A vague error, and I don't know what the problem is

[driftluo]

version: tokio 0.2.1

I even don’t know where this question should be mentioned, and I feel vague that this should be a rustc problem. If so, please tell me, I will write the problem over rustc again

Example：

use futures::{
    channel::mpsc::channel,
    stream::{iter, pending},
    SinkExt, StreamExt,
};
use std::future::Future;
use std::pin::Pin;

pub(crate) type BoxedFutureTask = Pin<Box<dyn Future<Output = ()> + 'static + Send>>;

fn main() {
    let mut rt = tokio::runtime::Runtime::new().unwrap();
    let (sender, mut receiver) = channel(128);
    rt.block_on(async move {
        tokio::spawn(async move {
            let mut tasks = iter(
                (1..100)
                    .map(|_| {
                        Box::pin(async {
                            let mut stream = pending::<()>();
                            loop {
                                stream.next().await;
                            }
                        }) as BoxedFutureTask
                    })
                    .collect::<Vec<_>>(),
            )
            .map(Ok);
            let _ = sender.send_all(&mut tasks).await;
        });
        loop {
            if receiver.next().await.is_none() {
                break
            }
        }
    })
}

will output：

error[E0308]: mismatched types
  --> src/main.rs:12:9
   |
12 |         tokio::spawn(async move {
   |         ^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected type `std::ops::FnOnce<(std::pin::Pin<std::boxed::Box<dyn core::future::future::Future<Output = ()> + std::marker::Send>>,)>`
              found type `std::ops::FnOnce<(std::pin::Pin<std::boxed::Box<dyn core::future::future::Future<Output = ()> + std::marker::Send>>,)>`

error: aborting due to previous error

but change BoxedFutureTask to () like follow, it works well

fn main () {
    let mut rt = tokio::runtime::Runtime::new().unwrap();
    let (mut sender, mut receiver) = channel(128);
    rt.block_on(async move {
        tokio::spawn(async move {
            let mut tasks = iter(
            (1..100)
                .map(|_| {
                    ()
                })
                .collect::<Vec<_>>(),
            ).map(Ok);
            let _ = sender.send_all(&mut tasks).await;
        });
        loop {
            if receiver.next().await.is_none() {
                break
            }
        }
    })
}

I am very puzzled about this, Vec<Pin<Box<dyn Future <Output = () >> + 'static + Send >> is a correct value, and the error here makes me overwhelmed.

[jebrosen]

Possibly related: rust-lang/rust#64650, rust-lang/rust#60658.

[carllerche]

Looks like a compiler bug

[LucioFranco]

is it the order of pub(crate) type BoxedFutureTask = Pin<Box<dyn Future<Output = ()> + 'static + Send>>;

Maybe it should be pub(crate) type BoxedFutureTask = Pin<Box<dyn Future<Output = ()> + Send + 'static>>;? I think the order in a trait object matter.

[ernestas-poskus]

Same here

48 |         let _ = tokio::spawn(watcher::watch_health(
   |                 ^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected type `std::ops::FnOnce<(&&str,)>`
              found type `std::ops::FnOnce<(&&str,)>`

[LucioFranco]

@ernestas-poskus seems like a compiler error, I would report this upstream.

[Aaron1011]

This seems to be another case of rust-lang/rust#64552. The compiler is "forgetting" that certain regions are 'static, and that certain regions are actually equal to each other. This leads to an incomprehensible error being reported - the two types really are equal, but we've erased the information that would allow us to determine this.

[Darksonn]

Closing as this is not our bug.