poor error message with question mark applied to non-Result

[durka]

This code

#![feature(question_mark)]

fn main() {
    42?;
}

prints the error

<anon>:4:5: 4:9 error: mismatched types:
 expected `_`,
    found `core::result::Result<_, _>`
(expected integral variable,
    found enum `core::result::Result`) [E0308]
<anon>:4     42?;
             ^~~~

which is confusing. The error is that I put a non-Result where a Result was expected, but the error says the opposite.

cc @japaric

[durka]

This is because it expands to

fn main() {
    match 42 {
        Err(err) => return Err(From::from(err)),
        Ok(val) => val,
    };
}

so the error comes from the match arms. I know the expansion is done before typechecking, but a possible way to fix it is with type ascription. This expansion produces a better error:

#![feature(type_ascription)]

fn main() {
    match 42: Result<_,_> {
        Err(err) => return Err(From::from(err)),
        Ok(val) => val,
    };
}

[japaric]

I think we can swap the expected and found types for these errors when they originate from desugared syntax and seems like we do something like that for if let. I'll give it a try.

[durka]

@japaric

Heh, but if you switch them what of this code?

#![feature(question_mark, type_ascription)]

fn main() {
    (42: Result<_,_>)?;
}

[japaric]

I think that error message would remain unchanged between there is no type mismatch between the "head" (42: Result<_, _>) and the arm patterns.

[pnkfelix]

cc me

[cbreeden]

I am now seeing a new error message. @durka, how do you feel about this?

error[E0277]: the trait bound `{integer}: std::ops::Carrier` is not satisfied
 --> src/main.rs:4:5
  |
4 |     42?;
  |     ^^^ trait `{integer}: std::ops::Carrier` not satisfied
  |
  = help: the following implementations were found:
  = help:   <std::ops::_DummyErrorType as std::ops::Carrier>
  = help:   <std::result::Result<U, V> as std::ops::Carrier>
  = note: required by `std::ops::Carrier::translate`

error[E0277]: the trait bound `(): std::ops::Carrier` is not satisfied
 --> src/main.rs:4:5
  |
4 |     42?;
  |     ^^^ trait `(): std::ops::Carrier` not satisfied
  |
  = note: required by `std::ops::Carrier::from_error`

[durka]

That's a pretty terrible error message from a user perspective. cc @nrc (about Carrier)

[carols10cents]

Just to clarify: the examples in this issue are also showing the error messages when you try to use ? in a function that does not return a Result, which is #35946/#36988, but this issue is for trying to call ? on a non-Result, correct? To distinguish this issue from the other issues, I think a better example would be:

fn something() -> Result<(), ()> {
    42?;
    Ok(())
}

which currently results in this single error:

error[E0277]: the trait bound `{integer}: std::ops::Carrier` is not satisfied
 --> src/main.rs:5:5
  |
5 |     42?;
  |     ^^^ trait `{integer}: std::ops::Carrier` not satisfied
  |
  = help: the following implementations were found:
  = help:   <std::ops::_DummyErrorType as std::ops::Carrier>
  = help:   <std::result::Result<U, V> as std::ops::Carrier>
  = note: required by `std::ops::Carrier::translate`

[cengiz-io]

Hello
Any updates? I've just bumped into this.

cc @steveklabnik

[steveklabnik]

hey @cengizio 👋

Not yet, no. There's some linked issues/PRs in the comments above, but it has not yet been fixed.

[nodakai]

If #37671 had to be closed as a duplicate of #35946, so does this

[carols10cents]

@nodakai no, this issue is for using the ? on something that doesn't support the question mark operator, and this can happen in a function that does have a return type that supports a question mark. Please see my example in this comment, this is a distinct issue from both #37671 and #35946.

[estebank]

Current output:

error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
 --> src/main.rs:2:5
  |
2 |     42?;
  |     ^^^ the `?` operator cannot be applied to type `{integer}`
  |
  = help: the trait `std::ops::Try` is not implemented for `{integer}`
  = note: required by `std::ops::Try::into_result`