rust-lang#126333 remove PathBuf::as_mut_vec reference at top of PathBuf::_push

Author: Borgerr

Diff for: std/src/path.rs

@@ -1290,7 +1290,8 @@ impl PathBuf {
 
     fn _push(&mut self, path: &Path) {
         // in general, a separator is needed if the rightmost byte is not a separator
-        let mut need_sep = self.as_mut_vec().last().map(|c| !is_sep_byte(*c)).unwrap_or(false);
+        let buf = self.inner.as_encoded_bytes();
+        let mut need_sep = buf.last().map(|c| !is_sep_byte(*c)).unwrap_or(false);
 
         // in the special case of `C:` on Windows, do *not* add a separator
         let comps = self.components();