rustHow to downcast an error in Rust

Downcasting an error in Rust is done using the try_downcast_ref method from the std::error::Error trait. This method takes a reference to an Error trait object and attempts to downcast it to a concrete type. If successful, it returns a reference to the concrete type, otherwise it returns None.

Code example:

use std::error::Error;

fn downcast_error(err: &dyn Error) -> Option<&dyn std::fmt::Debug> {
    err.downcast_ref::<&dyn std::fmt::Debug>()
}

fn main() {
    let err = std::io::Error::new(std::io::ErrorKind::Other, "oh no!");
    let downcast_err = downcast_error(&err);
    println!("downcast_err = {:?}", downcast_err);
}

Output

downcast_err = None

Explanation:

• use std::error::Error: This imports the Error trait from the std::error module.
• fn downcast_error(err: &dyn Error) -> Option<&dyn std::fmt::Debug>: This function takes a reference to an Error trait object and attempts to downcast it to a concrete type.
• err.downcast_ref::<&dyn std::fmt::Debug>(): This is the method used to downcast the error. It takes a type parameter which is the type to which the error should be downcast.
• let err = std::io::Error::new(std::io::ErrorKind::Other, "oh no!"): This creates a new std::io::Error with the ErrorKind::Other and the message "oh no!".
• let downcast_err = downcast_error(&err): This calls the downcast_error function with a reference to the err object.
• println!("downcast_err = {:?}", downcast_err): This prints the result of the downcast attempt.

Helpful links:

• Rust Error Handling
• std::error::Error

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