t_wの輪郭

fn returns_summarizable() -> impl Summary {
    Tweet {
        username: String::from("horse_ebooks"),
        content: String::from(
            "of course, as you probably already know, people",
        ),
        reply: false,
        retweet: false,
    }
}

https://doc.rust-jp.rs/book-ja/ch10-02-traits.html#トレイトを実装している型を返す