4 files changed, 27 insertions, 6 deletions
diff --git a/library/std/src/env.rs b/library/std/src/env.rs
index 4d649f8a6f1..295e782639b 100644
--- a/library/std/src/env.rs
+++ b/library/std/src/env.rs
@@ -323,8 +323,10 @@ impl Error for VarError {
 /// This function is also always safe to call on Windows, in single-threaded
 /// and multi-threaded programs.
 ///
-/// In multi-threaded programs on other operating systems, we strongly suggest
-/// not using `set_var` or `remove_var` at all. The exact requirement is: you
+/// In multi-threaded programs on other operating systems, the only safe option is
+/// to not use `set_var` or `remove_var` at all.
+///
+/// The exact requirement is: you
 /// must ensure that there are no other threads concurrently writing or
 /// *reading*(!) the environment through functions or global variables other
 /// than the ones in this module. The problem is that these operating systems
@@ -390,8 +392,10 @@ unsafe fn _set_var(key: &OsStr, value: &OsStr) {
 /// This function is also always safe to call on Windows, in single-threaded
 /// and multi-threaded programs.
 ///
-/// In multi-threaded programs on other operating systems, we strongly suggest
-/// not using `set_var` or `remove_var` at all. The exact requirement is: you
+/// In multi-threaded programs on other operating systems, the only safe option is
+/// to not use `set_var` or `remove_var` at all.
+///
+/// The exact requirement is: you
 /// must ensure that there are no other threads concurrently writing or
 /// *reading*(!) the environment through functions or global variables other
 /// than the ones in this module. The problem is that these operating systems
diff --git a/library/std/src/sys/pal/windows/rand.rs b/library/std/src/sys/pal/windows/rand.rs
index e427546222a..09f527a09bf 100644
--- a/library/std/src/sys/pal/windows/rand.rs
+++ b/library/std/src/sys/pal/windows/rand.rs
@@ -1,6 +1,6 @@
+use core::{mem, ptr};
+
 use crate::sys::c;
-use core::mem;
-use core::ptr;
 
 #[cfg(not(target_vendor = "win7"))]
 #[inline]
diff --git a/library/std/src/sys_common/wtf8.rs b/library/std/src/sys_common/wtf8.rs
index bb1e505285b..84128a4b595 100644
--- a/library/std/src/sys_common/wtf8.rs
+++ b/library/std/src/sys_common/wtf8.rs
@@ -477,6 +477,9 @@ impl Wtf8Buf {
     /// 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.is_known_utf8 = false;
         &mut self.bytes
     }
 }
diff --git a/library/std/tests/windows.rs b/library/std/tests/windows.rs
new file mode 100644
index 00000000000..9f7596f1bc2
--- /dev/null
+++ b/library/std/tests/windows.rs
@@ -0,0 +1,14 @@
+#![cfg(windows)]
+//! An external tests
+
+use std::{ffi::OsString, os::windows::ffi::OsStringExt, path::PathBuf};
+
+#[test]
+#[should_panic]
+fn os_string_must_know_it_isnt_utf8_issue_126291() {
+    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);
+    utf8.into_os_string().into_string().unwrap();
+}