Recommend `Option::as_deref` when given `Option<&String>` in place of `Option<&str>`

[NoraCodes]

&String can easily be converted to &str, but we give a somewhat opaque E0308 when faced with the following situation:

fn take_str_maybe(x: Option<&str>) -> Option<&str> { None }

fn main() {
    let string = String::from("Hello, world");
    let option = Some(&string);
    take_str_maybe(option);
}

This results in:

error[E0308]: mismatched types
 --> src/main.rs:8:20
  |
8 |     take_str_maybe(option);
  |                    ^^^^^^ expected `str`, found struct `String`
  |
  = note: expected enum `Option<&str>`
             found enum `Option<&String>`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error

playground

Which is not totally clear to people who do not understand the Deref trait and/or do not know that it is implemented for these types.

We should suggest that the user use Option::as_deref to convert between these types.

[terrarier2111]

@rustbot claim

[terrarier2111]

@estebank Option::as_deref only works for String and not for &String
There are a couple of options I thought about, if there is a better one i'd be happy to use it:

• tell the user to pass a String instead of a &String and use Option::deref
• try to find where the &String parameter comes from and tell the user to remove the & sign and use Option::deref
  if this is the last instance where the String was used
• just handle the cases in which a String is passed
• try to find a Copy or Clone impl on the struct and suggest to use it and use Option::deref (this one seems like the most unsafe for arbitrary types, but in this particular case it would work)

sorry for the inconvenience, I am new to rustc and I am currently trying to get started with this issue.

[estebank]

@terrarier2111 another option that would remain more local than finding where the Option<&String> was constructed is to try and suggest .map(|x| &**x) for that case in particular. Having said that, finding where the Option<&String> was constructed would be great because it's a type that is almost always wrong to use, unless it comes from a place where you end up with that because of type params (like a hashmap or another container), so it'd be good to lead people towards better code.

What you can do is indeed split your work and focus on making the Option<String> case work on its own PR, and then follow up with an attempt to handle Option<&String>.