Why should I prefer `Option::ok_or_else` instead of `Option::ok_or`?

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The primary reason to use ok_or_else or any of the ..._or_else methods is to avoid executing a function when it’s not needed. In the case of Option::ok_or_else or Option::unwrap_or_else, there’s no need to run extra code when the Option is Some. This can make code faster, depending on what happens in the error case

In this example, Error::new likely performs allocation, but it could also write to standard out, make a network request, or anything any piece of Rust code can do; it’s hard to tell from the outside. It’s generally safer to place such code in a closure so you don’t have to worry about extraneous side effects when the success case happens.

Clippy lints this for you as well:

fn main() {
    let foo = None;
    foo.unwrap_or("hello".to_string());
}

warning: use of `unwrap_or` followed by a function call
 --> src/main.rs:3:9
  |
3 |     foo.unwrap_or("hello".to_string());
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(|| "hello".to_string())`
  |
  = note: `#[warn(clippy::or_fun_call)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#or_fun_call

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