Remove the From<Dispaly> trait for FieldError

[david0u0]

Is your feature request related to a problem? Please describe.
This may be a wild, breaking change, but I found it error-prone for FieldError to implement From<Display> trait.

According to the document, FieldError now implements From<Display> trait, which can transform anything implementing Display into FieldError. This is dangerous with the ? operator.

For example, I may have something like

impl Mutation {
    fn some_mutation(context: &Ctx) -> Result<bool, FieldError> {
        do_the_mutation()?;  // Danger!!
        // Something else...
        Ok(true)
    }
}
fn do_the_mutation() -> Result<(), SuperSecretError> {
    // implementation
}

where SuperSecretError implements Display trait, and contains some critical information I don't like users to know.

However, when i use the ? operator in some_mutaion function, rust will automatically transform SuperSecretError into FieldError, and send it to users. There WON'T be any compile error or warning. Such transformation simply relies on format! macro.

As a backend programmer, I implement the Display trait for better logging, not for web users. The leakage of information is counter-intuitive. Even if there's no critical information leakage, such transformation is not what a programmer would expect.

Describe the solution you'd like
Remove the From<Dispaly> trait for FieldError

Describe alternatives you've considered
Now in my own project, I use the trait IntoFieldError, and strictly forbid anyone to use the raw FieldError.

We can document the danger of FieldError + Display, so people can still take advantage of such transformation, while it is encouraged not to do so.

[davidpdrsn]

I see what you're saying, however I think it makes sense to keep because of ergonomics. Users who want more control can use their own error type and implement IntoFieldError, like you're saying.

I think documenting the possible danger is a good idea. PRs for that very welcome 😊

Relevant discussion can be found here. There we ran into issues implementing std::error::Error for FieldError because of this, but eventually decided not to implement std::error::Error because we think the ergonomics of From<Display> are more valuable. I agree with that decision.

[LegNeato]

@davidpdrsn should this one be closed out?

[davidpdrsn]

Yep.