Macro assert_err

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

Asserts that the expression matches an Err(_) variant, returning the contained value.

Uses

Assertions are always checked in both debug and release builds, and cannot be disabled. See debug_assert_err! for assertions that are not enabled in release builds by default.

Custom messages

This macro has a second form, where a custom panic message can be provided with or without arguments for formatting. See std::fmt for syntax for this form.

Examples

let res: Result<i32, ()> = Err(());

assert_err!(res);

// With a custom message.
assert_err!(res, "we are checking if there was an error with {:?}", res);

An Ok(_) variant will panic:

let res: Result<i32, ()> = Ok(42);

assert_err!(res);  // Will panic