t_wの輪郭

pub fn notify(item: &(impl Summary + Display)) {

pub fn notify<T: Summary + Display>(item: &T) {

pub fn notify<T>(item: &T) where T:Summary + Display {

→pub fn notify<T>(item: &T) where T:Summary + Display {

https://doc.rust-jp.rs/book-ja/ch10-02-traits.html#複数のトレイト境界を構文で指定する

error[E0782]: trait objects must include the `dyn` keyword 
  --> src\main.rs:26:42
   |
26 | fn run(config: Config) -> Result<(), Box<Error>> {    
   |                                          ^^^^^        
   |
help: add `dyn` keyword before this trait
   |
26 | fn run(config: Config) -> Result<(), Box<dyn Error>> {
   |                                          +++

pub fn notify(item: &impl Summary) {
    println!("Breaking news! {}", item.summarize());
}

https://doc.rust-jp.rs/book-ja/ch10-02-traits.html#引数としてのトレイト

特質境界構文の糖衣構文となっている

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