+Added the C<<PL_curstackinfo->si_cxsubix>> field. This records the stack
+index of the most recently pushed sub/format/eval context. It is set and
+restored automatically by C<cx_pushsub()>, C<cx_popsub()> etc., but would
+need to be manually managed if you do any unusual manipulation of the
+context stack.
+
+=item *
+
+Various macros dealing with character type classification and changing
+case where the input is encoded in UTF-8 now require an extra parameter
+to prevent potential reads beyond the end of the buffer. Use of these
+has generated a deprecation warning since 5.26. Details are in
+L<perldeprecation/In XS code, use of various macros dealing with UTF-8.>