Show better error when blocking conftest files via -p (#14018)

Show a clear message when `-p` is used for a  `conftest.py` file, instead of raising an internal assertion error.

Fixes #13634

Author: Bubble-Interface

AUTHORS

@@ -312,6 +312,7 @@ Michael Goerz
 Michael Krebs
 Michael Seifert
 Michael Vogt
+Michael Reznik
 Michal Wajszczuk
 Michał Górny
 Michał Zięba

changelog/13634.bugfix.rst

@@ -0,0 +1,5 @@
+Blocking a ``conftest.py`` file using the ``-p no:`` option is now explicitly disallowed.
+
+Previously this resulted in an internal assertion failure during plugin loading.
+
+Pytest now raises a clear ``UsageError`` explaining that conftest files are not plugins and cannot be disabled via ``-p``.

src/_pytest/config/__init__.py

@@ -816,6 +816,12 @@ def consider_pluginarg(self, arg: str) -> None:
             if name in essential_plugins:
                 raise UsageError(f"plugin {name} cannot be disabled")
 
+            if name.endswith("conftest.py"):
+                raise UsageError(
+                    f"Blocking conftest files using -p is not supported: -p no:{name}\n"
+                    "conftest.py files are not plugins and cannot be disabled via -p.\n"
+                )
+
             # PR #4304: remove stepwise if cacheprovider is blocked.
             if name == "cacheprovider":
                 self.set_blocked("stepwise")

testing/test_config.py

@@ -2475,6 +2475,10 @@ def test_config_does_not_load_blocked_plugin_from_args(pytester: Pytester) -> No
     result.stderr.fnmatch_lines(["*: error: unrecognized arguments: -s"])
     assert result.ret == ExitCode.USAGE_ERROR
 
+    result = pytester.runpytest(str(p), "-p no:/path/to/conftest.py", "-s")
+    result.stderr.fnmatch_lines(["ERROR:*Blocking conftest files*"])
+    assert result.ret == ExitCode.USAGE_ERROR
+
 
 def test_invocation_args(pytester: Pytester) -> None:
     """Ensure that Config.invocation_* arguments are correctly defined"""