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

Copy特質を持たない変数を関数に渡すと、所有権の移動が発生し、関数に渡した変数は使用不可能になる。

fn main() {
    let s = String::from("hello");
    test(s);
    println!("{}", s);   //error
    /*
    error[E0382]: borrow of moved value: `s`
    --> src\main.rs:4:20
    |
    2 |     let s = String::from("hello");
    |         - move occurs because `s` has type `String`, which does not implement the `Copy` trait
    3 |     test(s);
    |          - value moved here
    4 |     println!("{}", s);   //error
    |                    ^ value borrowed here after move
    */
}

fn test(s:String) {
    println!("{}", s);   //hello
}

fn main() {
    let reference_to_nothing = dangle();
}

fn dangle() -> &String {
    let s = String::from("hello");

    &s
}

error[E0106]: missing lifetime specifier
 --> src\main.rs:5:16
  |
5 | fn dangle() -> &String {
  |                ^ expected named lifetime parameter
  |
  = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime
  |
5 | fn dangle() -> &'static String {
  |                ~~~~~~~~

関数に変数を渡すことによって所有権の移動が発生するが、戻り値によっても所有権の移動が発生する。

fn main() {
    let s1 = String::from("hello");
    let s2 = test(s1);
    println!("{}", s2);   //hello
}

fn test(s:String) -> String {
    s
}