Mastering Rust - Second Edition

FnMut closures

When the compiler figures out that a closure mutates a value referenced from the environment, the closure implements the FnMut trait. Adapting the same code as before, we have the following:

// fn_mut_closure.rsfn main() {    let mut a = String::from("Hey!");    let fn_mut_closure = || {        a.push_str("Alice");        };    fn_mut_closure();    println!("Main says: {}", a);}

The previous closure adds the "Alice" string to a. fn_mut_closure mutates its environment.