生锈期货 - 使用Box :: PIN或PIN_MUT外部期货:: SELECT

Question

我已经阅读了Futures :: Select的使用情况，但是我遇到了障碍。这是我的混乱：在expression in select！中，我们应该使用类型实现unpin and fusedfuture >，但是这里pin unpin在使用pin的使用情况下无效。

1.为什么我们应该在unpin上包装pin如果call fuse> fuse（） select！select！，如果没有效果2.为什么pin可以在的表达式中使用！

这是代码：

async fn async_identity_fn(arg: usize) -> usize {
    arg
}
let fut_1 = async_identity_fn(1).fuse();
let fut_2 = async_identity_fn(2).fuse();
let mut fut_1 = Box::pin(fut_1); // Pins the Future on the heap
pin_mut!(fut_2); // Pins the Future on the stack
let res = select! {
    a_res = fut_1 => a_res,
    b_res = fut_2 => b_res,
};

这是一些链接

用法代码> ;

关于使用pin> pin> pin> pin

select> select！

/docs.rs/futures/latest/futures/macro.select.html“ rel =” nofollow noreferrer “ sstatic.net/p0o29.png“ rel =“ nofollow noreferrer”>关于使用选择条件的混乱！

Answer 1

为什么我们应该在上包装 pin> pin> uncin 如果call fuse> fuse（） select> select！，如果它没有效果

是因为它不是umpin：

error[E0277]: `from_generator::GenFuture<[static generator@src/lib.rs:3:49: 5:2]>` cannot be unpinned
  --> src/lib.rs:12:15
   |
12 |       let res = select! {
   |  _______________^
13 | |         a_res = fut_1 => a_res,
14 | |         b_res = fut_2 => b_res,
15 | |     };
   | |_____^ within `futures::future::future::fuse::_::__Origin<'_, impl futures::Future<Output = usize>>`, the trait `Unpin` is not implemented for `from_generator::GenFuture<[static generator@src/lib.rs:3:49: 5:2]>`
   |
   = note: consider using `Box::pin`
   = note: required because it appears within the type `impl futures::Future<Output = usize>`
   = note: required because it appears within the type `impl futures::Future<Output = usize>`
   = note: required because it appears within the type `Option<impl futures::Future<Output = usize>>`
   = note: required because it appears within the type `futures::future::future::fuse::_::__Origin<'_, impl futures::Future<Output = usize>>`
   = note: required because of the requirements on the impl of `Unpin` for `futures::future::Fuse<impl futures::Future<Output = usize>>`
note: required by a bound in `futures_util::async_await::assert_unpin`
  --> /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.21/src/async_await/mod.rs:50:24
   |
50 | pub fn assert_unpin<T: Unpin>(_: &T) {}
   |                        ^^^^^ required by this bound in `futures_util::async_await::assert_unpin`
   = note: this error originates in the macro `$crate::select_internal` (in Nightly builds, run with -Z macro-backtrace for more info)

因此，它对于选择！无效。但是，即使对于！umpin值，如果我们将其固定，则固定值确实实现了umpin（不是因为pin> pin本身，而是因为>框和引用始终是uncin）。

为什么pin可以在的表达中使用！


因为宏可以固定呼叫本身的结果。但是，当直接使用变量时，它不能很好地做到这一点。