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

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

Fn:

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

FnMut:

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

FnOnce:

• Consumes captured values.
• Can be called once.

1// Fn
2let s = String::from("hello");
3let closure = || println!("{}", s);
4closure();
5closure();
6
7// FnMut
8let mut v = vec![];
9let mut closure = || v.push(1);
10closure();
11closure();
12
13// FnOnce
14let s = String::from("hello");
15let closure = || drop(s);
16closure();
17// closure();  // Error: already consumed

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