--- a/src/lib/strfuncs.c	Sun Feb 21 18:32:03 2016 +0200
+++ b/src/lib/strfuncs.c	Sun Feb 21 18:32:50 2016 +0200
@@ -5,6 +5,7 @@
 #include "lib.h"
 #include "printf-format-fix.h"
 #include "strfuncs.h"
+#include "array.h"
 
 #include <stdio.h>
 #include <limits.h>
@@ -726,3 +727,11 @@
 	i_assert(pos >= 0);
 	return buffer + pos;
 }
+
+char *p_array_const_string_join(pool_t pool, const ARRAY_TYPE(const_string) *arr,
+				const char *separator)
+{
+	if (array_count(arr) == 0)
+		return "";
+	return p_strarray_join_n(pool, array_idx(arr, 0), array_count(arr), separator);
+}

--- a/src/lib/strfuncs.h	Sun Feb 21 18:32:03 2016 +0200
+++ b/src/lib/strfuncs.h	Sun Feb 21 18:32:50 2016 +0200
@@ -100,6 +100,12 @@
 const char **p_strarray_dup(pool_t pool, const char *const *arr)
 	ATTR_MALLOC ATTR_RETURNS_NONNULL;
 
+/* Join ARRAY_TYPE(const_string) to a string, similar to t_strarray_join() */
+char *p_array_const_string_join(pool_t pool, const ARRAY_TYPE(const_string) *arr,
+				const char *separator);
+#define t_array_const_string_join(arr, separator) \
+	((const char *)p_array_const_string_join(unsafe_data_stack_pool, arr, separator))
+
 /* FIXME: v2.3 - sort and search APIs belong into their own header, not here */
 #include "sort.h"
 

--- a/src/lib/test-strfuncs.c	Sun Feb 21 18:32:03 2016 +0200
+++ b/src/lib/test-strfuncs.c	Sun Feb 21 18:32:50 2016 +0200
@@ -1,7 +1,7 @@
 /* Copyright (c) 2009-2016 Dovecot authors, see the included COPYING file */
 
 #include "test-lib.h"
-
+#include "array.h"
 
 static void test_p_strarray_dup(void)
 {
@@ -190,6 +190,31 @@
 	test_end();
 }
 
+static void test_p_array_const_string_join(void)
+{
+	ARRAY_TYPE(const_string) arr;
+	unsigned int i;
+	char *res;
+
+	test_begin("p_array_const_string_join()");
+
+	i_array_init(&arr, 2);
+	/* empty array -> empty string */
+	test_assert(strcmp(t_array_const_string_join(&arr, " "), "") == 0);
+
+	array_append(&arr, test_strarray_input,
+		     str_array_length(test_strarray_input));
+	for (i = 0; i < N_ELEMENTS(test_strarray_outputs); i++) {
+		res = p_array_const_string_join(default_pool, &arr,
+						test_strarray_outputs[i].separator);
+		test_assert_idx(strcmp(res, test_strarray_outputs[i].output) == 0, i);
+		i_free(res);
+	}
+
+	array_free(&arr);
+	test_end();
+}
+
 void test_strfuncs(void)
 {
 	test_p_strarray_dup();
@@ -200,4 +225,5 @@
 	test_t_str_ltrim();
 	test_t_str_rtrim();
 	test_t_strarray_join();
+	test_p_array_const_string_join();
 }