or_else family of functions such as
Option::map_or_else accept a closure as an argument as a means of delaying the "else"
computation. However, simple closures such as
|| 5 add extra overhead.
Consider using the "eager" variants of these functions instead, as it reduces laziness and improves readability:
let x = Err(2); x.unwrap_or_else(|| 0);
let x = Err(2); x.unwrap_or(0);