Crate dyn_error

This crate provides error-related utilities.

In particular, the check_err_box function can test an Err result wrapping a Box with an Error implementation, using equality test via PartialEq:

use dyn_error::*;
use std::fmt::Display;
use std::error::Error;

//
// Declaring a custom Error type
//
#[derive(Debug, PartialEq, Eq)]
struct MyErr(pub u8);

impl Display for MyErr {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "Custom error: {}", self.0)
    }
}

impl Error for MyErr {};

// Declaring a Result that can hold any Error implementation
let mut result: Result<String, Box<dyn Error>>;

//
// Test scenarios
//
result = Err(Box::new(MyErr(90)));
assert_eq!(
    check_err_box(result, MyErr(90)),
    Ok(())
);

result = Err(Box::new(MyErr(90)));
assert_eq!(
    check_err_box(result, MyErr(7)),
    Err(ErrBoxCheckFailure::NotEqual {
        expected: "Custom error: 7".to_string(),
        actual: "Custom error: 90".to_string()
    })
);

For more details - including different Error taxonomies, please refer to check_err_box.

You can simplify the check even more, by using the panic-based assert_err_box macro:

use dyn_error::*;
use std::fmt::Display;
use std::error::Error;

#[derive(Debug, PartialEq, Eq)]
struct MyErr(pub u8);

impl Display for MyErr {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "Custom error: {}", self.0)
    }
}

impl Error for MyErr {};

let result: Result<String, Box<dyn Error>> = Err(Box::new(MyErr(90)));

assert_err_box!(result, MyErr(90));

Macros

• assert_err_box
  Macro that internally calls assert_err_box, issuing panic in case of failure.

Enums

• ErrBoxCheckFailure
  Unsuccessful outcome of check_err_box.

Functions

• check_err_box
  Easily tests whether the given Result is actually an Err with a Box containing an Error implementation equal to the expected instance: