diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index 734b3ba7c6bba..e5b8412e117e8 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -2658,7 +2658,8 @@ impl str {
     ///
     /// It's important to remember that [`char`] represents a Unicode Scalar
     /// Value, and may not match your idea of what a 'character' is. Iteration
-    /// over grapheme clusters may be what you actually want.
+    /// over grapheme clusters may be what you actually want. This functionality
+    /// is not provided by Rust's standard library, check crates.io instead.
     ///
     /// # Examples
     ///