What is the difference between Fn FnMut FnOnce

The difference lies in how many times a closure can be called and whether it mutates captured data. Fn can be called multiple times and borrows captures immutably; FnMut can be called multiple times and mutates captures; FnOnce can be called exactly once and takes ownership of captures. Use Fn for read-only logic, FnMut for stateful logic, and FnOnce for one-time actions like moving resources.

let x = 5;
let read_only = || println!("{}", x); // Fn
let mut state = 0;
let mutator = || state += 1; // FnMut
let owned = String::from("hello");
let consumer = || println!("{}", owned); // FnOnce

These traits define how a closure handles the variables it uses from its surrounding code. Think of them as permission levels: Fn is like reading a book without writing in it, FnMut is like writing notes in the margins, and FnOnce is like burning the book after reading it once. You choose the trait based on whether your code needs to read, change, or consume those variables.