Output a warning to stderr when an invalid key is read from an INI config file

src/_pytest/config/__init__.py

@@ -1020,6 +1020,7 @@ class Config:
             )
 
         self._checkversion()
+        self._validatekeys()
         self._consider_importhook(args)
         self.pluginmanager.consider_preparse(args, exclude_only=False)
         if not os.environ.get("PYTEST_DISABLE_PLUGIN_AUTOLOAD"):
@@ -1072,6 +1073,14 @@ class Config:
                     )
                 )
 
+    def _validatekeys(self):
+        for key in self._get_unknown_ini_keys():
+            sys.stderr.write("WARNING: unknown config ini key: {}\n".format(key))
+
+    def _get_unknown_ini_keys(self) -> List[str]:
+        parser_inicfg = self._parser._inidict
+        return [name for name in self.inicfg if name not in parser_inicfg]
+
     def parse(self, args: List[str], addopts: bool = True) -> None:
         # parse given cmdline arguments into this config object.
         assert not hasattr(

testing/test_config.py

@@ -147,6 +147,52 @@ class TestParseIni:
         result = testdir.inline_run("--confcutdir=.")
         assert result.ret == 0
 
+    @pytest.mark.parametrize(
+        "ini_file_text, invalid_keys, stderr_output",
+        [
+            (
+                """
+          [pytest]
+          unknown_ini = value1
+          another_unknown_ini = value2
+          """,
+                ["unknown_ini", "another_unknown_ini"],
+                [
+                    "WARNING: unknown config ini key: unknown_ini",
+                    "WARNING: unknown config ini key: another_unknown_ini",
+                ],
+            ),
+            (
+                """
+          [pytest]
+          unknown_ini = value1
+          minversion = 5.0.0
+          """,
+                ["unknown_ini"],
+                ["WARNING: unknown config ini key: unknown_ini"],
+            ),
+            (
+                """
+          [pytest]
+          minversion = 5.0.0
+          """,
+                [],
+                [],
+            ),
+        ],
+    )
+    def test_invalid_ini_keys_generate_warings(
+        self, testdir, ini_file_text, invalid_keys, stderr_output
+    ):
+        testdir.tmpdir.join("pytest.ini").write(textwrap.dedent(ini_file_text))
+        config = testdir.parseconfig()
+        assert config._get_unknown_ini_keys() == invalid_keys, str(
+            config._get_unknown_ini_keys()
+        )
+
+        result = testdir.runpytest()
+        result.stderr.fnmatch_lines(stderr_output)
+
 
 class TestConfigCmdlineParsing:
     def test_parsing_again_fails(self, testdir):