3 +580,43 @@ def test_passenv(projdir, passenv):
     assert result.returncode == 0
     assert "\nassertme\n" in result.stdout
     assert "\nNone\n" not in result.stdout
+
+
+def test_assert_config_option_with_config(projdir):
+    result = tox("-l", "--assert-config", check=False)
+    assert result.returncode == 0
+
+
+def test_assert_config_option_without_config(projdir):
+    (projdir / "tox.ini").unlink()
+    result = tox("-l", "--assert-config", check=False)
+    assert result.returncode > 0
+    assert "config" in result.stderr
+    assert "not found" in result.stderr
+
+
+def test_assert_config_option_with_setup_cfg(projdir):
+    (projdir / "tox.ini").unlink()
+    setup_cfg = projdir / "setup.cfg"
+    setup_cfg.write_text(textwrap.dedent("""
+        [tox:tox]
+        env_list =
+            py310
+    """))
+    result = tox("-l", "--assert-config", check=False)
+    assert result.returncode == 0
+
+
+def test_assert_config_option_with_pyproject_toml(projdir):
+    (projdir / "tox.ini").unlink()
+    pyproject_toml = projdir / "pyproject.toml"
+    pyproject_toml.write_text(textwrap.dedent('''
+        [tool.tox]
+        legacy_tox_ini = """
+        [tox]
+        envlist =
+            py310
+        """
+    '''))
+    result = tox("-l", "--assert-config", check=False)
+    assert result.returncode == 0

tests/test_integration_tox4.py

@@ -591,3 +591,54 @@ def test_report_installed(projdir):
     assert result.returncode == 0
     assert "tox==" in result.stdout
     assert "pytest==" in result.stdout
+
+
+def test_assert_config_option_with_config(projdir):
+    result = tox("-l", "--assert-config", check=False)
+    assert result.returncode == 0
+
+
+def test_assert_config_option_without_config(projdir):
+    (projdir / "tox.ini").unlink()
+    result = tox("-l", "--assert-config", check=False)
+    assert result.returncode > 0
+    assert "config" in result.stderr
+    assert "not found" in result.stderr
+
+
+def test_assert_config_option_with_setup_cfg(projdir):
+    (projdir / "tox.ini").unlink()
+    setup_cfg = projdir / "setup.cfg"
+    setup_cfg.write_text(textwrap.dedent("""
+        [tox:tox]
+        env_list =
+            py310
+    """))
+    result = tox("-l", "--assert-config", check=False)
+    assert result.returncode == 0
+
+
+def test_assert_config_option_with_pyproject_toml(projdir):
+    (projdir / "tox.ini").unlink()
+    pyproject_toml = projdir / "pyproject.toml"
+    pyproject_toml.write_text(textwrap.dedent('''
+        [tool.tox]
+        legacy_tox_ini = """
+        [tox]
+        envlist =
+            py310
+        """
+    '''))
+    result = tox("-l", "--assert-config", check=False)
+    assert result.returncode == 0
+
+
+def test_assert_config_option_with_pyproject_toml_native(projdir):
+    (projdir / "tox.ini").unlink()
+    pyproject_toml = projdir / "pyproject.toml"
+    pyproject_toml.write_text(textwrap.dedent("""
+        [tool.tox]
+        env_list = ["3.13"]
+    """))
+    result = tox("-l", "--assert-config", check=False)
+    assert result.returncode == 0