core :: marker :: Sized没有为Foo实现

Question

我有这个相当简单的Rust程序：

use std::ops::Deref;

trait Foo {
  fn foo(&self);
}

impl Foo for () {
  fn foo(&self) {
    println!("hello world");
  }
}

impl<F> Foo for Box<F> where F: Foo {
  fn foo(&self) {
    let f: &F = self.deref();
    f.foo()
  }
}

fn call_foo<F>(foo: &F) where F: Foo {
  foo.foo()
}

fn main() {
  let foo: Box<Foo> = Box::new(());
  call_foo(&foo);
}

但是我收到了编译错误：

$ rustc main.rs
main.rs:26:3: 26:11 error: the trait `core::marker::Sized` is not implemented for the type `Foo` [E0277]
main.rs:26   call_foo(&foo);
             ^~~~~~~~
main.rs:26:3: 26:11 help: run `rustc --explain E0277` to see a detailed explanation
main.rs:26:3: 26:11 note: `Foo` does not have a constant size known at compile-time
main.rs:26   call_foo(&foo);
             ^~~~~~~~
main.rs:26:3: 26:11 note: required by `call_foo`
main.rs:26   call_foo(&foo);
             ^~~~~~~~
error: aborting due to previous error

E0277的错误解释似乎无关。我该如何解决这个问题？