diff --git a/src/vim.h b/src/vim.h
index cd54d69..c78b12b 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -822,6 +822,8 @@
 #define EXPAND_BREAKPOINT	51
 #define EXPAND_SCRIPTNAMES	52
 #define EXPAND_RUNTIME		53
+#define EXPAND_STRING_SETTING	54
+#define EXPAND_SETTING_SUBTRACT	55
 
 // Values for exmode_active (0 is no exmode)
 #define EXMODE_NORMAL		1
@@ -2330,6 +2332,20 @@
  */
 typedef char *(*opt_did_set_cb_T)(optset_T *args);
 
+/*
+ * Type for the callback function that is invoked when expanding possible
+ * string option values during cmdline completion.
+ *
+ * Strings in returned matches will be managed and freed by caller.
+ *
+ * Returns OK if the expansion succeeded (numMatches and matches have to be
+ * set). Otherwise returns FAIL.
+ *
+ * Note: If returned FAIL or *numMatches is 0, *matches will NOT be freed by
+ * caller.
+ */
+typedef int (*opt_expand_cb_T)(optexpand_T *args, int *numMatches, char_u ***matches);
+
 // Flags for assignment functions.
 #define ASSIGN_VAR	0     // ":var" (nothing special)
 #define ASSIGN_FINAL	0x01  // ":final"