pythongh-125196: Use PyUnicodeWriter in symtable.c

Python/symtable.c

@@ -3128,25 +3128,23 @@ _Py_Mangle(PyObject *privateobj, PyObject *ident)
  return NULL;
  }
 
- Py_UCS4 maxchar = PyUnicode_MAX_CHAR_VALUE(ident);
- if (PyUnicode_MAX_CHAR_VALUE(privateobj) > maxchar) {
- maxchar = PyUnicode_MAX_CHAR_VALUE(privateobj);
- }
-
- PyObject *result = PyUnicode_New(1 + nlen + plen, maxchar);
- if (!result) {
+ PyUnicodeWriter *writer = PyUnicodeWriter_Create(1 + nlen + plen);
+ if (!writer) {
  return NULL;
  }
  /* ident = "_" + priv[ipriv:] + ident # i.e. 1+plen+nlen bytes */
- PyUnicode_WRITE(PyUnicode_KIND(result), PyUnicode_DATA(result), 0, '_');
- if (PyUnicode_CopyCharacters(result, 1, privateobj, ipriv, plen) < 0) {
- Py_DECREF(result);
- return NULL;
+ if (PyUnicodeWriter_WriteChar(writer, '_') < 0) {
+ goto error;
  }
- if (PyUnicode_CopyCharacters(result, plen+1, ident, 0, nlen) < 0) {
- Py_DECREF(result);
- return NULL;
+ if (PyUnicodeWriter_WriteSubstring(writer, privateobj, ipriv, plen) < 0) {
+ goto error;
  }
- assert(_PyUnicode_CheckConsistency(result, 1));
- return result;
+ if (PyUnicodeWriter_WriteStr(writer, ident) < 0) {
+ goto error;
+ }
+ return PyUnicodeWriter_Finish(writer);
+
+error:
+ PyUnicodeWriter_Discard(writer);
+ return NULL;
 }