diff --git a/src/ast/ast.rs b/src/ast/ast.rs
index 65e3c6bd51117f6440af5ac612b0502d0b1b43b1..c0a361aea716645b65919224d51958c778197193 100644
--- a/src/ast/ast.rs
+++ b/src/ast/ast.rs
@@ -26,6 +26,10 @@ fn check(t: Type) {
     }
 }
 
+// This requires unstable (nightly) feature
+// and won't compile using the stable toolchain
+//
+// Please use `check` instead for matching boxed values
 fn check2(t: Type) {
     use Type::*;
     match t {