rust 中没有为类型 `<generic #0>` 实现的特性 `core::kinds::Sized` 是什么?

问题描述

我希望这能奏效:

trait Task<R, E> {
  fn run(&self) -> Result<R, E>;
}

mod test {

  use super::Task;

  struct Foo;
  impl<uint, uint> Task<uint, uint> for Foo {
    fn run(&self) -> Result<uint, uint> {
      return Err(0);
    }
  }

  fn can_have_task_trait() {
    Foo;
  }
}

fn main() {
  test::can_have_task_trait();
}

...但它没有:

<anon>:10:3: 14:4 error: the trait `core::kinds::Sized` is not implemented for the type `<generic #0>`
<anon>:10   impl<uint, uint> Task<uint, uint> for Foo {
<anon>:11     fn run(&self) -> Result<uint, uint> {
<anon>:12       return Err(0);
<anon>:13     }
<anon>:14   }
<anon>:10:3: 14:4 note: the trait `core::kinds::Sized` must be implemented because it is required by `Task`
<anon>:10   impl<uint, uint> Task<uint, uint> for Foo {
<anon>:11     fn run(&self) -> Result<uint, uint> {
<anon>:12       return Err(0);
<anon>:13     }
<anon>:14   }
<anon>:10:3: 14:4 error: the trait `core::kinds::Sized` is not implemented for the type `<generic #1>`
<anon>:10   impl<uint, uint> Task<uint, uint> for Foo {
<anon>:11     fn run(&self) -> Result<uint, uint> {
<anon>:12       return Err(0);
<anon>:13     }
<anon>:14   }
<anon>:10:3: 14:4 note: the trait `core::kinds::Sized` must be implemented because it is required by `Task`
<anon>:10   impl<uint, uint> Task<uint, uint> for Foo {
<anon>:11     fn run(&self) -> Result<uint, uint> {
<anon>:12       return Err(0);
<anon>:13     }
<anon>:14   }
error: aborting due to 2 previous errors
playpen: application terminated with error code 101
Program ended.

围栏:http://is.gd/kxDt0P

那么，怎么回事?

我不知道这个错误是什么意思.

I have no idea what this error means.

是不是我在使用 Result 并且要求 U、V 没有大小?在这种情况下，为什么要调整大小?我没有写:

Is it that I'm using Result and that requires that U, V are not Sized? In which case, why are they Sized? I didn't write:

Task<Sized? R, Sized? E>

现在所有的泛型都是动态调整大小的吗?(在这种情况下，Sized 是什么意思?甚至是什么意思?)

Are all generics now dynamically sized or something? (in which case, what does Sized? even mean?)

怎么回事?

推荐答案

你只需要去掉impl中的，因为类型参数在那里而不是具体类型:

You just need to remove the <uint, uint> in impl<uint, uint>, because type parameters go there and not concrete types:

impl Task<uint, uint> for Foo { ... }

我认为您遇到的错误是编译器对未使用的类型参数感到困惑.这个超精简版也会出现这种情况:

I think the errors you're getting are the compiler getting confused over unused type parameters. It also occurs with this super-reduced version:

trait Tr {}
impl<X> Tr for () {}

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