diff --git a/src/structs.h b/src/structs.h
index d718efe..009f16e 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -4915,6 +4915,34 @@
 } optset_T;
 
 /*
+ * Argument for the callback function (opt_expand_cb_T) invoked after a string
+ * option value is expanded for cmdline completion.
+ */
+typedef struct
+{
+    // Pointer to the option variable. It's always a string.
+    char_u	*oe_varp;
+    // The original option value, escaped.
+    char_u	*oe_opt_value;
+
+    // TRUE if using set+= instead of set=
+    int		oe_append;
+    // TRUE if we would like to add the original option value as the first
+    // choice.
+    int		oe_include_orig_val;
+
+    // Regex from the cmdline, for matching potential options against.
+    regmatch_T	*oe_regmatch;
+    // The expansion context.
+    expand_T	*oe_xp;
+
+    // The full argument passed to :set. For example, if the user inputs
+    // ':set dip=icase,algorithm:my<Tab>', oe_xp->xp_pattern will only have
+    // 'my', but oe_set_arg will contain the whole 'icase,algorithm:my'.
+    char_u	*oe_set_arg;
+} optexpand_T;
+
+/*
  * Spell checking variables passed from win_update() to win_line().
  */
 typedef struct {