pythongh-125196: Use PyUnicodeWriter for repr(structseq)

Replace the private _PyUnicodeWriter with the public PyUnicodeWriter.

* Avoid temporary PyUnicode_DecodeUTF8(): call
  PyUnicodeWriter_WriteUTF8() instead.
* Avoid temporary PyObject_Repr(): call PyUnicodeWriter_WriteRepr()
  instead.

Objects/structseq.c

@@ -266,83 +266,66 @@ static PyObject *
 structseq_repr(PyStructSequence *obj)
 {
  PyTypeObject *typ = Py_TYPE(obj);
- _PyUnicodeWriter writer;
 
- /* Write "typename(" */
- PyObject *type_name = PyUnicode_DecodeUTF8(typ->tp_name,
- strlen(typ->tp_name),
- NULL);
- if (type_name == NULL) {
+ // count 5 characters per item: "x=1, "
+ Py_ssize_t type_name_len = strlen(typ->tp_name);
+ Py_ssize_t prealloc = (type_name_len + 1
+ + VISIBLE_SIZE(obj) * 5 + 1);
+ PyUnicodeWriter *writer = PyUnicodeWriter_Create(prealloc);
+ if (writer == NULL) {
  return NULL;
  }
 
- _PyUnicodeWriter_Init(&writer);
- writer.overallocate = 1;
- /* count 5 characters per item: "x=1, " */
- writer.min_length = (PyUnicode_GET_LENGTH(type_name) + 1
- + VISIBLE_SIZE(obj) * 5 + 1);
-
- if (_PyUnicodeWriter_WriteStr(&writer, type_name) < 0) {
- Py_DECREF(type_name);
+ // Write "typename("
+ if (PyUnicodeWriter_WriteUTF8(writer, typ->tp_name, type_name_len) < 0) {
  goto error;
  }
- Py_DECREF(type_name);
-
- if (_PyUnicodeWriter_WriteChar(&writer, '(') < 0) {
+ if (PyUnicodeWriter_WriteChar(writer, '(') < 0) {
  goto error;
  }
 
  for (Py_ssize_t i=0; i < VISIBLE_SIZE(obj); i++) {
  if (i > 0) {
- /* Write ", " */
- if (_PyUnicodeWriter_WriteASCIIString(&writer, ", ", 2) < 0) {
+ // Write ", "
+ if (PyUnicodeWriter_WriteChar(writer, ',') < 0) {
+ goto error;
+ }
+ if (PyUnicodeWriter_WriteChar(writer, ' ') < 0) {
  goto error;
  }
  }
 
- /* Write "name=repr" */
+ // Write name
  const char *name_utf8 = typ->tp_members[i].name;
  if (name_utf8 == NULL) {
- PyErr_Format(PyExc_SystemError, "In structseq_repr(), member %zd name is NULL"
+ PyErr_Format(PyExc_SystemError,
+ "In structseq_repr(), member %zd name is NULL"
  " for type %.500s", i, typ->tp_name);
  goto error;
  }
-
- PyObject *name = PyUnicode_DecodeUTF8(name_utf8, strlen(name_utf8), NULL);
- if (name == NULL) {
- goto error;
- }
- if (_PyUnicodeWriter_WriteStr(&writer, name) < 0) {
- Py_DECREF(name);
+ if (PyUnicodeWriter_WriteUTF8(writer, name_utf8, -1) < 0) {
  goto error;
  }
- Py_DECREF(name);
 
- if (_PyUnicodeWriter_WriteChar(&writer, '=') < 0) {
+ // Write "=" + repr(value)
+ if (PyUnicodeWriter_WriteChar(writer, '=') < 0) {
  goto error;
  }
-
  PyObject *value = PyStructSequence_GetItem((PyObject*)obj, i);
  assert(value != NULL);
- PyObject *repr = PyObject_Repr(value);
- if (repr == NULL) {
- goto error;
- }
- if (_PyUnicodeWriter_WriteStr(&writer, repr) < 0) {
- Py_DECREF(repr);
+ if (PyUnicodeWriter_WriteRepr(writer, value) < 0) {
  goto error;
  }
- Py_DECREF(repr);
  }
 
- if (_PyUnicodeWriter_WriteChar(&writer, ')') < 0) {
+ if (PyUnicodeWriter_WriteChar(writer, ')') < 0) {
  goto error;
  }
 
- return _PyUnicodeWriter_Finish(&writer);
+ return PyUnicodeWriter_Finish(writer);
 
 error:
- _PyUnicodeWriter_Dealloc(&writer);
+ PyUnicodeWriter_Discard(writer);
  return NULL;
 }