Error converting Box<Error+Send> to &Error

[emk]

I'm trying to write an Error implementation which supports a set of "important" error, plus a catch-all for various minor errors caused by low-level subsystems. The code looks something like this:

use std::error::Error;

pub enum DecodingError {
    EncodingUnknown,
    //...several named errors here...
    /// Other errors, the details of which are probably unimportant.
    Unexpected(Box<Error+Send>)
}

impl Error for DecodingError {
    fn description(&self) -> &str { "decoding error" }
    fn detail(&self) -> Option<String> { None }
    fn cause(&self) -> Option<&Error> {
        match self {
            &DecodingError::Unexpected(cause) => Some(&*cause),
            _ => None
        }
    }
}

This yields the following error in cause:

:14:9: 17:10 error: mismatched types: expected core::option::Option<&std::error::Error+'static>, found core::option::Option<&std::error::Error+Send> (expected no bounds, found Send)

I should presumably be able to convert a Box<Error+Send> to an &Error safely, but I can't figure out a way to do it.

[emk]

Here's a demonstration of this error on play: http://is.gd/W3LG9a

[steveklabnik]

I believe this is a duplicate of #18524

[Guang1234567]

@emk

My solution(rust 2018 nightly):

#[derive(Debug)]
pub enum LibError {
    EmptyVec,
    DotEnvErr(Box<dyn Error>),
    LogErr(Box<dyn Error>),
    DatabaseErr(Box<dyn Error>),
}


impl Error for LibError {
    fn description(&self) -> &str {
        match self {
            EmptyVec => "empty vectors not allowed",
            DotEnvErr(e) => e.description(),
            LogErr(e) => e.description(),
            DatabaseErr(e) => e.description(),
        }
    }
    fn cause(&self) -> Option<&Error> {
        match self {
            EmptyVec => None,
            DotEnvErr(e) => Some(&**e),
            LogErr(e) => Some(&**e),
            DatabaseErr(e) => Some(&**e),   //<————————  here
        }
    }
}