cow_borrow.rs

use std::{
  borrow::{Borrow, Cow},
  fmt::Debug,
};

fn say<'a, C, B>(first_name: C, last_name: &B)
where
  C: Into<Cow<'a, str>> + Debug + ?Sized,
  B: Borrow<str> + Debug + ?Sized,
{
  print!("{:?} {:?}", first_name, last_name.borrow());
}

fn main() {
  say("John", "Smith");
  let first_name = "John";
  let last_name = "Smith";
  // let last_name = "Smith".to_owned(); // expected reference, found struct `std::string::String` help: consider borrowing here: `&last_name`
  say(first_name, last_name);
}