Trait Error

pub trait Error: Debug + Display {
    // Provided methods
    fn source(&self) -> Option<&(dyn Error + 'static)> { ... }
    fn description(&self) -> &str { ... }
    fn cause(&self) -> Option<&dyn Error> { ... }
}

Error is a trait representing the basic expectations for error values, i.e., values of type E in Result<T, E>. Errors must describe themselves through the Display and Debug traits, and may provide cause chain information:

The source method is generally used when errors cross “abstraction boundaries”. If one module must report an error that is caused by an error from a lower-level module, it can allow access to that error via the source method. This makes it possible for the high-level module to provide its own errors while also revealing some of the implementation for debugging via source chains.

Provided Methods

fn source(&self) -> Option<&(dyn Error + 'static)>

The lower-level source of this error, if any.

Examples

use std::error::Error;
use std::fmt;

#[derive(Debug)]
struct SuperError {
    side: SuperErrorSideKick,
}

impl fmt::Display for SuperError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "SuperError is here!")
    }
}

impl Error for SuperError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        Some(&self.side)
    }
}

#[derive(Debug)]
struct SuperErrorSideKick;

impl fmt::Display for SuperErrorSideKick {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "SuperErrorSideKick is here!")
    }
}

impl Error for SuperErrorSideKick {}

fn get_super_error() -> Result<(), SuperError> {
    Err(SuperError { side: SuperErrorSideKick })
}

fn main() {
    match get_super_error() {
        Err(e) => {
            println!("Error: {}", e);
            println!("Caused by: {}", e.source().unwrap());
        }
        _ => println!("No error"),
    }
}

fn description(&self) -> &str

👎Deprecated since 1.42.0: use Display

if let Err(e) = "xc".parse::<u32>() {
    // Print `e` itself, no need for description().
    eprintln!("Error: {}", e);
}

fn cause(&self) -> Option<&dyn Error>

👎Deprecated since 1.33.0: obsoleted by Error::source, which can support downcasting

Implementors