Improved messaging for ? within a function with () return type - E0277

[amanjeev]

This is mostly a suggestion if there can be some improvement to the diagnostics.

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=0aa16f03b3a56a8133bf2023fd8a0c4e

use std::num::ParseIntError;

fn retoptionres(x: &str) -> Result<Option<u32>, ParseIntError> {
    let n = x.parse::<u32>()?;
    Ok(Some(n))
}

fn main() {
    let x = retoptionres("4")??;
    println!("{:?}", x);
}

The current output is:

Compiling playground v0.0.1 (/playground)
error[[E0277]](https://doc.rust-lang.org/stable/error-index.html#E0277): the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
  --> src/main.rs:9:30
   |
8  | / fn main() {
9  | |     let x = retoptionres("4")??;
   | |                              ^ cannot use the `?` operator in a function that returns `()`
10 | |     println!("{:?}", x);
11 | | }
   | |_- this function should return `Result` or `Option` to accept `?`
   |
   = help: the trait `FromResidual<Result<Infallible, ParseIntError>>` is not implemented for `()`

Ideally the output should look like:

Compiling playground v0.0.1 (/playground)
error[[E0277]](https://doc.rust-lang.org/stable/error-index.html#E0277): the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
  --> src/main.rs:9:30
   |
8  | / fn main() {
9  | |     let x = retoptionres("4")??;
   | |                              ^ cannot use the `?` operator in a function that returns `()`
10 | |     println!("{:?}", x);
11 | | }
   | |_- this function (crate::main) should return `Result` or `Option` to accept `?`
   |    - Hint: add an `Option` or `Result` return type to `main`
   = help: the trait `FromResidual<Result<Infallible, ParseIntError>>` is not implemented for `()`

This error is easier to miss because busy-ness and not knowing which function needs the return type.

[compiler-errors]

I'm not sure if that hint: you suggested adds more value than the labeled message that's directly above it.

Regarding "not knowing which function needs the return type", it's already being pointed out in the error message? Do you have a better example for where this is particularly confusing?

[amanjeev]

Regarding "not knowing which function needs the return type", it's already being pointed out in the error message? Do you have a better example for where this is particularly confusing?

I think the pointed out part is using those ascii characters (| and /). I feel that having at least the function name that the error is talking about would be beneficial.

If you think the hint I suggested does not add more value, thats fine.

[compiler-errors]

This error now looks like:

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
 --> src/main.rs:9:30
  |
8 | fn main() {
  | --------- this function should return `Result` or `Option` to accept `?`
9 |     let x = retoptionres("4")??;
  |                              ^ cannot use the `?` operator in a function that returns `()`
  |
  = help: the trait `FromResidual<Result<Infallible, ParseIntError>>` is not implemented for `()`

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
 --> src/main.rs:9:31
  |
8 | fn main() {
  | --------- this function should return `Result` or `Option` to accept `?`
9 |     let x = retoptionres("4")??;
  |                               ^ cannot use the `?` operator in a function that returns `()`
  |
  = help: the trait `FromResidual<Option<Infallible>>` is not implemented for `()`

Is that better?

[amanjeev]

Thank you! I think this is better. Are you ok with this change?

[compiler-errors]

I'm gonna close this issue as completed then, please re-open an issue if you continue to find this error message confusing!