如何使用异步闭包捕获和接受引用

Question

我试图传递一个返回Future<Output=bool>到异步函数的闭包，并将此闭包称为异步谓词（类似于异步.filter或其他高阶函数）。

该谓词接收其输入作为引用。我找到了如何为不捕获其环境的纯谓词实现它：

type BoxFuture<'a, Out> = Pin<Box<dyn Future<Output=Out> + 'a + Send>>;

////////////////////////////////////////////////////////////////////////////////
// 1 -> Closure only uses the inner argument                                  //
////////////////////////////////////////////////////////////////////////////////

/// Run the computation and check that its output is not empty
fn run1<'a>(expected: &'a [u8]) -> impl Future<Output=()> + 'a {
  async move {
    let is_ok = compute_and_check1(|b: &[u8]| Box::pin(async move {
      b.len() > 0 // Only uses the inner argument
    }));
    let is_ok1 = is_ok.await;
    dbg!(is_ok1);
  }
}

/// Compute some bytes (may be complex / use async), then check their validity
/// with a user-supplied function, finally do some clean-up and return.
async fn compute_and_check1<F>(check: F) -> bool
  where
    F: for<'r> FnOnce(&'r [u8]) -> BoxFuture<'r, bool>
{
  let bytes = [0u8; 128];
  let is_ok = check(&bytes).await;
  drop(bytes);
  is_ok
}

游乐场链接

仅支持非捕获闭包是相当有限的。我想使用一个捕获其环境的闭包。通过更改函数的边界compute_and_check，我能够传递使用其环境的闭包 - 但不能传递其输入：

type BoxFuture<'a, Out> = Pin<Box<dyn Future<Output=Out> + 'a + Send>>;

////////////////////////////////////////////////////////////////////////////////
// 2 -> Closure only uses the outer argument                                  //
////////////////////////////////////////////////////////////////////////////////

/// Run the computation and assume that its output is not empty if `expected` is not empty
fn run2<'a>(expected: &'a [u8]) -> impl Future<Output=()> + 'a {
  async move {
    let is_ok = compute_and_check2(|b: &[u8]| Box::pin(async move {
      expected.len() > 0 // Only uses the environment
    }));
    let is_ok2 = is_ok.await;
    dbg!(is_ok2);
  }
}

/// Compute some bytes (may be complex / use async), then check their validity
/// with a user-supplied function, finally do some clean-up and return.
async fn compute_and_check2<'a, F>(check: F) -> bool
  where
    F: for<'r> FnOnce(&'r [u8]) -> BoxFuture<'a, bool>
{
  let bytes = [0u8; 128];
  let is_ok = check(&bytes).await;
  drop(bytes);
  is_ok
}

游乐场链接

我可以编写一种实现，其中闭包使用其输入，以及闭包使用其环境。但不能同时两者。

我如何接受一个使用其输入和环境引用的未来生产闭包？

我想写的是这样的：

type BoxFuture<'a, Out> = Pin<Box<dyn Future<Output=Out> + 'a + Send>>;

////////////////////////////////////////////////////////////////////////////////
// 3 -> Closure uses both the inner and outer arguments                       //
////////////////////////////////////////////////////////////////////////////////

/// Run the computation and check its output is the provided expected value
fn run3<'a>(expected: &'a [u8]) -> impl Future<Output=()> + 'a {
  async move {
    let is_ok = compute_and_check3(|b: &[u8]| Box::pin(async move {
      b == expected // Uses both the input and environment
    }));
    let is_ok2 = is_ok.await;
    dbg!(is_ok2);
  }
}

/// Compute some bytes (may be complex / use async), then check their validity
/// with a user-supplied function, finally do some clean-up and return.
async fn compute_and_check3<'a, F>(check: F) -> bool
  where
    F: for<'r> FnOnce(&'r [u8]) -> BoxFuture<'r + 'a, bool>
{
  let bytes = [0u8; 128];
  let is_ok = check(&bytes).await;
  drop(bytes);
  is_ok
}

游乐场链接

此代码无法编译，因为我要求闭包返回BoxFuture<'r + 'a, bool>，但这不是合法的语法：

error[E0226]: only a single explicit lifetime bound is permitted
  --> src/main.rs:89:51
   |
89 |     F: for<'r> FnOnce(&'r [u8]) -> BoxFuture<'r + 'a, bool>
   |                                                   ^^

据我了解，我的问题的核心是我需要限制我的高级特质束缚。'r我想说的不是“对于任何”，而是“对于任何'r不会过时的'a”，但我不知道如何写下来。

我尝试对我的BoxFuture类型别名使用两个生命周期和限制，或者定义一个辅助特征，但我没有设法解决这个问题，因为我未能对 HRTB 生命周期应用限制。

compute_and_check1为了完整起见，以下是将最终闭包传递给（仅输入）和compute_and_check2（仅环境）时出现的错误：

• 使用compute_and_check1（仅输入）playground 链接

  error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
    --> src/main.rs:17:67
     |
  17 |       let is_ok = compute_and_check1(|b: &[u8]| Box::pin(async move {
     |  ___________________________________________________________________^
  18 | |       b == expected
  19 | |     }));
     | |_____^
     |
  note: first, the lifetime cannot outlive the lifetime `'a` as defined on the function body at 15:9...
    --> src/main.rs:15:9
     |
  15 | fn run3<'a>(expected: &'a [u8]) -> impl Future<Output=()> + 'a {
     |         ^^
  note: ...so that the types are compatible
    --> src/main.rs:17:67
     |
  17 |       let is_ok = compute_and_check1(|b: &[u8]| Box::pin(async move {
     |  ___________________________________________________________________^
  18 | |       b == expected
  19 | |     }));
     | |_____^
     = note: expected `(&[u8], &[u8])`
                found `(&[u8], &'a [u8])`
  note: but, the lifetime must be valid for the anonymous lifetime #1 defined on the body at 17:36...
    --> src/main.rs:17:36
     |
  17 |       let is_ok = compute_and_check1(|b: &[u8]| Box::pin(async move {
     |  ____________________________________^
  18 | |       b == expected
  19 | |     }));
     | |______^
  note: ...so that the expression is assignable
    --> src/main.rs:17:47
     |
  17 |       let is_ok = compute_and_check1(|b: &[u8]| Box::pin(async move {
     |  _______________________________________________^
  18 | |       b == expected
  19 | |     }));
     | |______^
     = note: expected `Pin<Box<dyn Future<Output = bool> + Send>>`
                found `Pin<Box<dyn Future<Output = bool> + Send>>`
  

• 使用compute_and_check2（仅环境）playground 链接

  error: lifetime may not live long enough
    --> src/main.rs:17:47
     |
  17 |       let is_ok = compute_and_check2(|b: &[u8]| Box::pin(async move {
     |  ________________________________________-____-_^
     | |                                        |    |
     | |                                        |    return type of closure is Pin<Box<(dyn Future<Output = bool> + Send + '2)>>
     | |                                        let's call the lifetime of this reference `'1`
  18 | |       b == expected
  19 | |     }));
     | |______^ returning this value requires that `'1` must outlive `'2`
  

我还研究了夜间功能，unboxed_closure但未能解决我的问题。我希望我的代码能够在稳定的 Rust 上运行，但如果这是唯一的解决方案，我的代码要求 nightly 也是可以接受的。