Rust proper error handling (auto convert from one error type to another with question mark)

Question

I want to learn how to properly deal with errors in Rust. I have read the book and this example; now I would like to know how I should deal with errors in this function:

fn get_synch_point(&self) -> Result<pv::synch::MeasPeriods, reqwest::Error> {
    let url = self.root.join("/term/pv/synch"); // self.root is url::Url
    let url = match url {
        Ok(url) => url,
        // ** this err here is url::ParseError and can be converted to Error::Kind https://docs.rs/reqwest/0.8.3/src/reqwest/error.rs.html#54-57 **//
        Err(err) => {
            return Err(Error {
                kind: ::std::convert::From::from(err),
                url: url.ok(),
            })
        }
    };

    Ok(reqwest::get(url)?.json()?) //this return reqwest::Error or convert to pv::sych::MeasPeriods automaticly
}      

This code is improper; it causes a compilation error:

error[E0451]: field `kind` of struct `reqwest::Error` is private
  --> src/main.rs:34:42
   |
34 |             Err(err) => return Err(Error{kind: ::std::convert::From::from(err), url: url.ok()})
   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ field `kind` is private

error[E0451]: field `url` of struct `reqwest::Error` is private
  --> src/main.rs:34:81
   |
34 |             Err(err) => return Err(Error{kind: ::std::convert::From::from(err), url: url.ok()})
   |                                                                                 ^^^^^^^^^^^^^ field `url` is private

What is a proper pattern to deal with that case? For me, reqwest::Error in this case is a good solution so I would like to avoid defining my own error type:

enum MyError {
    Request(reqwest::Error),
    Url(url::ParseError) // this already a part of request::Error::Kind!!!
} 

Answer 1

Update 2020

The rust programming language is evolving quickly so a new answer can be added! I really liked custom_error but now I think thiserror will be my loved one!

use thiserror::Error;

#[derive(Error, Debug)]
pub enum DataStoreError {
    #[error("data store disconnected")]
    Disconnect(#[from] io::Error),
    #[error("the data for key `{0}` is not available")]
    Redaction(String),
    #[error("invalid header (expected {expected:?}, found {found:?})")]
    InvalidHeader {
        expected: String,
        found: String,
    },
    #[error("unknown data store error")]
    Unknown,
}

This allow change io::Error to DataStoreError::Disconnect with question mark ?. Go here for details

useful links:

• great blog about using thiserror in combine with anyhow

---

Other interesting crates:

• anyhow - Flexible concrete Error type built on std::error::Error
• snafu - Situation Normal: All Fouled Up - SNAFU is a library to easily assign underlying errors into domain-specific errors while adding context. (similar to thiserror)
• custom_error - This crate contains a macro that should make it easier to define custom errors without having to write a lot of boilerplate code.

for panics:

• proc-macro-error - This crate aims to make error reporting in proc-macros simple and easy to use.
• human-panic - Panic messages for humans. Handles panics by calling std::panic::set_hook to make errors nice for humans.