core::marker::Sized 未为 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);
}

但我得到一个编译错误：

$ 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？

Answer 1

当被问到这个问题时，错误消息并不明显。相同的代码 (Playground) 今天显示了更好的错误消息：

error[E0277]: the size for values of type `dyn Foo` cannot be known at compilation time
  --> src/main.rs:26:3
   |
26 |   call_foo(&foo);
   |   ^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `dyn Foo`
   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
   = note: required because of the requirements on the impl of `Foo` for `std::boxed::Box<dyn Foo>`
note: required by `call_foo`
  --> src/main.rs:20:1
   |
20 | fn call_foo<F>(foo: &F) where F: Foo {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

错误消息链接到 Rust 编程语言书，Chapter 19, section 4, "Dynamically sized types and the Sized trait"。 出现问题是因为默认情况下，泛型类型参数假定为Sized。但是，特征对象（Box<dyn Foo> 中的dyn Foo）没有已知的大小。在这种情况下这是可以接受的，所以我们修改了毯子实现以允许盒子大小未知：

impl<F: ?Sized> Foo for Box<F>
    where F: Foo

这里是固定代码，更新到 Rust 2018 版：

trait Foo {
    fn foo(&self);
}

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

impl<F: ?Sized> Foo for Box<F>
    where F: Foo
{
    fn foo(&self) {
        (**self).foo()
    }
}

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

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