Macro assert_err_eq

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

Asserts that the left expression contains an Err(E) variant and its contained value of type E equals the right expression.

Uses

Assertions are always checked in both debug and release builds, and cannot be disabled. See debug_assert_err_eq! 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(1);

assert_err_eq!(res, 1);

// With a custom message
assert_err_eq!(res, 1, "Everything is good with {:?}", res);

The contained value will be returned from the macro call:

let res: Result<(), i32> = Err(1);

let value = assert_err_eq!(res, 1);
assert_eq!(value, 1);

An Ok(_) variant will panic:

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

assert_err_eq!(res, 1);  // Will panic