expect_with

Adds expect_with() for Option<T> and Result<T, E> where E is Debug (all std types that has expect method).

This method functions exactly the same as except() but evaluates error message only when actual error occurred.

Example

fn some_heavy_function() -> String {
    return String::from("42");
}

let result: Result<(), String> = Err(String::from("some error"));
result.expect_with(|| format!("error {}", some_heavy_function()));

Motivation

Using expect has one major drawback. It will calculate it's argument every time. Even if no error occurred. This can be really slow if expect is called frequently and evaluating error message envolves some computing (even simple format can be awfully slow). expect_with removes this overhead by accepting lambda, which will be executed to get error message only when needed.