Some refactoring

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1180,7 +1180,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                         default.value.span,
                         "default values for const generic parameters are unstable",
                     );
-                    err.note("to enable them use #![feature(const_generic_defaults)]");
+                    err.help("add `#![feature(const_generic_defaults)]` to the crate attributes to enable");
                     err.emit();
                     break;
                 }