windows platforms accounted for, should close rust-lang#126333

library/std/src/sys/os_str/wtf8.rs

@@ -165,10 +165,20 @@ impl Buf {
         self.as_slice().into_rc()
     }
 
-    /// Part of a hack to make PathBuf::push/pop more efficient.
+    /// More well behaving alternative to allowing outer types
+    /// full mutable access to the core `Vec`.
+    /// Provides plumbing to core `Vec::truncate`.
     #[inline]
-    pub(crate) fn as_mut_vec_for_path_buf(&mut self) -> &mut Vec<u8> {
-        self.inner.as_mut_vec_for_path_buf()
+    pub(crate) fn truncate(&mut self, len: usize) {
+        self.inner.truncate(len);
+    }
+
+    /// More well behaving alternative to allowing outer types
+    /// full mutable access to the core `Vec`.
+    /// Provides plumbing to core `Vec::extend_from_slice`.
+    #[inline]
+    pub(crate) fn extend_from_slice(&mut self, other: &[u8]) {
+        self.inner.extend_from_slice(other);
     }
 }
 

library/std/src/sys_common/wtf8.rs

@@ -474,13 +474,12 @@ impl Wtf8Buf {
         Wtf8Buf { bytes: bytes.into_vec(), is_known_utf8: false }
     }
 
-    /// Part of a hack to make PathBuf::push/pop more efficient.
+    /// More well behaving alternative to allowing outer types
+    /// full mutable access to the core `Vec`.
+    /// Provides plumbing to core `Vec::extend_from_slice`.
     #[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
+    pub(crate) fn extend_from_slice(&mut self, other: &[u8]) {
+        self.bytes.extend_from_slice(other);
     }
 }