diff --git a/src/libcollectionstest/lib.rs b/src/libcollectionstest/lib.rs
index 32a07e3e7e62..878581a4f296 100644
--- a/src/libcollectionstest/lib.rs
+++ b/src/libcollectionstest/lib.rs
@@ -21,6 +21,7 @@
 #![feature(rand)]
 #![feature(step_by)]
 #![feature(str_escape)]
+#![feature(str_replacen)]
 #![feature(test)]
 #![feature(unboxed_closures)]
 #![feature(unicode)]
diff --git a/src/libcollectionstest/str.rs b/src/libcollectionstest/str.rs
index a61925cd3be5..62e164a569aa 100644
--- a/src/libcollectionstest/str.rs
+++ b/src/libcollectionstest/str.rs
@@ -218,6 +218,20 @@ fn test_is_empty() {
     assert!(!"a".is_empty());
 }
 
+#[test]
+fn test_replacen() {
+    assert_eq!("".replacen('a', "b", 5), "");
+    assert_eq!("acaaa".replacen("a", "b", 3), "bcbba");
+    assert_eq!("aaaa".replacen("a", "b", 0), "aaaa");
+
+    let test = "test";
+    assert_eq!(" test test ".replacen(test, "toast", 3), " toast toast ");
+    assert_eq!(" test test ".replacen(test, "toast", 0), " test test ");
+    assert_eq!(" test test ".replacen(test, "", 5), "   ");
+
+    assert_eq!("qwer123zxc789".replacen(char::is_numeric, "", 3), "qwerzxc789");
+}
+
 #[test]
 fn test_replace() {
     let a = "a";