async/await 状态机

  编译:成功
  运行(退出码 0):
    async fn nothing()     的 Future 大小:     1 字节
    buf 在 await 前用完(仍在作用域内):       1002 字节
    buf 放进内层 { } 块,await 前作用域结束:      2 字节
    buf[1000] 跨 await 存活:                  1002 字节
    a[1000] + b[2000] 跨 await 存活:          3002 字节
    两个 await,只有一个 1000 字节数组跨第二个 await: 1002 字节
    YieldNow 自己:                               1 字节

  编译:成功
  运行(退出码 0):
    第 1 次 poll:
      [状态0] 开始
    第 2 次 poll:
      [await 之后] i = 1 sum = 1
    第 3 次 poll:
      [await 之后] i = 2 sum = 3
    第 4 次 poll:
      [await 之后] i = 3 sum = 6
    Ready(6),共 poll 4 次

  编译:成功
  运行(退出码 101):
    第一次 poll: Ready(1)
    第二次 poll(已完成):
    
    thread 'main' (7203531) panicked at <实验目录>/src/u98c.rs:7:21:
    `async fn` resumed after completion
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

  源码:
      1 | async fn fib(n: u32) -> u32 { if n < 2 { n } else { fib(n - 1).await + fib(n - 2).await } }
      2 | fn main() { let _ = fib(5); }
  编译:失败
    error[E0733]: recursion in an async fn requires boxing
     --> <实验目录>/src/u98d.rs:1:1
      |
    1 | async fn fib(n: u32) -> u32 { if n < 2 { n } else { fib(n - 1).await + fib(n - 2).await } }
      | ^^^^^^^^^^^^^^^^^^^^^^^^^^^                         ---------------- recursive call here
      |
      = note: a recursive `async fn` call must introduce indirection such as `Box::pin` to avoid an infinitely sized future
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0733`.
  编译:成功
  运行(退出码 0):
    Ready(55)

  源码:
      1 | #![allow(unused)]
      2 | use std::{future::Future, pin::Pin, task::{Context, Poll, Waker}, mem::size_of_val};
      3 | struct YieldNow(bool);
      4 | impl Future for YieldNow { type Output = (); fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> { if self.0 { Poll::Ready(()) } else { self.0 = true; cx.waker().wake_by_ref(); Poll::Pending } } }
      5 | fn yield_now() -> YieldNow { YieldNow(false) }
      6 | 
      7 | fn is_unpin<T: Unpin>(_: &T) {}
      8 | async fn borrows() { let local = [1u8, 2, 3]; let r = &local; yield_now().await; println!("{}", r[0]); }
      9 | fn main() { let f = borrows(); is_unpin(&f); is_unpin(&yield_now()); }
  编译:失败
    error[E0277]: `{async fn body of borrows()}` cannot be unpinned
     --> <实验目录>/src/u98f.rs:9:41
      |
    8 | async fn borrows() { let local = [1u8, 2, 3]; let r = &local; yield_now().await; println!("{}", r[0]); }
      |                   - within this `impl Future<Output = ()>`
    9 | fn main() { let f = borrows(); is_unpin(&f); is_unpin(&yield_now()); }
      |                                -------- ^^ within `impl Future<Output = ()>`, the trait `Unpin` is not implemented for `{async fn body of borrows()}`
      |                                |
      |                                required by a bound introduced by this call
      |
      = note: consider using the `pin!` macro
              consider using `Box::pin` if you need to access the pinned value outside of the current scope
    note: required because it appears within the type `impl Future<Output = ()>`
     --> <实验目录>/src/u98f.rs:8:19
      |
    8 | async fn borrows() { let local = [1u8, 2, 3]; let r = &local; yield_now().await; println!("{}", r[0]); }
      |                   ^
    note: required by a bound in `is_unpin`
     --> <实验目录>/src/u98f.rs:7:16
      |
    7 | fn is_unpin<T: Unpin>(_: &T) {}
      |                ^^^^^ required by this bound in `is_unpin`
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0277`.