rustc suggests &None.as_ref() as well as None

[tgeoghegan]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=9d89810932e63366d4910a3d8cca200e

fn takes_option(_arg: Option<&String>) {}

fn main() {
    takes_option(&None)
}

The current output is:

Compiling playground v0.0.1 (/playground)
error[[E0308]](https://doc.rust-lang.org/stable/error-index.html#E0308): mismatched types
 --> src/main.rs:4:18
  |
4 |     takes_option(&None)
  |     ------------ ^^^^^ expected enum `Option`, found `&Option<_>`
  |     |
  |     arguments to this function are incorrect
  |
  = note:   expected enum `Option<&String>`
          found reference `&Option<_>`
note: function defined here
 --> src/main.rs:1:4
  |
1 | fn takes_option(_arg: Option<&String>) {}
  |    ^^^^^^^^^^^^ ---------------------
help: you can convert from `&Option<T>` to `Option<&T>` using `.as_ref()`
  |
4 |     takes_option(&None.as_ref())
  |                  ~~~~~~~~~~~~~~
help: consider removing the borrow
  |
4 -     takes_option(&None)
4 +     takes_option(None)
  |

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

The error is correct, but I think the diagnostic is wrong. The second recommendation to call takes_option(None) is sound, but the first one (&None.as_ref()) seems like bad advice.

This reproduces on rustc 1.61 (on my workstation), as well as 1.63 (stable channel in playground), 1.64 (beta channel in playground), and 1.65 (nightly channel in playground).

[chenyukang]

Intresting..

This code also make wrong help, https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=9d89810932e63366d4910a3d8cca200e

fn takes_option(_arg: Option<&String>) {}

fn main() {
    let x = String::from("x");
    let res = Some(x);
    takes_option(&res);
}

The current output is:

error[[E0308]](https://doc.rust-lang.org/stable/error-index.html#E0308): mismatched types
 --> src/main.rs:6:18
  |
6 |     takes_option(&res);
  |     ------------ ^^^^
  |     |            |
  |     |            expected enum `Option`, found `&Option<String>`
  |     |            help: you can convert from `&Option<T>` to `Option<&T>` using `.as_ref()`: `&res.as_ref()`
  |     arguments to this function are incorrect
  |
  = note:   expected enum `Option<&String>`
          found reference `&Option<String>`
note: function defined here
 --> src/main.rs:1:4
  |
1 | fn takes_option(_arg: Option<&String>) {}
  |    ^^^^^^^^^^^^ ---------------------

For more information about this error, try `rustc --explain E0308`.

Using &res.as_ref() is a bad advice.

[chenyukang]

@rustbot claim