Downcast to `&'static str ` an error returned on join thread

downcast_ref_error.rs

use std::thread;

fn main() {
    let numbers: Vec<usize> = vec![];

    let t = thread::spawn(move || {
        let len = numbers.len();
        let sum = numbers.into_iter().sum::<usize>();
        sum / len
    });

    let avg = match t.join() {
        Ok(v) => format!("{}", v),
        Err(err) => {
            match err.downcast_ref::<&'static str>() {
                Some(s) => *s,
                None => match err.downcast_ref::<String>() {
                    Some(s) => &s[..],
                    None => "Sorry, unknown payload type",
                },
            }.to_string()
        },
    };

    println!("{:?}", avg);
}