gh-146143: Fix the PyUnicodeWriter_WriteUCS4() signature

Doc/c-api/unicode.rst

@@ -1867,7 +1867,7 @@ object.
    On success, return ``0``.
    On error, set an exception, leave the writer unchanged, and return ``-1``.
 
-.. c:function:: int PyUnicodeWriter_WriteUCS4(PyUnicodeWriter *writer, Py_UCS4 *str, Py_ssize_t size)
+.. c:function:: int PyUnicodeWriter_WriteUCS4(PyUnicodeWriter *writer, const Py_UCS4 *str, Py_ssize_t size)
 
    Writer the UCS4 string *str* into *writer*.
 

Include/cpython/unicodeobject.h

@@ -496,7 +496,7 @@ PyAPI_FUNC(int) PyUnicodeWriter_WriteWideChar(
     Py_ssize_t size);
 PyAPI_FUNC(int) PyUnicodeWriter_WriteUCS4(
     PyUnicodeWriter *writer,
-    Py_UCS4 *str,
+    const Py_UCS4 *str,
     Py_ssize_t size);
 
 PyAPI_FUNC(int) PyUnicodeWriter_WriteStr(

Misc/NEWS.d/next/C_API/2026-03-18-23-44-29.gh-issue-146143.pwIrJq.rst

@@ -0,0 +1,2 @@
+:c:func:`PyUnicodeWriter_WriteUCS4` now accepts pointer to constant buffer
+of ``Py_UCS4``.

Objects/unicodeobject.c

@@ -2224,7 +2224,7 @@ _PyUnicode_FromUCS4(const Py_UCS4 *u, Py_ssize_t size)
 
 int
 PyUnicodeWriter_WriteUCS4(PyUnicodeWriter *pub_writer,
-                          Py_UCS4 *str,
+                          const Py_UCS4 *str,
                           Py_ssize_t size)
 {
     _PyUnicodeWriter *writer = (_PyUnicodeWriter*)pub_writer;