From 9ae1f8434fcd864a518a8cd939d53ada6ecf4a65 Mon Sep 17 00:00:00 2001
From: Jubilee Young <workingjubilee@gmail.com>
Date: Tue, 11 Jun 2024 22:55:55 -0700
Subject: [PATCH] Make PathBuf less Ok with adding UTF-16 then `into_string`

---
 library/std/src/sys/os_str/wtf8.rs |  3 +++
 library/std/tests/windows.rs       | 13 +++++++++++++
 2 files changed, 16 insertions(+)
 create mode 100644 library/std/tests/windows.rs

diff --git a/library/std/src/sys/os_str/wtf8.rs b/library/std/src/sys/os_str/wtf8.rs
index dfff4dd4fb023..e8590fb1fe1e0 100644
--- a/library/std/src/sys/os_str/wtf8.rs
+++ b/library/std/src/sys/os_str/wtf8.rs
@@ -167,6 +167,9 @@ impl Buf {
     /// Part of a hack to make PathBuf::push/pop more efficient.
     #[inline]
     pub(crate) fn as_mut_vec_for_path_buf(&mut self) -> &mut Vec<u8> {
+        // FIXME: this function should not even exist, as it implies violating Wtf8Buf invariants
+        // For now, simply assume that is about to happen.
+        self.inner.is_known_utf8 = false;
         self.inner.as_mut_vec_for_path_buf()
     }
 }
diff --git a/library/std/tests/windows.rs b/library/std/tests/windows.rs
new file mode 100644
index 0000000000000..0e70479434f0f
--- /dev/null
+++ b/library/std/tests/windows.rs
@@ -0,0 +1,13 @@
+#![cfg(windows)]
+//! An external tests
+
+use std::{ffi::OsString, os::windows::ffi::OsStringExt, path::PathBuf};
+
+#[test]
+fn os_string_must_know_it_isnt_utf8_issue_126291() -> Result<String, OsString> {
+    let mut utf8 = PathBuf::from(OsString::from("utf8".to_owned()));
+    let non_utf8: OsString =
+        OsStringExt::from_wide(&[0x6e, 0x6f, 0x6e, 0xd800, 0x75, 0x74, 0x66, 0x38]);
+    utf8.set_extension(&non_utf8);
+    let result = utf8.into_os_string().into_string();
+}