Calling `and_then` on `Option<String>` with a function that takes `&str` could provide a more helpful diagnostic

[Kampfkarren]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=d2e7d8643e1ae76544e6f0f1a117230b

fn produces_string() -> Option<String> {
    Some("my cool string".to_owned())
}

fn takes_str(x: &str) -> Option<()> {
    Some(())
}

fn main() {
    let x = produces_string().and_then(takes_str);
}

The current output is:

error[[E0631]](https://doc.rust-lang.org/stable/error-index.html#E0631): type mismatch in function arguments
  --> src/main.rs:10:40
   |
5  | fn takes_str(x: &str) -> Option<()> {
   | ----------------------------------- found signature defined here
...
10 |     let x = produces_string().and_then(takes_str);
   |                               -------- ^^^^^^^^^ expected due to this
   |                               |
   |                               required by a bound introduced by this call
   |
   = note: expected function signature `fn(String) -> _`
              found function signature `for<'a> fn(&'a str) -> _`
note: required by a bound in `Option::<T>::and_then`

The solution is to change:

-let x = produces_string().and_then(takes_str);
+let x = produces_string().as_deref().and_then(takes_str);

It would be nice if rustc told me this.

[y21]

@rustbot claim