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

}

error[E0631]: type mismatch in function arguments
 --> src/main.rs:6:29
  |
6 |     let _switcheroo = hello.map(String::as_str);
  |                             ^^^
  |                             |
  |                             expected signature of `fn(std::string::String) -> _`
  |                             found signature of `for<'r> fn(&'r std::string::String) -> _`

error: aborting due to previous error

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