Add note to use nightly when using expr in const generics

compiler/rustc_resolve/src/diagnostics.rs

@@ -480,6 +480,7 @@ impl<'a> Resolver<'a> {
                         "const parameters may only be used as standalone arguments, i.e. `{}`",
                         name
                     ));
+                    err.note("use feature(const_generics) and feature(const_evaluatable_checked) to enable this");
                 }
 
                 err