t_wの輪郭

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 {
  |                ~~~~~~~~

Rustの借用
関数の引数に参照を取ること
変数の参照によって生じる？

https://doc.rust-jp.rs/book-ja/ch15-02-deref.html#関数やメソッドで暗黙的な参照外し型強制

https://doc.rust-jp.rs/book-ja/ch15-02-deref.html#derefトレイトを実装して型を参照のように扱う

実体・参照のメソッド呼び出しは同じ.でできる

    let bmi1 = body.bmi_calc();
    let bmi2 = (&body).bmi_calc();