Document that DICT_MAX_INDEX_COL_LEN must not be changed.

dict/dict0dict.c

@@ -1531,6 +1531,12 @@ dict_index_add_col(
 	if (field->fixed_len > DICT_MAX_INDEX_COL_LEN) {
 		field->fixed_len = 0;
 	}
+#if DICT_MAX_INDEX_COL_LEN != 768
+	/* The comparison limit above must be constant.  If it were
+	changed, the disk format of some fixed-length columns would
+	change, which would be a disaster. */
+# error "DICT_MAX_INDEX_COL_LEN != 768"
+#endif
 
 	if (!(col->prtype & DATA_NOT_NULL)) {
 		index->n_nullable++;

include/dict0mem.h

@@ -161,7 +161,9 @@ struct dict_col_struct{
 /* DICT_MAX_INDEX_COL_LEN is measured in bytes and is the max index column
 length + 1. Starting from 4.1.6, we set it to < 3 * 256, so that one can
 create a column prefix index on 255 characters of a TEXT field also in the
-UTF-8 charset. In that charset, a character may take at most 3 bytes. */
+UTF-8 charset. In that charset, a character may take at most 3 bytes.
+This constant MUST NOT BE CHANGED, or the compatibility of InnoDB data files
+would be at risk! */
 
 #define DICT_MAX_INDEX_COL_LEN		768