Ch04-03 Slices - String is coerced to &str

[GeeF]

Regarding listing-04-09: &String is actually coerced to &str as of rust-lang/rust#21351.
Should this be mentioned at this point in the book?

fn first_word(s: &str) -> &str {
..

fn main() {
    let my_string = String::from("hello world");
    // first_word works on slices of `String`s
    let word = first_word(&my_string[..]);
    let word = first_word(&my_string); // this is also valid now
    ..

[steveklabnik]

Thanks for this! You're right that we could also mention this here, but there's already a lot going on, and understanding this requires knowledge we learn a lot later. I think i'd like to keep it as-is for now. Thank you!