Compile error for using bincode::Error on 1.7.1

[Leo1003]

After upgrading to 1.7.1, the compiler complained that error[E0277]: the trait bound 'std::boxed::Box<bincode::ErrorKind>: std::borrow::Borrow<dyn std::error::Error>' is not satisfied

I'm sure that it didn't occurred on 1.6.0 and 1.7.0. But this version breaks my codes... Note: The bincode::ErrorKind enum DO implement std::error::Error.

---

Details:

error[E0277]: the trait bound `std::boxed::Box<bincode::ErrorKind>: std::borrow::Borrow<dyn std::error::Error>` is not satisfied

custom_error! {pub SimpleError
    Binary { source: bincode::Error } = "Binary Serialize/Deserialize Error",
}
= help: the following implementations were found:
             <std::boxed::Box<T> as std::borrow::Borrow<T>>
   = note: required by `std::borrow::Borrow::borrow`
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

[lovasoa]

This must have been introduced by https://github.com/lovasoa/custom_error/commit/d55faa97ae200255002a610f0da246dc110f50ae ...

[lovasoa]

Can you please provide a failing example ?

[lovasoa]

OK, here is the failing example:

use bincode;
use custom_error::custom_error;

custom_error! {pub SimpleError
    Binary { source: bincode::Error } = "Binary Serialize/Deserialize Error",
}

I would say the problem comes from bincode, who do not implement std::error::Error on their Error type.

[lovasoa]

After some investigation, this indeed comes from d55faa9. This would be fixable with specialization for handling Box<dyn Error> in a particular way, but I don't see how it could be done in today's rust.

[Leo1003]

• std::boxed::Box do implement std::borrow::Borrow
• bincode::ErrorKind do implement std::error::Error

This would fail to compile:

let a: &dyn std::error::Error = std::borrow::Borrow::borrow(&e);

39 | let a: &dyn std::error::Error = std::borrow::Borrow::borrow(&e);
   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::borrow::Borrow<dyn std::error::Error>` is not implemented for `std::boxed::Box<bincode::ErrorKind>`

But this is ok:

let a: &bincode::ErrorKind = std::borrow::Borrow::borrow(&e);
let b: &dyn std::error::Error = a;

:frowning:

[lovasoa]

yes. But the thing is: the macro cannot generate the second code, because it does not have access to the underlying type (bincode::ErrorKind).

[lovasoa]

If this is in fact possible and you have an idea of how to implement the conversion that works for both Box<dyn Error> and Box<ConcreteErrorType>, let me know!

[Leo1003]

This is the most possible solution I could find... I don't know if this can be generated by macro.

let a: &dyn std::error::Error = std::borrow::Borrow::borrow(&*e);

I am sorry that I may not provide more help because I am not familiar with macro...

[lovasoa]

The problem is that &*e doesn't work for non-boxed errors.

[glehmann]

Is there a known workaround for this problem? Should we file a bug report at bincode?

[lovasoa]

The workaround is to manually implement From<bincode::Error> for your error, and to name the field something other than source...

You can open a bug report at bincode, as it is indeed unconventional not to implement std::error::Error on your main error type, but since it essentially just works in every other case, I'm not sure they'll be willing to do something...

[glehmann]

Thanks! I'll file a bug report tomorrow.

[glehmann]

I guess https://github.com/servo/bincode/issues/227 is related — would it be good enough for custom_error?

[lovasoa]

Yes, it's exactly what we would need!

[ZoeyR]

Hi, maintainer of bincode here. Error should be implemented for all Box<T> where T: Error by a blanket impl in std. I'm not sure what bincode could do to fix this.

[lovasoa]

An error type has to implement std::borrow::Borrow<dyn std::error::Error> for it to work with custom_error. This allows the library to implement source for both types that implement Error directly and Box<dyn Error>. Unfortunately, this does not work for other boxed error types.

[kkharji]

I'm having similar issue:

error: the trait bound tree_sitter::LanguageError: StdError is not satisfied required for the cast to the object type dyn StdError

LanguageError definition:

/// An error that occurred when trying to assign an incompatible `Language` to a `Parser`.
#[derive(Debug, PartialEq, Eq)]
pub struct LanguageError {
    version: usize,
}

My definition:

custom_error! {
    pub Error
    ParseError = "Unable to create tree_sitter tree.",
    Io{source: io::Error}         = "unable to read from the file",
    LangError{source: LanguageError} = "Unable to load language tree_sitter v{source}"

}

Thanks

cc @Leo1003

[lovasoa]

tree_sitter::LanguageError does not implement the standard Error trait. You should open an issue on their side.