Input functions

Since closures may be used as arguments, you might wonder if the same can be said
about functions. And indeed they can! If you declare a function that takes a
closure as parameter, then any function that satisfies the trait bound of that
closure can be passed as a parameter.

// Define a function which takes a generic `F` argument
// bounded by `Fn`, and calls it
fn call_me<F: Fn()>(f: F) {
    f();
}
// Define a wrapper function satisfying the `Fn` bound
fn function() {
    println!("I'm a function!");
}
fn main() {
    // Define a closure satisfying the `Fn` bound
    let closure = || println!("I'm a closure!");
    call_me(closure);
    call_me(function);
}

As an additional note, the Fn, FnMut, and FnOnce traits dictate how
a closure captures variables from the enclosing scope.

See also:

Fn, FnMut, and FnOnce