Closed
Description
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.