Rust: What is the difference between Fn, FnMut, and FnOnce?

Fn — immutable closure. FnMut — mutable closure. FnOnce — one-time closure.

Fn:

• Cannot capture by mutable reference.
• Can be called multiple times.
• Most restrictive.

FnMut:

• Can capture by mutable reference.
• Can be called multiple times.
• Intermediate.

FnOnce:

• Consumes captured values.
• Can be called once.
• Least restrictive.

1let s = String::from("hello");
2
3// Fn: cannot capture mutably
4let closure = || println!("{}", s);
5
6// FnMut: can capture mutably
7let mut v = vec![];
8let mut closure = || v.push(1);
9closure();
10closure();
11
12// FnOnce: consumes
13let s = String::from("hello");
14let closure = || drop(s);
15closure();
16// closure();  // Error: already consumed

Key: Fn for immutable, FnMut for mutable, FnOnce for consuming.