Code shared from the Rust Playground

playground.rs

use std::error::Error;

#[derive(Debug)]
struct CustomError(String);

impl Error for CustomError {}

impl std::fmt::Display for CustomError {
	fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
		<Self as std::fmt::Debug>::fmt(self, f) // delegate to Debug
	}
}

impl CustomError {
	pub fn shout(&self) {
		println!("{}", self.0);
	}
}

fn boom() -> Result<(), Box<dyn Error>> {
	// Err(String::from("string error").into())
	Err(Box::new(CustomError("custom error".to_owned())))
}

fn main() {
	let e = boom().unwrap_err();

	if let Some(e) = e.downcast_ref::<CustomError>() {
		e.shout();
	} else {
		println!("Other: {}", e)
	}
}