Function expect_err 

pub const fn expect_err<T, E>(res: Result<T, E>, message: &str) -> E

Const version of Result::expect_err without the Debug formatting.

Can also be used in combination with Result::is_err to match the result.

Example

use const_util::result::expect_err;
const fn double<T>(res: Result<T, i32>) -> Result<T, i32> {
    if res.is_err() {
        Err(2 * expect_err(res, "Unreachable"))
    } else {
        res
    }
}
assert_eq!(double(Err::<String, _>(1)), Err(2));