Macro test_err 

macro_rules! test_err {
    (
    $($target:tt)*
) => { ... };
}

Wraps a function that takes nothing and returns something, failing the test if the result is not Err. Shortcut for should_err!.

// This macro invocation:
test_err! { ... }
// Is equivalent to:
should_err! { #[test] ... }