tox 4: Abort the --print* options when no tox config was found

That way, Fedora packages using `%pyproject_buildrequires -t`
will fail to build when the upstream project does not use tox at all
or when the tox configuration is missing.

Author: hroncok

README.rst

@@ -167,6 +167,9 @@ The plugin is available also for tox 4. Differences in behavior between tox 3 an
 - The plugin does not check the requested Python version nor the environment name.
   If you let it run for multiple environments they'll all use the same Python.
 - Deprecated ``--print-deps-only`` option is no longer available.
+- Unlike tox 3, tox 4 can normally run without any tox configuration.
+  When tox 4 runs with ``--print-deps-to`` or ``--print-extras-to`` without a tox configuration, it will fail.
+  This was deliberately done to make Fedora packages misusing this option fail to build.
 
 Use an isolated environment
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/tox_current_env/hooks4.py

@@ -61,6 +61,11 @@ def tox_add_option(parser):
 @impl
 def tox_add_core_config(core_conf, state):
     opt = state.conf.options
+    if (opt.print_deps_to or opt.print_extras_to) and not core_conf.loaders:
+        raise RuntimeError(
+            "--print-deps-to and/or --print-extras-to cannot be used without a tox config. "
+            "Seeing this error indicates this project either does not use tox at all or the tox configuration is missing."
+        )
 
     if opt.current_env or opt.print_deps_to or opt.print_extras_to:
         # We do not want to install the main package.

tests/test_integration_tox4.py

@@ -430,3 +430,24 @@ def test_report_installed(projdir):
     assert result.returncode == 0
     assert "tox==" in result.stdout
     assert "pytest==" in result.stdout
+
+
+@pytest.mark.parametrize("toxenv", envs_from_tox_ini())
+def test_print_deps_without_config_fails(projdir, toxenv, print_deps_stdout_arg):
+    tox_ini = projdir / "tox.ini"
+    tox_ini.unlink()
+    # note: tox will traverse the filesystem up to find a config,
+    # so if this (or the next) test fails,
+    # check if you don't have a stray tox.ini in /tmp
+    result = tox("-e", toxenv, print_deps_stdout_arg, check=False)
+    assert result.returncode > 0
+    assert "tox configuration is missing" in result.stderr
+
+
+@pytest.mark.parametrize("toxenv", envs_from_tox_ini())
+def test_print_extras_without_config_fails(projdir, toxenv):
+    tox_ini = projdir / "tox.ini"
+    tox_ini.unlink()
+    result = tox("-e", toxenv, "--print-extras-to=-", check=False)
+    assert result.returncode > 0
+    assert "tox configuration is missing" in result.stderr