Okay in the details block is a standalone version suitable for the playground:

#![crate_type="lib"]

use std::future::Future;
use std::io::Result;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::time::Duration;

pub fn timeout<T>(_duration: Duration, _future: T) -> Timeout<T>
where
    T: Future,
{
    loop { }
}

#[derive(Debug)]
pub struct Elapsed(());

impl std::fmt::Display for Elapsed {
    fn fmt(&self, w: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(w, "Elapsed")
    }
}

impl std::error::Error for Elapsed {}

impl From<Elapsed> for std::io::Error {
    fn from(_err: Elapsed) -> std::io::Error {
        std::io::ErrorKind::TimedOut.into()
    }
}

pub struct Timeout<T> {
    _value: T,
}

impl<T> Future for Timeout<T>
where
    T: Future,
{
    type Output = std::result::Result<T::Output, Elapsed>;

    fn poll(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Self::Output> {
        loop { }
    }
}

static mut TIMEOUT: u64 = 2000;

use std::marker::PhantomData;

pub struct JoinHandle<T> {
    _p: PhantomData<T>,
}

pub fn spawn<T>(_task: T) -> JoinHandle<T::Output>
where
    T: Future + Send + 'static,
    T::Output: Send + 'static,
{
    loop { }
}

pub async fn my_main() {
    spawn(proxy());
}

// This version of `fn proxy` demonstrates the error
pub async fn proxy() -> Result<Vec<u8>> {
    timeout
        (Duration::from_millis(unsafe { TIMEOUT }),
         async { Result::Ok(vec![0u8]) })
        .await?
        .unwrap()
        ;

    Result::Ok(vec![0u8])
}

// This variant does not generate an error
pub async fn proxy_2() -> Result<Vec<u8>> {
    let dur = Duration::from_millis(unsafe { TIMEOUT });

    timeout
        (dur,
         async { Result::Ok(vec![0u8]) })
        .await?
        .unwrap()
        ;

    Result::Ok(vec![0u8])
}

// This variant does not generate an error
pub async fn proxy_3() -> Result<Vec<u8>> {
    let tv = unsafe { TIMEOUT };

    timeout
        (Duration::from_millis(tv),
         async { Result::Ok(vec![0u8]) })
        .await?
        .unwrap()
        ;

    Result::Ok(vec![0u8])
}

and the error it generates:

error: future cannot be sent between threads safely
  --> src/lib.rs:65:5
   |
56 | pub fn spawn<T>(_task: T) -> JoinHandle<T::Output>
   |        -----
57 | where
58 |     T: Future + Send + 'static,
   |                 ---- required by this bound in `spawn`
...
65 |     spawn(proxy());
   |     ^^^^^ future returned by `proxy` is not `Send`
   |
   = help: within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `*mut u64`
note: future is not `Send` as this value is used across an await
  --> src/lib.rs:70:5
   |
70 | /     timeout
71 | |         (Duration::from_millis(unsafe { TIMEOUT }),
   | |                                         ------- has type `*mut u64`
72 | |          async { Result::Ok(vec![0u8]) })
73 | |         .await?
   | |______________^ await occurs here, with `TIMEOUT` maybe used later
74 |           .unwrap()
75 |           ;
   |           - `TIMEOUT` is later dropped here

error: aborting due to previous error

not Send due to await retainment #67611

Description

Mark-Simulacrumopenedon Dec 25, 2019

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Mark-Simulacrum
openedon Dec 25, 2019