Code shared from the Rust Playground

playground.rs

fn do_twice(f: fn(i32) -> i32, arg: i32) -> i32 { // definition
    f(arg) + f(arg)
}

fn main() {
    do_twice(|x| x + 1, 5); // call
    let inc = 2;
    do_twice(|x| x + inc, 5); // fail
}