クロージャーとFn/FnMut/FnOnceの使い分け(@Rust)

// A function which takes a closure as an argument and calls it.
// <f> denotes that F is a "Generic type parameter"
fn apply<f>(f: F) where
    // The closure takes no input and returns nothing.
    F: FnOnce() {
    // ^ TODO: Try changing this to `Fn` or `FnMut`.

    f();
}

// A function which takes a closure and returns an `i32`.
fn apply_to_3<f>(f: F) -> i32 where
    // The closure takes an `i32` and returns an `i32`.
    F: Fn(i32) -> i32 {

    f(3)
}

fn main() {
    use std::mem;

    let greeting = "hello";
    // A non-copy type.
    // `to_owned` creates owned data from borrowed one
    let mut farewell = "goodbye".to_owned();

    // Capture 2 variables: `greeting` by reference and
    // `farewell` by value.
    let diary = || {
        // `greeting` is by reference: requires `Fn`.
        println!("I said {}.", greeting);

        // Mutation forces `farewell` to be captured by
        // mutable reference. Now requires `FnMut`.
        farewell.push_str("!!!");
        println!("Then I screamed {}.", farewell);
        println!("Now I can sleep. zzzzz");

        // Manually calling drop forces `farewell` to
        // be captured by value. Now requires `FnOnce`.
        mem::drop(farewell);
    };

    // Call the function which applies the closure.
    apply(diary);

    // `double` satisfies `apply_to_3`'s trait bound
    let double = |x| 2 * x;

    println!("3 doubled: {}", apply_to_3(double));
}

pub trait FnOnce
where
    Args: Tuple,
{
    type Output;

    // Required method
    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
}

pub trait FnMut: FnOnce
where
    Args: Tuple,
{
    // Required method
    extern "rust-call" fn call_mut(
        &mut self,
        args: Args
    ) -> Self::Output;
}

pub trait Fn: FnMut
where
    Args: Tuple,
{
    // Required method
    extern "rust-call" fn call(&self, args: Args) -> Self::Output;
}