Macro format_err 

macro_rules! format_err {
    ($($arg:tt)*) => { ... };
}

Constructs an Error using the standard string interpolation syntax.

#[macro_use] extern crate amethyst_error;

fn main() {
    let err = format_err!("number: {}", 42);
    assert_eq!("number: 42", err.to_string());
}