Rust: What is the difference between unwrap_or and unwrap_or_else?

unwrap_or — default value. unwrap_or_else — lazy default.

unwrap_or:

• Always evaluates default.
• Eager evaluation.
• Simpler.

unwrap_or_else:

• Lazy evaluation.
• Only when needed.
• More efficient.

1let value: Option<i32> = None;
2
3// Always evaluates 0
4let v1 = value.unwrap_or(0);
5
6// Only evaluates when None
7let v2 = value.unwrap_or_else(|| {
8    println!("Computing default");
9    0
10});

Key: unwrap_or for simple defaults, unwrap_or_else for expensive defaults.