Lint *Error types that don't implement core::error::Error

[SpecificProtagonist]

fixes #1291

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See the linked issue.
Running linkcheck, this lint triggered 6 3 times

• 2 of which are genuine missing error impls
• 3 of which are triggered because the crate supports Rust 1.80 or older
• 1 of which is because of public item in a private module, which should probably be fixed regardless of this lint.

I could see this lint generating a false positives if there is e.g. a pub struct CalcError(f32); that describes the amount of error in some calculation, but I that's the exception.

changelog: [missing_error_implementations]: New lint that checks for exported types that should likely implement Error, but don't.

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[blyxyas]

Very good first patch! I have some comments. Thanks for the contribution and sorry for the delay. I didn't see that you were a first-time contributor, I like to prioritize first-timers here 🤠

[SpecificProtagonist]

Thanks for the review ❤️

Noticed this suggestion, should I add that too?
#1291 (comment)

Additionally, we could lint this for types which are used for associated Error and Err types on traits. For example, someone might have an error type called BadValue and use type Err = BadValue on a FromStr impl, and we'd still want to catch that too.

[alice-i-cecile]

Hey, just wanted to chime in as a user: thanks a ton for implementing and reviewing this!

[alice-i-cecile]

There should be a comment here about why an item named "Error" shouldn't be caught by this lint.

[SpecificProtagonist]

Originally added that because I didn't want clippy to recommend a change that triggers another lint, but thinking about it I'll remove that check.

[alice-i-cecile]

Should the docs for this lint link back to the error-impl-error lint?

[blyxyas]

Additionally, we could lint this for types which are used for associated Error and Err types on traits

I can see how this would be very useful, but there can also exist error types unrelated. An associated Error type could be an i32 with an error code and I'm not sure if we can really detect those.

[SpecificProtagonist]

I also want to add a check that skips the lint if there's a impl<T: Error> From<T> because that would result in a conflicting implementations error, but I haven't yet figured out how to do that.

[blyxyas]

For that, you can use implements_trait, the From<T> trait has a diagnostic item associated, so we can use tcx.get_diagnostic_item with sym::From (because the trait also has an associated symbol).

imeplements_trait would collect a single GenericArg, we need to check that it implements Error.