Macro assert_err

macro_rules! assert_err {
    ($expr:expr $(,)?) => { ... };
}

Panic if the expression is not Err(_).

Examples

// Successful
assert_err!(Err::<u32, &str>("failed"));

// Panics
assert_err!(Ok::<u32, &str>(2025));

// Compile error: `assert_err!` requires a `Result` value
assert_err!(Ok::<u32, &str>Some(42));