use posix error messages under 127 (#6420)

* use posix error messages under 127

* use posix error message for unix

* unmark successful tests

Lib/test/test_argparse.py

@@ -6747,8 +6747,6 @@ class TestExitOnError(TestCase):
                 "ambiguous option: --foob=1 could match --foobaz, --fooble$",
             self.parser.parse_args, ['--foob=1', '--foogle', '2'])
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_os_error(self):
         self.parser.add_argument('file')
         self.assertRaisesRegex(argparse.ArgumentError,

Lib/test/test_py_compile.py

@@ -266,7 +266,6 @@ class PyCompileCLITestCase(unittest.TestCase):
         self.assertEqual(stdout, b'')
         self.assertEqual(stderr, b'')
 
-    @unittest.expectedFailureIfWindows("TODO: RUSTPYTHON")
     def test_file_not_exists(self):
         should_not_exists = os.path.join(os.path.dirname(__file__), 'should_not_exists.py')
         rc, stdout, stderr = self.pycompilecmd_failure(self.source_path, should_not_exists)

Lib/test/test_subprocess.py

@@ -1878,8 +1878,6 @@ class POSIXProcessTestCase(BaseTestCase):
                       self._nonexistent_dir)
         return desired_exception
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_exception_cwd(self):
         """Test error in the child raised in the parent for a bad cwd."""
         desired_exception = self._get_chdir_exception()
@@ -1895,8 +1893,6 @@ class POSIXProcessTestCase(BaseTestCase):
         else:
             self.fail("Expected OSError: %s" % desired_exception)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_exception_bad_executable(self):
         """Test error in the child raised in the parent for a bad executable."""
         desired_exception = self._get_chdir_exception()
@@ -1912,8 +1908,6 @@ class POSIXProcessTestCase(BaseTestCase):
         else:
             self.fail("Expected OSError: %s" % desired_exception)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_exception_bad_args_0(self):
         """Test error in the child raised in the parent for a bad args[0]."""
         desired_exception = self._get_chdir_exception()

crates/vm/src/stdlib/io.rs

@@ -20,10 +20,41 @@ pub use _io::{OpenArgs, io_open as open};
 impl ToPyException for std::io::Error {
     fn to_pyexception(&self, vm: &VirtualMachine) -> PyBaseExceptionRef {
         let errno = self.posix_errno();
+        #[cfg(windows)]
+        let msg = 'msg: {
+            // On Windows, use C runtime's strerror for POSIX errno values
+            // For Windows-specific error codes, fall back to FormatMessage
+
+            // UCRT's strerror returns "Unknown error" for invalid errno values
+            // Windows UCRT defines errno values 1-42 plus some more up to ~127
+            const MAX_POSIX_ERRNO: i32 = 127;
+            if errno > 0 && errno <= MAX_POSIX_ERRNO {
+                let ptr = unsafe { libc::strerror(errno) };
+                if !ptr.is_null() {
+                    let s = unsafe { std::ffi::CStr::from_ptr(ptr) }.to_string_lossy();
+                    if !s.starts_with("Unknown error") {
+                        break 'msg s.into_owned();
+                    }
+                }
+            }
+            self.to_string()
+        };
+        #[cfg(unix)]
+        let msg = {
+            let ptr = unsafe { libc::strerror(errno) };
+            if !ptr.is_null() {
+                unsafe { std::ffi::CStr::from_ptr(ptr) }
+                    .to_string_lossy()
+                    .into_owned()
+            } else {
+                self.to_string()
+            }
+        };
+        #[cfg(not(any(windows, unix)))]
         let msg = self.to_string();
+
         #[allow(clippy::let_and_return)]
         let exc = vm.new_errno_error(errno, msg);
-
         #[cfg(windows)]
         {
             use crate::object::AsObject;