如何使错误链错误与失败错误兼容？

Question

Given the following code:

extern crate dotenv; // v0.11.0
#[macro_use]
extern crate failure; // v0.1.1

#[derive(Debug, Fail)]
#[fail(display = "oh no")]
pub struct Error(dotenv::Error);

Due to the fact that error-chain (v0.11, the latest) does not guarantee that its wrapped error is Sync ( open PR to fix the issue ) I get an error saying that Sync is not implemented for dotenv::Error :

error[E0277]: the trait bound `std::error::Error + std::marker::Send + 'static: std::marker::Sync` is not satisfied
 --> src/main.rs:5:17
  |
5 | #[derive(Debug, Fail)]
  |                 ^^^^ `std::error::Error + std::marker::Send + 'static` cannot be shared between threads safely
  |
  = help: the trait `std::marker::Sync` is not implemented for `std::error::Error + std::marker::Send + 'static`
  = note: required because of the requirements on the impl of `std::marker::Sync` for `std::ptr::Unique<std::error::Error + std::marker::Send + 'static>`
  = note: required because it appears within the type `std::boxed::Box<std::error::Error + std::marker::Send + 'static>`
  = note: required because it appears within the type `std::option::Option<std::boxed::Box<std::error::Error + std::marker::Send + 'static>>`
  = note: required because it appears within the type `error_chain::State`
  = note: required because it appears within the type `dotenv::Error`
  = note: required because it appears within the type `Error`

What's the minimal amount of boilerplate/work I can get away with to make these types play nicely together? The shortest thing I've been able to come up with is adding an ErrorWrapper newtype so that I can implement Error on an Arc<Mutex<ERROR_CHAIN_TYPE>> :

use std::fmt::{self, Debug, Display, Formatter};
use std::sync::{Arc, Mutex};

#[derive(Debug, Fail)]
#[fail(display = "oh no")]
pub struct Error(ErrorWrapper<dotenv::Error>);

#[derive(Debug)]
struct ErrorWrapper<T>(Arc<Mutex<T>>)
where
    T: Debug;

impl<T> Display for ErrorWrapper<T>
where
    T: Debug,
{
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        write!(f, "oh no!")
    }
}

impl<T> ::std::error::Error for ErrorWrapper<T>
where
    T: Debug,
{
    fn description(&self) -> &str {
        "whoops"
    }
}

// ... plus a bit more for each `From<T>` that needs to be converted into
// `ErrorWrapper`

Is this the right way to go about it? Is there something that will have less code/runtime cost for converting into an Arc<Mutex<T>> for things that don't need it?

Answer 1

The only thing that I can suggest is to remove the Arc type.

If you have something that is not Sync then wrap into a Mutex type, so to integrate error-chain with failure just wrap dotenv::Error :

#[macro_use]
extern crate failure;
extern crate dotenv;
use std::sync::Mutex;

#[derive(Debug, Fail)]
#[fail(display = "oh no")]
pub struct Error(Mutex<dotenv::Error>);

fn main() {
    match dotenv::dotenv() {
        Err(e) => {
            let err = Error(Mutex::new(e));
            println!("{}", err)
        }
        _ => (),
    }
}