Rustの勉強[高度な関数とクロージャ その2]

fn add_one(x: i32) -> i32 {
    x + 1
}

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

fn main() {
    let answer = do_twice(add_one, 5);

    // 答えは{}
    println!("The answer is: {}", answer);
}

fn returns_closure() -> Fn(i32) -> i32 {
    |x| x + 1
}

error[E0277]: the trait bound `std::ops::Fn(i32) -> i32 + 'static:
std::marker::Sized` is not satisfied
-->
|
1 | fn returns_closure() -> Fn(i32) -> i32 {
|                         ^^^^^^^^^^^^^^ `std::ops::Fn(i32) -> i32 + 'static`
does not have a constant size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for
`std::ops::Fn(i32) -> i32 + 'static`
= note: the return type of a function must have a statically known size

fn returns_closure() -> Box<Fn(i32) -> i32> {
    Box::new(|x| x + 1)
}