Downcasting boxed errors in Rust

Today I was trying to write polymorphic error handling in Rust, something achievable with dynamic_cast in C++, and incredibly easy in Go. In Rust, of course, it’s overcomplicated.

After a couple frustrating hours of searching with no success, I stumbled upon the downcast_ref method of the Error trait, which finally allowed me to write what I wanted. Oddly enough, I couldn’t find this solution anywhere.

	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)
	}
	}

view raw playground.rs hosted with ❤ by GitHub

Note that the String concrete type cannot be retrieved.

Playground link here.