特征返回特征：在某些情况下有效，在其他情况下无效

我需要实现一个返回的特征futures::StreamExt trait.

一般来说，这听起来很简单，并且有几个答案，例如这里 https://stackoverflow.com/questions/60143046/how-can-a-rust-trait-object-return-another-trait-object.

我尝试过这个StreamExt但这确实 - 由于某种原因 - 不起作用。这是我的示例代码：

// [dependencies]
// futures = "0.3.6"
// async-std = { version = "1.6.5", features = ["attributes", "unstable"] } // interval function is unstable atm.
use std::time::Duration;

use futures::StreamExt;

trait StreamProvidingTrait {
  fn returnastream(self: &Self) -> Box<dyn StreamExt<Item=i32>>;
}

struct StreamProvider {}

impl StreamProvidingTrait for StreamProvider {
  fn returnastream(self: &Self) -> Box<dyn StreamExt<Item=i32>> {
    return Box::new(async_std::stream::interval(Duration::from_millis(1000)).map(|_| 1));
  }
}

#[async_std::main]
async fn main() {
  let mut counter = 0;
  let object = StreamProvider {};

  // creates a stream
  let worx = object.returnastream();
  // mappes the stream into something....
  let mut mapped_stream = worx.map(|_| {
    counter = counter + 1;
    counter
  });
  // subscribing to the items
  while let item = mapped_stream.next().await {
    match item {
      Some(value) => println!("{}", value),
      _ => {}
    }
  }
}

这里是错误：

Compiling traittest v0.1.0 (/Users/andre/repos/traittest)
warning: the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
--> /Users/andre/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.6/src/stream/stream/mod.rs:1326:8
|
1326 |     fn poll_next_unpin(&mut self, cx: &mut Context<'_>) -> Poll<Option<Self::Item>>
|        ^^^^^^^^^^^^^^^ the trait cannot be made into an object because method `poll_next_unpin` references the `Self` type in its `where` clause
|
= note: `#[warn(where_clauses_object_safety)]` on by default
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #51443 <https://github.com/rust-lang/rust/issues/51443>

error[E0038]: the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
--> src/main.rs:6:36
|
6    |   fn returnastream(self: &Self) -> Box<dyn StreamExt<Item=i32>>;
|                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
|
::: /Users/andre/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.6/src/stream/stream/mod.rs:226:8
|
226  |     fn next(&mut self) -> Next<'_, Self>
|        ---- the trait cannot be made into an object because method `next` references the `Self` type in its return type
...
976  |     fn by_ref(&mut self) -> &mut Self {
  |        ------ the trait cannot be made into an object because method `by_ref` references the `Self` type in its return type
  ...
      1381 |     fn select_next_some(&mut self) -> SelectNextSome<'_, Self>
  |        ---------------- the trait cannot be made into an object because method `select_next_some` references the `Self` type in its return type

  error[E0038]: the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
      --> src/main.rs:12:36
      |
      12   |   fn returnastream(self: &Self) -> Box<dyn StreamExt<Item=i32>> {
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
        |
    ::: /Users/andre/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.6/src/stream/stream/mod.rs:226:8
        |
        226  |     fn next(&mut self) -> Next<'_, Self>
    |        ---- the trait cannot be made into an object because method `next` references the `Self` type in its return type
    ...
        976  |     fn by_ref(&mut self) -> &mut Self {
      |        ------ the trait cannot be made into an object because method `by_ref` references the `Self` type in its return type
      ...
          1381 |     fn select_next_some(&mut self) -> SelectNextSome<'_, Self>
      |        ---------------- the trait cannot be made into an object because method `select_next_some` references the `Self` type in its return type

      error: aborting due to 2 previous errors; 1 warning emitted

      For more information about this error, try `rustc --explain E0038`.
      error: could not compile `traittest`.

      To learn more, run the command again with --verbose.

          Process finished with exit code 101

当我交换StreamExt与我自己的特征相比，这段代码运行得很好。

trait SomeTrait {
  fn returnatrait(self: &Self) -> Box<dyn SomeOtherTrait>;
}

trait SomeOtherTrait {
  fn sayHelloWorld(&self);
}

struct DummyStruct {}

impl SomeOtherTrait for DummyStruct {
  fn sayHelloWorld(&self) {
    println!("hello world");
  }
}

struct Implementation {}

impl SomeTrait for Implementation {
  fn returnatrait(self: &Self) -> Box<dyn SomeOtherTrait> {
    return Box::new(DummyStruct{});
  }
}

fn main() {
  let implementation = Implementation{};

  let worx = implementation.returnatrait();

  worx.sayHelloWorld();
}

这是怎么回事？显然有一些我不明白的事情。请帮助我理解这一点！

返回特征的函数可以使用impl Trait返回实现该特征的不透明类型的语法。当前返回特征的特征方法不支持 https://rust-lang.github.io/rfcs/2071-impl-trait-existential-types.html#motivation此功能并要求将特征作为特征对象返回 - 动态分派的引用或智能指针，例如Box or Rc。并不是所有的特质都是对象安全 https://doc.rust-lang.org/book/ch17-02-trait-objects.html#object-safety-is-required-for-trait-objects不过，坏消息是StreamExt由于编译器指出的原因，例如引用，属于那些不属于Self在方法返回类型和where条款。

然而，好消息是这不是问题：StreamExt是一种扩展特征，它为所有实现Stream。所以你不需要费心退货dyn StreamExt特征对象，你可以返回一个dyn Stream之一，您仍然可以访问StreamExt只需通过请求方法即可use StreamExt。换句话说，您只需替换Box<dyn StreamExt> with Box<dyn Stream>在你的特质的返回类型中。

您可能遇到的另一个问题是Box<dyn Stream>不适用于需要移动流的方法，其中包括由StreamExt。这些将需要固定流，这可以通过返回来修复Pin<Box<dyn Stream>>代替Box<dyn Stream>。甚至还有一个boxed()方法上StreamExt在一次操作中固定和装箱流；使用它的代码看起来像这样（):

use futures::stream::{Stream, StreamExt};
use std::pin::Pin;

trait StreamProvidingTrait {
    fn returnastream(&self) -> Pin<Box<dyn Stream<Item = i32>>>;
}

struct StreamProvider {}

impl StreamProvidingTrait for StreamProvider {
    fn returnastream(&self) -> Pin<Box<dyn Stream<Item = i32>>> {
        return tokio::stream::once(0).boxed();
    }
}

fn main() {
    let provider = StreamProvider {};
    let stream = provider.returnastream();
    let _fut = stream.into_future();
}