Mastering Rust - Second Edition

Fn closures

Closures that access variables only for read access implement the Fn trait. Any value they access are as reference types (&T). This is the default mode of borrowing the closures assumes. Consider the following code:

// fn_closure.rsfn main() {    let a = String::from("Hey!");    let fn_closure = || {        println!("Closure says: {}", a);        };    fn_closure();    println!("Main says: {}", a);}

We get the following output upon compilation:

Closure says: Hey!Main says: Hey!

The a variable was still accessible even after invoking the closure as the closure used a by reference.