fix: validate explicit config file paths

AUTHORS

@@ -507,6 +507,7 @@ Vlad Radziuk
 Vladyslav Rachek
 Volodymyr Kochetkov
 Volodymyr Piskun
+Wan Xiankai
 Warren Markham
 Wei Lin
 Wil Cooley

changelog/14716.bugfix.rst

@@ -0,0 +1 @@
+Explicit ``-c``/``--config-file`` paths now raise a clean usage error when the file is missing or uses an unsupported extension.

src/_pytest/config/findpaths.py

@@ -43,6 +43,9 @@ class ConfigValue:
 ConfigDict: TypeAlias = dict[str, ConfigValue]
 
 
+SUPPORTED_CONFIG_FILE_EXTENSIONS = {".cfg", ".ini", ".toml"}
+
+
 def _parse_ini_config(path: Path) -> iniconfig.IniConfig:
     """Parse the given generic '.ini' file using legacy IniConfig parser, returning
     the parsed object.
@@ -313,6 +316,17 @@ def determine_setup(
 
     if inifile:
         inipath_ = absolutepath(inifile)
+        if not inipath_.is_file():
+            raise UsageError(
+                f"Config file '{inipath_}' not found. "
+                "Check your '-c/--config-file' option."
+            )
+        if inipath_.suffix not in SUPPORTED_CONFIG_FILE_EXTENSIONS:
+            supported = ", ".join(sorted(SUPPORTED_CONFIG_FILE_EXTENSIONS))
+            raise UsageError(
+                f"Config file '{inipath_}' has unsupported extension "
+                f"{inipath_.suffix!r}. Supported extensions are: {supported}."
+            )
         inipath: Path | None = inipath_
         inicfg = load_config_dict_from_file(inipath_) or {}
         if rootdir_cmd_arg is None:

testing/test_config.py

@@ -787,6 +787,40 @@ def test_absolute_win32_path(self, pytester: Pytester) -> None:
         ret = pytest.main(["--config-file", temp_ini_file_norm])
         assert ret == ExitCode.NO_TESTS_COLLECTED
 
+    @pytest.mark.parametrize(
+        ("filename", "contents", "expected_error"),
+        [
+            pytest.param(
+                "custom.in",
+                "[pytest]",
+                "unsupported extension '.in'",
+                id="unsupported-extension",
+            ),
+            pytest.param(
+                "missing.ini",
+                None,
+                "not found",
+                id="missing-file",
+            ),
+        ],
+    )
+    @pytest.mark.parametrize("option", ("-c", "--config-file"))
+    def test_explicitly_specified_invalid_config_file_errors(
+        self,
+        pytester: Pytester,
+        option: str,
+        filename: str,
+        contents: str | None,
+        expected_error: str,
+    ) -> None:
+        if contents is not None:
+            pytester.path.joinpath(filename).write_text(contents, encoding="utf-8")
+
+        result = pytester.runpytest(option, filename)
+
+        assert result.ret == ExitCode.USAGE_ERROR
+        result.stderr.fnmatch_lines([f"ERROR: *{expected_error}*"])
+
 
 class TestConfigAPI:
     def test_config_trace(self, pytester: Pytester) -> None:
@@ -2058,6 +2092,38 @@ def test_with_specific_inifile(
         assert inipath == p
         assert ini_config["x"] == ConfigValue("10", origin="file", mode="ini")
 
+    @pytest.mark.parametrize(
+        ("filename", "contents", "expected_error"),
+        [
+            pytest.param(
+                "config.in",
+                "[pytest]\nx=10",
+                "unsupported extension '.in'",
+                id="unsupported-extension",
+            ),
+            pytest.param("missing.ini", None, "not found", id="missing-file"),
+        ],
+    )
+    def test_with_invalid_specific_inifile(
+        self,
+        tmp_path: Path,
+        filename: str,
+        contents: str | None,
+        expected_error: str,
+    ) -> None:
+        p = tmp_path / filename
+        if contents is not None:
+            p.write_text(contents, encoding="utf-8")
+
+        with pytest.raises(UsageError, match=re.escape(expected_error)):
+            determine_setup(
+                inifile=str(p),
+                override_ini=None,
+                args=[str(tmp_path)],
+                rootdir_cmd_arg=None,
+                invocation_dir=Path.cwd(),
+            )
+
     def test_explicit_config_file_sets_rootdir(
         self, tmp_path: Path, monkeypatch: pytest.MonkeyPatch
     ) -> None: