core::marker::Sized 没有为 Foo 实现 [英] core::marker::Sized not implemented for Foo
问题描述
我有一个相当简单的 Rust 程序:
I have this fairly straightforward Rust program:
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);
}
但是我得到一个编译错误:
But I get a compilation errors:
$ 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?
The error explanation for E0277 seems unrelated. How do I fix this?
推荐答案
问这个问题时,错误信息并不明显.相同的代码(Playground) 今天显示了一个更好的错误消息:
The error message wasn't stellar when this question was asked. The same code (Playground) shows a better error message today:
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,第 19 章,第 4 节,动态大小的类型和 Sized
trait".出现问题是因为默认情况下,泛型类型参数假定为 Sized
.然而,trait 对象(Box
中的dyn Foo
)没有已知的大小.在这种情况下这是可以接受的,因此我们修改了整体实现以允许使用未知大小的盒子:
The error message links to The Rust Programming Language Book, Chapter 19, section 4, "Dynamically sized types and the Sized
trait".
The problem arises because by default, a generic type parameter is assumed to be Sized
. However, a trait object (the dyn Foo
in Box<dyn Foo>
) does not have a known size. That's acceptable in this case, so we modify the blanket implementation to allow boxes things with an unknown size:
impl<F: ?Sized> Foo for Box<F>
where F: Foo
这是固定代码,更新到 Rust 2018 版:
Here is the fixed code, updated to the Rust 2018 edition:
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);
}
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