Add a test showing `#![feature(default_field_values)]` using `#[const_trait] trait Default` (`#![feature(const_default)]` + `#![feature(const_trait_impl)]`).
Add a test showing `#![feature(default_field_values)]` using `#[const_trait] trait Default` (`#![feature(const_default)]` + `#![feature(const_trait_impl)]`).