fn main() {
    let hello = Some("hello");
    let _switcheroo = hello.map(str::to_string); //ok

    let hello = Some("hello".to_string());
    let _switcheroo = hello.map(String::as_str); //not ok
}

let hello = Some("hello".to_string());
let _switcheroo = hello.as_ref().map(String::as_str); //ok