Snippet

Dynamic Error Downcasting and Context Contextualization

This snippet demonstrates how to define a custom error type and perform dynamic downcasting on a trait object of dyn Error. Using downcast_ref, the code inspects concrete error types at runtime, enabling specialized error recovery or context aggregation for specific failure modes.

snippet.rs

rust

use std::error::Error;
use std::fmt;
 
#[derive(Debug)]
pub struct QueryError {
    pub query: String,
}
 
impl fmt::Display for QueryError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Query failed: {}", self.query)
    }
}
 
impl Error for QueryError {}
 
pub fn process_error(err: &dyn Error) -> Option<&QueryError> {
    err.downcast_ref::<QueryError>()
}

Breakdown

pub struct QueryError

Defines a custom error struct containing domain-specific context metadata.

impl fmt::Display for QueryError

Implements formatting to print a human-readable description of the error.

impl Error for QueryError {}

Marks the struct as a standard library Error, enabling its use as a trait object.

err.downcast_ref::<QueryError>()

Uses the downcast_ref method to check if the dynamic error matches QueryError.

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Dynamic Error Downcasting and Context Contextualization

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